during prophase 1 homologous chromosomes exchange segments of dna in a process called
synapsis
737967
Chromosomal crossover Chromosomal crossover (or crossing over) is the exchange of genetic material between 2 homologous chromosomes non-sister chromatids that results in recombinant chromosomes during sexual reproduction. It is one of the final phases of genetic recombination, which occurs in the "pachytene" stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome. Crossing over was described, in theory, by Thomas Hunt Morgan. He relied
Chromosomal crossover
6309954
by a protein complex along the whole length of the chromosomes called the synaptonemal complex. Sex chromosomes also undergo synapsis; however, the synaptonemal protein complex that holds the homologous chromosomes together is only present at one end of each sex chromosome. This is not to be confused with mitosis. Mitosis also has prophase, but does not ordinarily do pairing of two homologous chromosomes. When the non-sister chromatids intertwine, segments of chromatids with similar sequence may break apart and be exchanged in a process known as genetic recombination or "crossing-over". This exchange produces a chiasma, a region that is shaped like
Synapsis
2088519
helps make a population more stable by providing a wider range of genetic traits for natural selection to act on. In prophase I of meiosis I, each chromosome is aligned with its homologous partner and pairs completely. In prophase I, the DNA has already undergone replication so each chromosome consists of two identical chromatids connected by a common centromere. During the zygotene stage of prophase I, the homologous chromosomes pair up with each other. This pairing occurs by a synapsis process where the synaptonemal complex - a protein scaffold - is assembled and joins the homologous chromosomes along their lengths.
Homologous chromosome
11013183
the chromosomes initially present is now composed of two copies called chromatids. These chromosomes (paired chromatids) then pair with the homologous chromosome (also paired chromatids) present in the same nucleus (see prophase I in the meiosis diagram). The process of alignment of paired homologous chromosomes is called synapsis (see Synapsis). During synapsis, genetic recombination usually occurs. Some of the recombination events occur by crossing over (involving physical exchange between two chromatids), but most recombination events involve information exchange but not physical exchange between two chromatids (see Synthesis-dependent strand annealing (SDSA)). Following recombination, chromosome segregation occurs as indicated by the stages
Chromosome segregation
737967
Chromosomal crossover Chromosomal crossover (or crossing over) is the exchange of genetic material between 2 homologous chromosomes non-sister chromatids that results in recombinant chromosomes during sexual reproduction. It is one of the final phases of genetic recombination, which occurs in the "pachytene" stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome. Crossing over was described, in theory, by Thomas Hunt Morgan. He relied
Chromosomal crossover
251332
process of pairing the homologous chromosomes is called synapsis. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma). Prophase I has historically been divided into a series of substages which are named according to the appearance of chromosomes. The first stage of prophase I is the "leptotene" stage, also known as "leptonema", from Greek words meaning "thin threads".In this stage of prophase I, individual chromosomes—each consisting of two sister chromatids—become "individualized" to form visible strands within the nucleus. The two sister chromatids closely associate and are visually indistinguishable from one another. During leptotene, lateral elements
Meiosis
1292941
ovulation. In the first stage of prophase I, lepotene (from the Greek for “delicate”), chromosomes begin to condense. Each chromosome is in a haploid state and consists of two sister chromatids; however, the chromatin of the sister chromatids is not yet condensed enough to be resolvable in microscopy. Homologous regions within homologous chromosome pairs begin to associate with each other. In the second phase of prophase I, zygotene (from the Greek for “conjugation”), all maternally and paternally derived chromosomes have found their homologous partner. The homologous pairs then undergo synapsis, a process by which the synaptonemal complex (a proteinaceous structure)
Prophase
1292942
aligns corresponding regions of genetic information on maternally and paternally derived non-sister chromatids of homologous chromosome pairs. The paired homologous chromosome bound by the synaptonemal complex are referred to as bivalents or tetrads. Sex (X and Y) chromosomes do not fully synapse because only a small region of the chromosomes are homologous. The nucleolus moves from a central to a peripheral position in the nucleus. The third phase of prophase I, pachytene (from the Greek for “thick”), begins at the completion of synapsis. Chromatin has condensed enough that chromosomes can now be resolved in microscopy. Structures called recombination nodules form
Prophase
6309953
Synapsis Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a protein-RNA complex called the synaptonemal complex. Autosomes undergo synapsis during meiosis, and are held together
Synapsis
6702968
Chromomere A chromomere, also known as an idiomere, is one of the serially aligned beads or granules of a eukaryotic chromosome, resulting from local coiling of a continuous DNA thread. In areas of chromatin with the absence of transcription, condensing of DNA and protein complexes will result in the formation of chromomeres. It is visible on a chromosome during the prophase of meiosis and mitosis. Giant banded (Polytene) chromosomes resulting from the replication of the chromosomes and the synapsis of homologs without cell division is a process called endomitosis. These chromosomes consist of more than 1000 copies of the same
Chromomere
5835662
of the adult female salamander. Synapsis and recombination during meiotic prophase I in these unisexual females is thought to ordinarily occur between identical sister chromosomes and occasionally between homologous chromosomes. Thus little, if any, genetic variation is produced. Recombination between homeologous chromosomes occurs rarely, if at all. Since production of genetic variation is weak, at best, it is unlikely to provide a benefit sufficient to account for the maintenance of meiosis for millions of years. Perhaps the efficient recombinational repair of DNA damages at each generation provided by meiosis has been a sufficient advantage to maintain meiosis. Endoreduplication Endoreduplication (also
Endoreduplication
6309958
appear to be initiation sites for meiotic silencing. In female "Drosophila melanogaster" fruit flies, meiotic chromosome synapsis occurs in the absence of recombination. Thus synapsis in "Drosophila" is independent of meiotic recombination, consistent with the view that synapsis is a precondition required for the initiation of meiotic recombination. Meiotic recombination is also unnecessary for homologous chromosome synapsis in the nematode "Caenorhabditis elegans". Synapsis Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I
Synapsis
475190
rice have been characterized. Studies of rice gene OsRAD51C showed that this gene is necessary for homologous recombinational repair of DNA, particularly the accurate repair of DNA double-strand breaks during meiosis. Rice gene OsDMC1 was found to be essential for pairing of homologous chromosomes during meiosis, and rice gene OsMRE11 was found to be required for both synapsis of homologous chromosomes and repair of double-strand breaks during meiosis. Rice plays an important role in certain religions and popular beliefs. In many cultures relatives will scatter rice during or towards the end of a wedding ceremony in front of the bride
Rice
251331
1c). Prophase I is typically the longest phase of meiosis. During prophase I, homologous chromosomes pair and exchange DNA (homologous recombination). This often results in chromosomal crossover. This process is critical for pairing between homologous chromosomes and hence for accurate segregation of the chromosomes at the first meiosis division. The new combinations of DNA created during crossover are a significant source of genetic variation, and result in new combinations of alleles, which may be beneficial. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. The
Meiosis
2088520
