DNA, short for deoxyribonucleic acid, is a double-stranded, helical molecule that forms the molecular basis for heredity. For DNA replication to occur, this molecule must first unwind, or "unzip," itself to allow the information-encoding bases to become accessible. The base pairing within DNA is of a complementary nature and, consequently, when the molecule unzips, due to the action of enzymes, two strands are temporarily produced, each of which acts as a template. A replication fork is first made—the DNA molecule separates at a small region and then the enzyme DNA polymerase adds complementary nucleotides to each side of the freshly separated strands. The DNA polymerase adds nucleotides only to one end of the DNA. As a result, one strand (the leading strand) is replicated continuously, while the other strand (the lagging strand) is replicated discontinuously, in short bursts. Each of these small sections is finally joined to its neighbor by the action of another enzyme, DNA ligase, to give a complete strand. This whole process gives rise to two completely new and identical daughter strands of DNA.
In the semi-conservative method, two strands of the parent molecule unwind and each becomes a template for the synthesis of the complementary strand of the daughter molecule. A competing hypothesis, which would eventually be disproved, was the conservative hypothesis that states no unzipping occurs and a new DNA molecule is formed alongside the original parent molecule. Consequently, of the two molecules of DNA produced after a round of replication, one of them is the intact parent molecule. By using radioactively labeled nitrogen to produce new DNA over several generations of cell replication by a bacillus species, all of the DNA in the daughter cells contained labeled nitrogen. The bacilli were then placed in media containing unlabeled nitrogen. After a further round of DNA replication the DNA was examined and it was found to contain equal amounts of labeled and unlabeled nitrogen. In the second generation two types of DNA were found—half was identical to the DNA from the first generation and the remaining half was found to consist of entirely unlabeled nitrogen. These results are consistent with the zip fastener model of the semi-conservative hypothesis, but not at all consistent with the conservative hypothesis. Thus, it was shown that DNA replication proceeds via the semi-conservative replication method.
This method of replication, known as the semi-conservative hypothesis, was proposed from the outset of the discovery, with the description of the structure of DNA by biochemists James D. Watson and Francis Harry Compton Crick in 1953. In 1957, biochemist Arthur Kornberg first produced new DNA from the constituent parts and a parent strand, forming synthetic but not biologically active molecules of DNA outside the cell. However, not until the work of Matthew Meselson and Franklin W. Stahl in the late 1950s was the semi-conservative hypothesis conclusively proven true.
- DNA Synthesis
- DNA Fingerprinting - The Mechanics Of Genetic Fingerprinting, Genetic Fingerprinting As A Forensic Tool, Historical Uses Of Genetic Fingerprinting
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DNA (Deoxyribonucleic acid) ni molekuli ya spiral yenye nyuzi mbili ambayo hufanyiza msingi wa molekuli kwa ajili ya urithi. Ili DNA ifanikiwe, molekuli hiyo lazima kwanza ijifungue ili kuwezesha misingi ya habari ipatikane. Kuunganishwa kwa msingi ndani ya DNA ni ya asili ya ziada na, kwa hivyo, wakati molekuli inapofunguliwa, kwa sababu ya hatua ya enzymes, nyuzi mbili hutengenezwa kwa muda, kila moja ikifanya kazi kama template. Kwanza, mviringo wa kuiga hutengenezwa - molekuli ya DNA hutenganishwa katika eneo dogo na kisha enzyme ya DNA polymerase inaongeza nucleotides za ziada kwa kila upande wa nyuzi zilizotenganishwa hivi karibuni. DNA polymerase huongeza nucleotidi kwenye ncha moja tu ya DNA. Matokeo yake, kamba moja (kamba inayoongoza) hufanyizwa kwa kuendelea, huku kamba nyingine (kamba inayobaki) ikifanyizwa kwa njia isiyo ya kuendelea, kwa vipindi vifupi. Kila sehemu ndogo huunganishwa na jirani yake kwa utendaji wa enzyme nyingine, DNA ligase, ili kutokeza nyuzi kamili. Utaratibu huo wote hutokeza nyuzi mbili mpya kabisa na zinazofanana za DNA. Katika njia ya nusu-conservative, nyuzi mbili za molekuli ya mzazi hufunguliwa na kila moja inakuwa template kwa ajili ya synthesis ya nyuzi ya ziada ya molekuli ya binti. Nadharia ya kupinga, ambayo hatimaye ingethibitishwa kuwa ni ya uwongo, ilikuwa nadharia ya kihafidhina ambayo inasema hakuna kufungua na molekuli mpya ya DNA huundwa pamoja na molekuli ya asili ya mzazi. Kwa hiyo, kati ya molekuli mbili za DNA zinazotokezwa baada ya mzunguko mmoja wa kuiga, moja ni molekuli ya mzazi isiyoharibiwa. Kwa kutumia nitrojeni iliyotiwa alama ya mnururisho kutengeneza DNA mpya kwa vizazi kadhaa vya kuzaliana kwa chembe za spishi ya bacillus, DNA zote katika chembe za binti zilikuwa na nitrojeni iliyotiwa alama. Kisha bakteria hizo ziliwekwa katika vyombo vya habari vilivyo na nitrojeni isiyo na alama. Baada ya mzunguko mwingine wa replication ya DNA, DNA ilichunguzwa na iligunduliwa kuwa na kiasi sawa cha nitrojeni iliyo na alama na isiyo na alama. Katika kizazi cha pili aina mbili za DNA zilipatikana - nusu ilikuwa sawa na DNA kutoka kizazi cha kwanza na nusu iliyobaki iligunduliwa kuwa inajumuisha nitrojeni isiyo na alama kabisa. Matokeo haya ni sawa na mfano zip fastener ya semi-conservative nadharia, lakini si kabisa sawa na nadharia ya kihafidhina. Hivyo, ilionyeshwa kwamba DNA replication inaendelea kupitia njia ya replica nusu-conservative. "Kama ilivyoelezwa hapo juu, ""semi-conservative hypothesis"" ni nadharia ya kuunda DNA iliyoanzishwa na James D. Watson na Francis Harry Compton Crick mnamo 1953." Katika 1957, Arthur Kornberg, mwanakemia wa kibiolojia, alitoa DNA mpya kutoka kwa sehemu za msingi na nyuzi ya mzazi, na kuunda molekuli za DNA za synthetic lakini zisizo na utendaji wa kibiolojia nje ya seli. Hata hivyo, hadi kazi ya Matthew Meselson na Franklin W. Stahl mwishoni mwa miaka ya 1950, nadharia ya nusu-mshikamanifu haikuthibitishwa kuwa kweli. - DNA Synthesis - DNA Fingerprinting - The Mechanics Of Genetic Fingerprinting, Genetic Fingerprinting As A Forensic Tool, Historical Uses Of Genetic Fingerprinting - Other Free Encyclopedias