Cohesin crosslinking occurs between the homologous chromosomes and helps them resist being pulled apart until anaphase. Genetic crossing-over, a type of recombination, occurs during the pachytene stage of prophase I. In addition, another type of recombination referred to as synthesis-dependent strand annealing (SDSA) frequently occurs. SDSA recombination involves information exchange between paired homologous chromatids, but not physical exchange. SDSA recombination does not cause crossing-over. In the process of crossing-over, genes are exchanged by the breaking and union of homologous portions of the chromosomes’ lengths. Structures called chiasmata are the site of the exchange. Chiasmata physically link the homologous chromosomes once
Homologous chromosome
11013185
DNA replication. Thus the two chromatids comprising each chromosome separate into different nuclei, so that each nucleus gets a single set of chromatids (now called chromosomes) and each nucleus becomes included in a haploid gamete (see stages following prophase II in the meiosis diagram). This segregation process is also facilitated by cohesin. Failure of proper segregation during prophase II can also lead to aneuploid gametes. Aneuploid gametes can undergo fertilization to form aneuploid zygotes and hence to serious adverse consequences for progeny. Meiotic chromosomal crossover (CO) recombination facilitates the proper segregation of homologous chromosomes. This is because, at the end
Chromosome segregation
1626661
during mitosis or meiosis. SCEs appear to primarily reflect DNA recombinational repair processes responding to DNA damage (see articles Sister chromatids and Sister chromatid exchange). "Non-sister chromatids", on the other hand, refers to either of the two chromatids of paired homologous chromosomes, that is, the pairing of a paternal chromosome and a maternal chromosome. In chromosomal crossovers, non-sister (homologous) chromatids form chiasmata to exchange genetic material during the prophase I of meiosis (See Homologous recombination). Chromatid A chromatid (Greek "khrōmat-" 'color' + "-id") is one copy of a newly copied chromosome which is still joined to the original chromosome by
Chromatid
8489983
Bivalent (genetics) A bivalent, sometimes referred to as a tetrad, is the association of a pair of homologous chromosomes physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division. The formation of a bivalent occurs during the first division of meiosis (in the pachynema stage of meiotic prophase 1). In most organisms, each replicated chromosome (composed of two identical sister chromatids) elicits formation of DNA double-strand breaks during the leptotene phase. These breaks are repaired by homologous recombination, that uses the homologous chromosome as
Bivalent (genetics)
251329
chromosome pairs along the metaphase plate during metaphase I & orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I & II, it allows a random and independent distribution of chromosomes to each daughter cell (and ultimately to gametes); and (2) Crossing Over. The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of DNA within chromosomes. During meiosis, specific genes are more highly transcribed. In addition to strong meiotic stage-specific expression of mRNA, there are also pervasive translational controls (e.g. selective usage
Meiosis
1292940
exchange genetic information. Prophase II is very similar to mitotic prophase. Prophase I is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In addition to the events that occur in mitotic prophase, several crucial events occur within these phases such as pairing of homologous chromosomes and the reciprocal exchange of genetic material between these homologous chromosomes. Prophase I occurs at different speeds dependent on species and sex. Many species arrest meiosis in diplotene of prophase I until ovulation. In humans, decades can pass as oocytes remain arrested in prophase I only to quickly complete meiosis I prior to
Prophase
8489987
highlighting the robustness of the chiasma and the cohesion that hold the bivalents together. Bivalent (genetics) A bivalent, sometimes referred to as a tetrad, is the association of a pair of homologous chromosomes physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division. The formation of a bivalent occurs during the first division of meiosis (in the pachynema stage of meiotic prophase 1). In most organisms, each replicated chromosome (composed of two identical sister chromatids) elicits formation of DNA double-strand breaks during the leptotene
Bivalent (genetics)
1292943
on the synaptonemal complex of bivalents. These recombination nodules facilitate genetic exchange between the non-sister chromatids of the synaptonemal complex in an event known as crossing-over or genetic recombination. Multiple recombination events can occur on each bivalent. In humans, an average of 2-3 events occur on each chromosome. In the fourth phase of prophase I, diplotene (from the Greek for “twofold”), crossing-over is completed. Homologous chromosomes retain a full set of genetic information; however, the homologous chromosomes are now of mixed maternal and paternal descent. Visible junctions called chiasmata hold the homologous chromosomes together at locations where recombination occurred as
Prophase
2088521
crossing over occurs and throughout the process of chromosomal segregation during meiosis. Both the non-crossover and crossover types of recombination function as processes for repairing DNA damage, particularly double-strand breaks. At the diplotene stage of prophase I the synaptonemal complex disassembles before which will allow the homologous chromosomes to separate, while the sister chromatids stay associated by their centromeres. In metaphase I of meiosis I, the pairs of homologous chromosomes, also known as bivalents or tetrads, line up in a random order along the metaphase plate. The random orientation is another way for cells to introduce genetic variation. Meiotic spindles
Homologous chromosome
11013182
as cohesin. Upon proper segregation, a complete set of chromatids ends up in each of two nuclei, and when cell division is completed, each DNA copy previously referred to as a chromatid is now called a chromosome. Chromosome segregation occurs at two separate stages during meiosis called prophase I and prophase II (see meiosis diagram). In a diploid cell there are two sets of homologous chromosomes of different parental origin (e.g. a paternal and a maternal set). During the phase of meiosis labeled “interphase s” in the meiosis diagram there is a round of DNA replication, so that each of
Chromosome segregation
251318
of chromosomes as the original parent cell. The two meiotic divisions are known as Meiosis I and Meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA is
Meiosis
1292939
the spindle apparatus at locations called foci. The mitotic spindle is of great importance in the process of mitosis and will eventually segregate the sister chromatids in metaphase. The nucleoli begin to break down in prophase, resulting in the discontinuation of ribosome production. This indicates a redirection of cellular energy from general cellular metabolism to cellular division. The nuclear envelope stays intact during this process. Meiosis involves two rounds of chromosome segregation and thus undergoes prophase twice, resulting in prophase I and prophase II. Prophase I the most complex phase in all of meiosis because homologous chromosomes must pair and
Prophase
11013184
metaphase I and anaphase I in the meiosis diagram. Different pairs of chromosomes segregate independently of each other, a process termed “independent assortment of non-homologous chromosomes”. This process results in each gamete usually containing a mixture of chromosomes from both original parents. Improper chromosome segregation can result in aneuploid gametes having either too few or too many chromosomes. The second stage at which segregation occurs during meiosis is prophase II (see meiosis diagram). During this stage, segregation occurs by a process similar to that during mitosis, except that in this case prophase II is not preceded by a round of
Chromosome segregation
14022034
spindle pole bodies can separate. The event is triggered by the activation of M-Cdk in late prophase I. Then the spindle assembly checkpoint examines the attachment of microtubules at kinetochores and APC can initiate the metaphase I. The special chromosome separation in meiosis, homologous chromosomes separation in meiosis I and chromatids separation in meiosis II, requires special tension between homologous chromatids and non-homologous chromatids for distinguishing microtubule attachment and it relies on the programmed DNA double strand break (DSB) and repair in prophase I. Therefore meiotic recombination checkpoint can be a kind of DNA damage response at specific time spot.
Meiotic recombination checkpoint
1292946
preprophase, and additional step in plant mitosis that results in formation of the preprophase band, a structure composed of microtubules. In mitotic prophase I of plants this band disappears. Prophase I in mitosis is the most complex iteration of prophase that occurs in both plant cells and animal cells. To ensure pairing of homologous chromosomes and recombination of genetic material occurs properly, there are cellular checkpoints in place. The meiotic checkpoint network is a DNA damage response system that controls double strand break repair, chromatin structure, and the movement and pairing of chromosomes. The system consists of multiple pathways (including
Prophase
8489984
a template for repair. The search for the homologous target, helped by numerous proteins collectively referred as the synaptonemal complex, cause the two homologs to pair, between the leptotene and the pachytene phases of meiosis I. Resolution of the DNA recombination intermediate into a crossover exchanges DNA segments between the two homologous chromosomes at a site called a chiasma (plural: chiasmata). This physical strand exchange and the cohesion between the sister chromatids along each chromosome ensure robust pairing of the homologs in diplotene phase. The structure, visible by microscopy, is called a bivalent. A bivalent is the association of two
Bivalent (genetics)
9354234
finally splitting, resulting in two haploid daughter cells each with a single copy of every chromosome. While homologous chromosomes are lined up, they are free to exchange corresponding segments of their own DNA with that of their homolog. This results in a chromosomes that carry both maternal and paternal DNA. Through recombination, daughter cells have the greatest amount of genetic diversity. One powerful tool for interpreting and graphing linkage data sets is called Hierarchical Clustering. Clustering organizes things into groups based on similarity. In the case of linkage, similarity equates to physical proximity on a chromosome. Hierarchical clustering is a
Complete linkage
251336
known as "diplonema", from Greek words meaning "two threads", the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at the transition to anaphase I. In human fetal oogenesis, all developing oocytes develop to this stage and are arrested in prophase I before birth. This suspended state is referred to as the "dictyotene stage" or dictyate. It lasts until
Meiosis
111288
combinations with new and complex functions. For example, polyketide synthases are large enzymes that make antibiotics; they contain up to one hundred independent domains that each catalyse one step in the overall process, like a step in an assembly line. In asexual organisms, genes are inherited together, or "linked", as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called homologous recombination, sexual organisms exchange DNA between two matching chromosomes. Recombination and reassortment do not
Evolution
12882055
strands to move closer together and the junction adopts the stacked X structure. It is during this state that resolution will be optimal, allowing RuvC to bind to the junction. Branch migration Branch migration is the process by which base pairs on homologous DNA strands are consecutively exchanged at a Holliday junction, moving the branch point up or down the DNA sequence. Branch migration is the second step of genetic recombination, following the exchange of two single strands of DNA between two homologous chromosomes. The process is random, and the branch point can be displaced in either direction on the
Branch migration
4160287
the stability of the DNA during the replication process. Lack of BLM protein or protein activity leads to an increase in mutations; however, the molecular mechanism(s) by which BLM maintains stability of the chromosomes is still a very active area of research. Persons with Bloom syndrome have an enormous increase in exchange events between homologous chromosomes or sister chromatids (the two DNA molecules that are produced by the DNA replication process); and there are increases in chromosome breakage and rearrangements compared to persons who do not have Bloom's syndrome. Direct connections between the molecular processes in which BLM operates and
Bloom syndrome
11288272
heterotrimer, although RPA has a more uniform 1:1:1 stoichiometry. Replication protein A Replication protein A (RPA) is the major protein that binds to single-stranded DNA (ssDNA) in eukaryotic cells. In vitro, RPA shows a much higher affinity for ssDNA than RNA or double-stranded DNA. During DNA replication, RPA prevents single-stranded DNA (ssDNA) from winding back on itself or from forming secondary structures. This keeps DNA unwound for the polymerase to replicate it. RPA also binds to ssDNA during the initial phase of homologous recombination, an important process in DNA repair and prophase I of meiosis. Hypersensitivity to DNA damaging agents
Replication protein A
251232
is the pairing of homologous chromosomes and homologous recombination (the exchange of genetic information) between homologous chromosomes. This process promotes the production of increased genetic diversity among progeny and the recombinational repair of damages in the DNA to be passed on to progeny. To explain the adaptive function of meiosis in flowering plants, some authors emphasize diversity and others emphasize DNA repair. Apomixis (reproduction via asexually formed seeds) is found naturally in about 2.2% of angiosperm genera. One type of apomixis, gametophytic apomixis found in a dandelion species involves formation of an unreduced embryo sac due to incomplete meiosis (apomeiosis)
Flowering plant
6702969
chromatid that are aligned and produce alternating dark and light bands when stained. The dark bands are the chromomere. It is unknown when chromomeres first appear on the chromosome. Chromomeres can be observed best when chromosomes are highly condensed. The chromomeres are present during leptotene phase of prophase I during meiosis. During zygotene phase of prophase I, the chromomeres of homologs align with each other to form homologous rough pairing (homology searching). These chromomeres helps provide a unique identity for each homologous pairs. There are more than 2000 chromomeres on 20 chromosomes of maize. Chromomeres are organized in a discontinuous
Chromomere
20907289
DNA fragmentation in the sperm, the embryo will die after day 3 of growth. Oocyte quality is also a main contributor to overall embryo quality since it is the DNA of the oocyte that is mainly involved in the first 3 days of embryo growth following fertilization. A major source of genetic abnormalities are balanced translocations (Figure 1). A translocation involves the exchange of segments of chromosomes that are not a homologous pair. In most cases, this leads to balanced translocations, in which no DNA is lost therefore is usually asymptomatic. However, as female gametes are formed, it is probable
Repeated implantation failure
13566593
"Thermococcus gammatolerans" shows comparable radioresistance. "Deinococcus radiodurans" also has a unique ability to repair damaged DNA. It isolates the damaged segments in a controlled area and repairs it. These bacteria can also repair many small fragments from an entire chromosome. "Deinococcus" accomplishes its resistance to radiation by having multiple copies of its genome and rapid DNA repair mechanisms. It usually repairs breaks in its chromosomes within 12–24 hours by a 2-step process. First, "D. radiodurans" reconnects some chromosome fragments by a process called single-stranded annealing. In the second step, multiple proteins mend double-strand breaks through homologous recombination. This process does
Deinococcus radiodurans
12967442
material from both gametes. In a process called genetic recombination, genetic material (DNA) joins up so that homologous chromosome sequences are aligned with each other, and this is followed by exchange of genetic information. Two rounds of cell division then produce four daughter cells with half the number of chromosomes from each original parent cell, and the same number of chromosomes as both parents. For instance, in human reproduction each human cell contains 46 chromosomes in 23 pairs. Meiosis in the parents' gonads produce gamete cells which only contain 23 chromosomes each. When the gametes are combined via sexual intercourse
Sexual reproduction
2088535
of the pollen tube (pollen bicellular in the anther and in the stigma). The female gamete is produced inside the embryo sac of the ovule. Meiosis is a central feature of gametogenesis, but the adaptive function of meiosis is currently a matter of debate. A key event during meiosis is the pairing of homologous chromosomes and recombination (exchange of genetic information) between homologous chromosomes. This process promotes the production of increased genetic diversity among progeny and the recombinational repair of damage in the DNA to be passed on to progeny. To explain the adaptive function of meiosis (as well as
Gametogenesis
9354231
absence of recombination, only parental phenotypes are expected. Genetic Linkage is the tendency of alleles, which are located closely together on a chromosome, to be inherited together during the process of meiosis in sexually reproducing organisms. During the process of meiosis, homologous chromosomes pair up, and can exchange corresponding sections of DNA. As a result, genes that were originally on the same chromosome can finish up on different chromosomes. This process is known as genetic recombination. The rate of recombination of two discrete loci corresponds to their physical proximity. Alleles that are closer together have lower rates of recombination than
Complete linkage
12882050
Branch migration Branch migration is the process by which base pairs on homologous DNA strands are consecutively exchanged at a Holliday junction, moving the branch point up or down the DNA sequence. Branch migration is the second step of genetic recombination, following the exchange of two single strands of DNA between two homologous chromosomes. The process is random, and the branch point can be displaced in either direction on the strand, influencing the degree of which the genetic material is exchanged. Branch migration can also be seen in DNA repair and replication, when filling in gaps in the sequence. It
Branch migration
1292936
and animal cells. These stains do not band chromosomes, but instead allow for DNA probing of specific regions and genes. Use of fluorescent microscopy has vastly improved spatial resolution. Prophase is the first stage of mitosis in animal cells, and the second stage of mitosis in plant cells. At the start of prophase there are two identical copies of each chromosome in the cell due to replication in interphase. These copies are referred to as sister chromatids and are attached by DNA element called the centromere. The main events of prophase are: the condensation of chromosomes, the movement of the
Prophase
1292934
Prophase Prophase (from the Greek πρό, "before" and φάσις, "stage") is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin and the disappearance of the nucleolus. Microscopy can be used to visualize condensed chromosomes as they move through meiosis and mitosis. Various DNA stains are used to treat cells such that condensing chromosomes can be visualized as the move through prophase. The giemsa G-banding technique is commonly used to identify mammalian chromosomes, utilizing the
Prophase
17701378
breaks. Delmas et al. proposed that nucleoid compaction is part of a DNA damage response that accelerates cell recovery by helping DNA repair proteins to locate targets, and by facilitating the search for intact DNA sequences during homologous recombination. "H. volcanii" cells can undergo a pairwise process of genetic exchange which involves cell fusion resulting in a heterodiploid cell (containing two different chromosomes in one cell). Cells of a related species, "Haloferax mediterranei", can similarly undergo genetic exchange with each other. "H. volcanii" has an average nucleotide sequence identity with "H. mediterranei" of 86.6%. At a reduced frequency, cells of
Haloferax volcanii
737982
When a high correlation between the two is found, it is likely that the appropriate gene sequence is really closer. Crossovers typically occur between homologous regions of matching chromosomes, but similarities in sequence and other factors can result in mismatched alignments. Most DNA is composed of base pair sequences repeated very large numbers of times. These repetitious segments, often referred to as satellites, are fairly homogenous among a species. During DNA replication, each strand of DNA is used as a template for the creation of new strands using a partially-conserved mechanism; proper functioning of this process results in two identical,
Chromosomal crossover
251334
and may start at the centromere (procentric), at the chromosome ends (proterminal), or at any other portion (intermediate). Individuals of a pair are equal in length and in position of the centromere. Thus pairing is highly specific and exact. The paired chromosomes are called bivalent or tetrad chromosomes. The "pachytene" stage ( ), also known as "pachynema", from Greek words meaning "thick threads". At this point a tetrad of the chromosomes has formed known as a bivalent. This is the stage when homologous recombination, including chromosomal crossover (crossing over), occurs. Nonsister chromatids of homologous chromosomes may exchange segments over regions
Meiosis
1292937
centrosomes, the formation of the mitotic spindle, and the beginning of nucleoli break down. DNA that was replicated in interphase is condensed from molecules with lengths reaching 4 cm to chromosomes that are measured in micrograms. This process employs the condensin complex. Condensed chromosomes consist of two sister chromatids joined at the centromere. During prophase in animal cells, centrosomes move far enough apart to be resolved using a light microscope. Microtubule activity in each centrosome is increased due to recruitment of γ-tubulin. Replicated centrosomes from interphase move apart towards opposite poles of the cell, powered by centrosome associated motor proteins.
Prophase
7550840
breaks. This repair system is referred to as homologous recombinational repair, and it is especially active during meiosis. Meiosis is the general process by which germ cells are formed in eukaryotes, and it appears to be an adaptation for efficiently removing damages in germ line DNA by homologous recombinational repair (see Origin and function of meiosis also). Human primary oocytes are present at an intermediate stage of meiosis, that is prophase I (see Oogenesis). Titus et al. also showed that expression of four key DNA repair genes that are necessary for homologous recombinational repair ("BRCA1", "MRE11", "Rad51" and "ATM") decline
Advanced maternal age
4862615
due to base mismatch repair during homologous recombination: if one of the four chromatids during meiosis pairs up with another chromatid, as can occur because of sequence homology, DNA strand transfer can occur followed by mismatch repair. This can alter the sequence of one of the chromosomes, so that it is identical to the other. Meiotic recombination is initiated through formation of a double-strand break (DSB). The 5’ ends of the break are then degraded, leaving long 3’ overhangs of several hundred nucleotides. One of these 3’ single stranded DNA segments then invades a homologous sequence on the homologous chromosome,
Gene conversion
2453345
in humans and mice in primordial follicles. Primordial follicles contain oocytes that are at an intermediate (prophase I) stage of meiosis. Meiosis is the general process in eukaryotic organisms by which germ cells are formed, and it is likely an adaptation for removing DNA damages, especially double-strand breaks, from germ line DNA. (Also see article Meiosis). Homologous recombinational repair employing BRCA1 is especially promoted during meiosis. It was found that expression of 4 key genes necessary for homologous recombinational repair of DNA double-strand breaks ("BRCA1, MRE11, RAD51" and "ATM") decline with age in the oocytes of humans and mice, leading
BRCA1
14808764
genetic sequences are exchanged between two similar segments of DNA. HR plays a major role in eukaryotic cell division, promoting genetic diversity through the exchange between corresponding segments of DNA to create new, and potentially beneficial combinations of genes. HR performs a second vital role in DNA repair, enabling the repair of double-strand breaks in DNA which is a common occurrence during a cell's lifecycle. It is this process which is artificially triggered by the above technologies and bootstrapped in order to engender 'knock-ins' or 'knockouts' in specific genes5, 7. A recent key advance was discovered using AAV-homologous recombination vectors,
Isogenic human disease models
473716
stage of division. The nuclear envelope is broken down, long strands of chromatin condense to form shorter more visible strands called chromosomes, the nucleolus disappears, and microtubules attach to the chromosomes at the kinetochores present in the centromere. Microtubules associated with the alignment and separation of chromosomes are referred to as the spindle and spindle fibers. Chromosomes will also be visible under a microscope and will be connected at the centromere. During this condensation and alignment period, homologous chromosomes may swap portions of their DNA in a process known as crossing over. Metaphase is the stage in cell division when
Cell division
1292947
the meiotic recombination checkpoint) that prevent the cell from entering metaphase I with errors due to recombination. Prophase Prophase (from the Greek πρό, "before" and φάσις, "stage") is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin and the disappearance of the nucleolus. Microscopy can be used to visualize condensed chromosomes as they move through meiosis and mitosis. Various DNA stains are used to treat cells such that condensing chromosomes can be visualized as
Prophase
11288270
Replication protein A Replication protein A (RPA) is the major protein that binds to single-stranded DNA (ssDNA) in eukaryotic cells. In vitro, RPA shows a much higher affinity for ssDNA than RNA or double-stranded DNA. During DNA replication, RPA prevents single-stranded DNA (ssDNA) from winding back on itself or from forming secondary structures. This keeps DNA unwound for the polymerase to replicate it. RPA also binds to ssDNA during the initial phase of homologous recombination, an important process in DNA repair and prophase I of meiosis. Hypersensitivity to DNA damaging agents can be caused by mutations in the RPA gene.
Replication protein A
2088528
reactive oxygen species. Homologous chromosomes can repair this damage by aligning themselves with chromosomes of the same genetic sequence. Once the base pairs have been matched and oriented correctly between the two strands, the homologous chromosomes perform a process that is very similar to recombination, or crossing over as seen in meiosis. Part of the intact DNA sequence overlaps with that of the damaged chromosome's sequence. Replication proteins and complexes are then recruited to the site of damage, allowing for repair and proper replication to occur. Through this functioning, double-strand breaks can be repaired and DNA can function normally. Current
Homologous chromosome
11013181
Chromosome segregation Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. During mitosis chromosome segregation occurs routinely as a step in cell division (see mitosis diagram). As indicated in the mitosis diagram, mitosis is preceded by a round of DNA replication, so that each chromosome forms two copies called chromatids. These chromatids separate to opposite poles, a process facilitated by a protein complex referred to
Chromosome segregation
1380521
Genetic recombination During meiosis in eukaryotes, genetic recombination involves the pairing of homologous chromosomes. This may be followed by information transfer between the chromosomes. The information transfer may occur without physical exchange (a section of genetic material is copied from one chromosome to another, without the donating chromosome being changed) (see SDSA pathway in Figure); or by the breaking and rejoining of DNA strands, which forms new molecules of DNA (see DHJ pathway in Figure). Recombination may also occur during mitosis in eukaryotes where it ordinarily involves the two sister chromosomes formed after chromosomal replication. In this case, new combinations
Genetic recombination
15065775
Replication timing Replication timing refers to the order in which segments of DNA along the length of a chromosome are duplicated. In eukaryotic cells (cells that package their DNA within a nucleus), chromosomes consist of very long linear double-stranded DNA molecules. During the S-phase of each cell cycle (Figure 1), all of the DNA in a cell is duplicated in order to provide one copy to each of the daughter cells after the next cell division. The process of duplicating DNA is called DNA replication, and it takes place by first unwinding the duplex DNA molecule, starting at many locations
Replication timing
9832283
first division of meiosis. In humans, a common variant of the "MLH1" gene is associated with increased risk of sperm damage and male infertility. MLH1 protein appears to localize to sites of crossing over in meiotic chromosomes. Recombination during meiosis is often initiated by a DNA double-strand break (DSB) as illustrated in the accompanying diagram. During recombination, sections of DNA at the 5' ends of the break are cut away in a process called "resection". In the "strand invasion" step that follows, an overhanging 3' end of the broken DNA molecule then "invades" the DNA of an homologous chromosome that
MLH1
12967464
damaging conditions induces cellular aggregation accompanied by high frequency genetic marker exchange. Ajon et al. hypothesized that this cellular aggregation enhances species-specific DNA repair by homologous recombination. DNA transfer in Sulfolobus may be an early form of sexual interaction similar to the more well-studied bacterial transformation systems that also involve species-specific DNA transfer leading to homologous recombinational repair of DNA damage. Sexual reproduction Sexual reproduction is a kind of life cycle where generations alternate between cells with a single set of chromosomes (haploid) and cells with a double set of chromosomes (diploid). "Sexual reproduction" is by far the most common
Sexual reproduction
20163266
status. This is usually achieved by elimination of duplicated genes. The main goals of diploidization are: (1) To ensure proper gene dosage; and (2) to maintain stable cellular division processes. It should be noted that this process does not need to occur rapidly for all chromosomes in one or few steps. In recent polyploid events, segments of the genome may still remain in a tetraploid status. In other words, diploidization is a long ongoing process that is shaped by both intrinsic and evolutionary drives. Normally, homologous chromosomes pair up in bivalents during meiosis and separate into different daughter cells. However,
Diploidization
1380525
between the paired chromosomes inherited from each of one's parents, generally occurring during meiosis. During prophase I (pachytene stage) the four available chromatids are in tight formation with one another. While in this formation, homologous sites on two chromatids can closely pair with one another, and may exchange genetic information. Because recombination can occur with small probability at any location along chromosome, the frequency of recombination between two locations depends on the distance separating them. Therefore, for genes sufficiently distant on the same chromosome, the amount of crossover is high enough to destroy the correlation between alleles. Tracking the movement
Genetic recombination
97597
reproduction, when genetic recombination occurs. Chromosomal crossover is when two DNA helices break, swap a section and then rejoin. Recombination allows chromosomes to exchange genetic information and produces new combinations of genes, which increases the efficiency of natural selection and can be important in the rapid evolution of new proteins. Genetic recombination can also be involved in DNA repair, particularly in the cell's response to double-strand breaks. The most common form of chromosomal crossover is homologous recombination, where the two chromosomes involved share very similar sequences. Non-homologous recombination can be damaging to cells, as it can produce chromosomal translocations and
DNA
4862624
studied are associated with non-crossover of outside markers. Non-crossover gene conversion events are mainly produced by Synthesis Dependent Strand Annealing (SDSA). This process involves limited informational exchange, but not physical exchange of DNA, between the two participating homologous chromosomes at the site of the conversion event, and little genetic variation is produced. Thus, explanations for the adaptive function of meiotic recombination that focus exclusively on the adaptive benefit of producing new genetic variation or physical exchange seem inadequate to explain the majority of recombination events during meiosis. However, the majority of meiotic recombination events can be explained by the proposal
Gene conversion
9358012
mice and humans, the BRCA2 complex primarily mediates orderly assembly of RAD51 on ssDNA, the form that is active for homologous pairing and strand invasion. BRCA2 also redirects RAD51 from dsDNA and prevents dissociation from ssDNA. However, in the presence of a BRCA2 mutation, human RAD52 can mediate RAD51 assembly on ssDNA and substitute for BRCA2 in homologous recombinational DNA repair, though with lower efficiency than BRCA2. Further steps are detailed in the article Homologous recombination. Rad51 has a crucial function in meiotic prophase in mice and its loss leads to depletion of late prophase I spermatocytes. During meiosis, the
RAD51
11330623
homologous double-stranded DNA (dsDNA). Once the homologous sequence is found, the recombinases facilitate invasion of the ssDNA end into the homologous dsDNA to form a D-loop. After strand exchange, homologous recombination intermediates are processed by either of two distinct pathways (see diagram) to form the final recombinant chromosomes. D-loop In molecular biology, a displacement loop or D-loop is a DNA structure where the two strands of a double-stranded DNA molecule are separated for a stretch and held apart by a third strand of DNA. An R-loop is similar to a D-loop, but in this case the third strand is RNA
D-loop
11013191
ability depends on the microtubule motor dynein that regulates the movement of chromosomes to the poles of the meiotic spindle. Chromosome segregation Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. During mitosis chromosome segregation occurs routinely as a step in cell division (see mitosis diagram). As indicated in the mitosis diagram, mitosis is preceded by a round of DNA replication, so that each chromosome
Chromosome segregation
16998172
Bloom syndrome protein, a RecQ helicase. These mutations may be frameshift, missense, non-sense, or mutations of other kinds and are likely to cause deletions in the gene product. Apart from helicase activity that is common to all RecQ helices, it also acts to prevent inappropriate homologous recombination. During replication of the genome, the two copies of DNA, called sister chromatids, are held together through a structure called the centromere. During this time, the homologous (corresponding) copies are in close physical proximity to each other, allowing them to 'cross' and exchange genetic information, a process called homologous recombination. Defective homologous recombination
Progeroid syndromes
12437200
more common mechanisms of gene duplication are retroposition, unequal crossing over, and non-homologous segmental duplication. Retroposition is when the mRNA transcript of a gene is reverse transcribed back into DNA and inserted into the genome at a different location. During unequal crossing over, homologous chromosomes exchange uneven portions of their DNA. This can lead to the transfer of one chromosome's gene to the other chromosome, leaving two of the same gene on one chromosome, and no copies of the gene on the other chromosome. Non-homologous duplications result from replication errors that shift the gene of interest into a new position.
Gene redundancy
14795961
to condense and undergo pairing. Stress-induced condensation occurs by a non-random, zipper-like convergence of sister chromosomes. This convergence appears to depend on the ability of identical double-stranded DNA molecules to specifically identify each other, a process that culminates in the proximity of homologous sites along the paired chromosomes. Diverse stress conditions appear to prime bacteria to effectively cope with severe DNA damages such as double-strand breaks. The apposition of homologous sites associated with stress-induced chromosome condensation helps explain how repair of double-strand breaks and other damages occurs. Eukaryotic DNA with a typical length of dozens of centimeters should be orderly
DNA condensation
8810543
for the discovery of sex chromosomes or she discovered them at the same time as Edmund Beecher Wilson. In humans, each cell nucleus contains 23 pairs of chromosomes, a total of 46 chromosomes. The first 22 pairs are called autosomes. Autosomes are homologous chromosomes i.e. chromosomes which contain the same genes (regions of DNA) in the same order along their chromosomal arms. The chromosomes of the 23rd pair are called allosomes consisting of two X chromosomes in most females, and an X chromosome and a Y chromosome in most males. Females therefore have 23 homologous chromosome pairs, while males have
Sex chromosome
11013187
during the second equational division of meiosis II. Both proper initial segregation of chromosomes in prophase I and the next chromosome segregation during equational division in meiosis II are required to generate gametes with the correct number of chromosomes. CO recombinants are produced by a process involving the formation and resolution of Holliday junction intermediates. As indicated in the figure titled "A current model of meiotic recombination", the formation of meiotic crossovers can be initiated by a double-strand break (DSB). The introduction of DSBs in DNA often employs the topoisomerase-like protein SPO11. CO recombination may also be initiated by external
Chromosome segregation
10126806
target. The bacterioruberin radical produced is less reactive than the initial radical, and will likely react with another radical, resulting in termination of the radical chain reaction. "H. salinarum" is polyploid and highly resistant to ionizing radiation and desiccation, conditions that induce DNA double-strand breaks. Although chromosomes are initially shattered into many fragments, complete chromosomes are regenerated by making use of over-lapping fragments. Regeneration occurs by a process involving DNA single-stranded binding protein, and is likely a form of homologous recombinational repair. Whole genome sequences are available for two strains of "H. salinarum", NRC-1 and R1. The Halobacterium sp. NRC-1
Halobacterium salinarum
4505703
tetrasomic inheritance (four chromosomes pairing at meiosis), intergenomic recombination, and reduced fertility. Gametology denotes the relationship between homologous genes on non-recombining, opposite sex chromosomes. The term was coined by García-Moreno and Mindell. 2000. Gametologs result from the origination of genetic sex determination and barriers to recombination between sex chromosomes. Examples of gametologs include CHDW and CHDZ in birds. Sequence homology Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a speciation event
Sequence homology
15911955
cycle. During the first stage or prophase, the microtubules required for cell division begins to form and grow towards the newly formed chromosomes forming a bundle of microtubules called the mitotic spindle. During prometaphase and metaphase this spindle attaches itself to the chromosomes at a particular point called the kinetochore and undergoes several growing and shortening periods in tuning with the back and forth oscillations of the chromosomes. In anaphase also, the microtubules attached to the chromosomes maintain a carefully regulated shortening and lengthening process. Thus the presence of a drug which can suppress the microtubule dynamics is sufficient to
Discovery and development of tubulin inhibitors
15552842
by asymmetric double strand breaks. PRDM9 mediates the process of meiosis by directing the sites of homologous recombination. In humans and mice, recombination does not occur evenly throughout the genome but at particular sites along the chromosomes called recombination hotspots. Hotspots are regions of DNA about 1-2kb in length. There are approximately 30,000 to 50,000 hotspots within the human genome corresponding to one for every 50-100kb DNA on average. In humans, the average number of crossover recombination events per hotspot is one per 1,300 meioses, and the most extreme hotspot has a crossover frequency of one per 110 meioses. These
PRDM9
1380537
observed in many species. In mammals, females most often have higher rates of recombination. The "Haldane-Huxley rule" states that achiasmy usually occurs in the heterogametic sex. Genetic recombination During meiosis in eukaryotes, genetic recombination involves the pairing of homologous chromosomes. This may be followed by information transfer between the chromosomes. The information transfer may occur without physical exchange (a section of genetic material is copied from one chromosome to another, without the donating chromosome being changed) (see SDSA pathway in Figure); or by the breaking and rejoining of DNA strands, which forms new molecules of DNA (see DHJ pathway in
Genetic recombination
304059
oocytes are repaired less efficiently in these women, and this inefficiency leads to early reproductive failure. The BRCA1 protein plays a key role in a type of DNA repair termed homologous recombinational repair that is the only known cellular process that can accurately repair DNA double-strand breaks. Titus et al. showed that DNA double-strand breaks accumulate with age in humans and mice in primordial follicles. Primordial follicles contain oocytes that are at an intermediate (prophase I) stage of meiosis. Meiosis is the general process in eukaryotic organisms by which germ cells are formed, and it is likely an adaptation for
Ovary
8249734
syndrome. The formation of dicentric chromosomes and their implications on centromere function are studied in certain clinical cytogenetics laboratories. Most dicentric chromosomes are known to form through chromosomal inversions, which are rotations in regions of a chromosome due to chromosomal breakages or intra-chromosomal recombinations. Inversions that exclude the centromere are known as paracentric inversions, which result in unbalanced gametes after meiosis. During prophase of meiosis I, homologous chromosomes form an inversion loop and crossover occurs. If a paracentric inversion has occurred, one of the products will be acentric, while the other product will be dicentric. The dicentric chromatid is pulled
Dicentric chromosome
7971329
mushroom cap. Meiosis is a specialized cell division process, occurring in diploid cells, in which a single round of DNA replication occurs, and is followed by two divisions to produce four haploid daughter nuclei. During meiosis homologous chromosomes pair with each other and undergo a DNA repair process in which DNA damage is removed and genetic information is recombined. Burns et al. studied the expression of genes involved in the 15-hour meiotic process encompassing time points prior to the haploid nuclear fusion that forms the diploid zygote to the final formation of the four haploid products. They compared expression of
Coprinopsis cinerea
7596125
of chromomeres with large chromatin loops extended laterally. Amphibian and avian lampbrush chromosomes can be microsurgically isolated from oocyte nucleus (germinal vesicle) with either forceps or needles. Each lateral loop contains one or several transcription units with polarized RNP-matrix coating the DNA axis of the loop. Giant chromosomes in the lampbrush form are useful model for studying chromosome organization, genome function and gene expression during meiotic prophase, since they allow the individual transcription units to be visualized. Moreover, lampbrush chromosomes are widely used for high-resolution mapping of DNA sequences and construction of detail cytological maps of individual chromosomes. it is
Lampbrush chromosome
1380522
of alleles are not produced since the sister chromosomes are usually identical. In meiosis and mitosis, recombination occurs between similar molecules of DNA (homologous sequences). In meiosis, non-sister homologous chromosomes pair with each other so that recombination characteristically occurs between non-sister homologues. In both meiotic and mitotic cells, recombination between homologous chromosomes is a common mechanism used in DNA repair. Genetic recombination and recombinational DNA repair also occurs in bacteria and archaea, which use asexual reproduction. Recombination can be artificially induced in laboratory (in vitro) settings, producing recombinant DNA for purposes including vaccine development. V(D)J recombination in organisms with an
Genetic recombination
15949254
may form by any number of processes wherein chromosomes remain topologically entangled during mitosis. One way in which this may occur is the failure to resolve joint molecules formed during homologous recombination mediated DNA repair, a process that ensures that replicated chromosomes are intact before chromosomes are segregated during cell division. In particular, genetic studies have demonstrated that the loss of the enzymes BLM (Bloom's Syndrome Helicase) or FANCM each result in a dramatic increase in the number of chromatin bridges. This occurs because loss of these genes causes an increase in chromosome fusions, either in an end-to-end manner or
Chromatin bridge
20901305
adjacent. This process is common for eukaryotic cells and tends to act as a repair mechanism, but can lead to these mutations if illegitimate recombination occurs. The illegitimate recombination will often take the form of large chromosomal aberrations within a eukaryotic organism as it has much larger segments of DNA than prokaryotic cells. As of such non-homologous end joining can cause illegitimate recombination which creates insertion and deletion mutations in chromosomes as well as translocation of one chromosomal segment to that of another chromosome. These large scale changes in the chromosome in eukaryotic organisms tend to have deleterious effects on
Illegitimate recombination
2453324
sister chromatid. In the nucleus of many types of normal cells, the BRCA1 protein interacts with RAD51 during repair of DNA double-strand breaks. These breaks can be caused by natural radiation or other exposures, but also occur when chromosomes exchange genetic material (homologous recombination, e.g., "crossing over" during meiosis). The BRCA2 protein, which has a function similar to that of BRCA1, also interacts with the RAD51 protein. By influencing DNA damage repair, these three proteins play a role in maintaining the stability of the human genome. BRCA1 is also involved in another type of DNA repair, termed mismatch repair. BRCA1
BRCA1
411953
repair of damaged DNA by means of homologous recombination. Van Wolferen et al., in discussing DNA exchange in the hyperthermophiles under extreme conditions, noted that DNA exchange likely plays a role in repair of DNA via homologous recombination. They suggested that this process is crucial under DNA damaging conditions such as high temperature. Also it has been suggested that DNA transfer in "Sulfolobus" may be a primitive form of sexual interaction similar to the more well-studied bacterial transformation systems that are associated with species-specific DNA transfer between cells leading to homologous recombinational repair of DNA damage [see Transformation (genetics)]. Thermophile
Thermophile
97596
with human DNA-dependent DNA polymerases, RNA polymerase II, the enzyme that transcribes most of the genes in the human genome, operates as part of a large protein complex with multiple regulatory and accessory subunits. A DNA helix usually does not interact with other segments of DNA, and in human cells, the different chromosomes even occupy separate areas in the nucleus called "chromosome territories". This physical separation of different chromosomes is important for the ability of DNA to function as a stable repository for information, as one of the few times chromosomes interact is in chromosomal crossover which occurs during sexual
DNA
2088511
Homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same loci where they provide points along each chromosome which enable a pair of chromosomes to align correctly with each other before separating during meiosis. This is the basis for Mendelian inheritance which characterizes inheritance patterns of genetic material from an organism to its offspring parent developmental cell at the given time and area. Chromosomes are linear arrangements of condensed deoxyribonucleic acid (DNA)
Homologous chromosome
8254137
chromosomes. Therefore, techniques like G‑banding were developed that made "bands" appear on the chromosomes. These bands were the same in appearance on the homologous chromosomes, thus, identification became easier and more accurate. The less condensed the chromosomes are, the more bands appear when G-banding. This means that the different chromosomes are more distinct in prophase than they are in metaphase. Other types of cytogenetic banding are listed below: G banding G-banding, G banding, or Giemsa banding is a technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes. It is useful for identifying genetic diseases through the
G banding
11922290
by exonucleases to complete the initial resection stage of the overall process. MRX complex The MRX complex is a heterotrimeric protein complex consisting of Mre11, Rad50, and Xrs2. It is a budding yeast homolog of the mammalian Mre11-Rad50-Nbs1 (MRN) DNA damage repair complex. Cells are able to accurately repair DNA double-strand breaks using a process called homologous recombination. By this process DNA sequence information that is lost because of the breakage can be recovered from a second homologous DNA molecule. Homologous recombinational repair is important for removing DNA damage both during mitosis and meiosis. The repair process begins with the
MRX complex
5981371
protein interacts with entering single-stranded DNA (ssDNA) to form RecA/ssDNA nucleofilaments that scan the resident chromosome for regions of homology and bring the entering ssDNA to the corresponding region, where strand exchange and homologous recombination occur. Thus the process of homologous recombination during bacterial transformation has fundamental similarities to homologous recombination during meiosis. Homologous recombination occurs in several groups of viruses. In DNA viruses such as herpesvirus, recombination occurs through a break-and-rejoin mechanism like in bacteria and eukaryotes. There is also evidence for recombination in some RNA viruses, specifically positive-sense ssRNA viruses like retroviruses, picornaviruses, and coronaviruses. There is controversy
Homologous recombination
9358005
of meiosis-specific DMC1, are essential for development in mammals. Rad51 is a member of the RecA-like NTPases. In humans, RAD51 is a 339-amino acid protein that plays a major role in homologous recombination of DNA during double strand break repair. In this process, an ATP dependent DNA strand exchange takes place in which a template strand invades base-paired strands of homologous DNA molecules. RAD51 is involved in the search for homology and strand pairing stages of the process. Unlike other proteins involved in DNA metabolism, the RecA/Rad51 family forms a helical nucleoprotein filament on DNA. This protein can interact with
RAD51
365928
the products digested with the restriction enzyme. This method has been called Cleaved Amplified Polymorphic Sequence (CAPS). Alternatively, the amplified segment can be analyzed by Allele specific oligonucleotide (ASO) probes, a process that can often be done by a simple Dot blot. Restriction fragment length polymorphism In molecular biology, restriction fragment length polymorphism (RFLP) is a technique that exploits variations in homologous DNA sequences. It refers to a difference between samples of homologous DNA molecules from differing locations of restriction enzyme sites, and to a related laboratory technique by which these segments can be illustrated. In RFLP analysis, the DNA
Restriction fragment length polymorphism
4447813
proliferating; Hayflick called this the time of "luxuriant growth". After months of doubling the cells eventually reach phase three, a phenomenon he named "senescence", where cell replication rate slows before halting altogether. The Hayflick limit has been found to correlate with the length of the telomeric region at the end of chromosomes. During the process of DNA replication of a chromosome, small segments of DNA within each telomere are unable to be copied and are lost. This occurs due to the uneven nature of DNA replication, where leading and lagging strands are not replicated symmetrically. The telomeric region of DNA
Hayflick limit
1292945
occurs with a haploid number of chromosomes as opposed to the diploid number in mitotic prophase. In both animal and plant cells chromosomes may de-condense during telophase I requiring them to re-condense in prophase II. If chromosomes do not need to re-condense, prophase II often proceeds very quickly as is seen in the model organism Arabidopsis. The most notable difference between prophase in plant cells and animal cells occurs because plant cells lack centrioles. The organization of the spindle apparatus is associated instead with foci at opposite poles of the cell or is mediated by chromosomes. Another notable difference is
Prophase
175328
gene; different genes have different sequences of bases. Within cells, the long strands of DNA form condensed structures called chromosomes. Organisms inherit genetic material from their parents in the form of homologous chromosomes, containing a unique combination of DNA sequences that code for genes. The specific location of a DNA sequence within a chromosome is known as a locus. If the DNA sequence at a particular locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect
Heredity
2326608
phases, prophase, metaphase, anaphase, and telophase. Prophase is the initial phase when spindle fibers appear that function to move the chromosomes toward opposite poles. This spindle apparatus consists of microtubules, microfilaments and a complex network of various proteins. During metaphase, the chromosomes line up using the spindle apparatus in the middle of the cell along the equatorial plate. The chromosomes move to opposite poles during anaphase and remain attached to the spindle fibers by their centromeres. Animal cell cleavage furrow formation is caused by a ring of actin microfilaments called the contractile ring, which forms during early anaphase. Myosin is
Cleavage furrow
11922288
MRX complex The MRX complex is a heterotrimeric protein complex consisting of Mre11, Rad50, and Xrs2. It is a budding yeast homolog of the mammalian Mre11-Rad50-Nbs1 (MRN) DNA damage repair complex. Cells are able to accurately repair DNA double-strand breaks using a process called homologous recombination. By this process DNA sequence information that is lost because of the breakage can be recovered from a second homologous DNA molecule. Homologous recombinational repair is important for removing DNA damage both during mitosis and meiosis. The repair process begins with the degradation of the 5’ end on either side of the double-strand break
MRX complex