Prophase Prophase is the first stage of division. The nuclear envelope begins to be broken down in this stage, long strands of chromatin condense to form shorter more visible strands called chromosomes, the nucleolus disappears, and the mitotic spindle begins to assemble from the two centrosomes. 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 in meiosis, the homologous chromosomes undergo a break in their double-stranded DNA at the same locations, followed by a recombination of the now fragmented parental DNA strands into non-parental combinations, known as crossing over. This process is evidenced to be caused in a large part by the highly conserved Spo11 protein through a mechanism similar to that seen with topoisomerase in DNA replication and transcription.
ProphAse prophase iS The fiRSt stage of divisiON. THe nUclEar EnvelOPe BeginS tO BE bRoKen DOWn in ThiS StAGe, loNG strandS of chromaTIN condeNse to foRm ShoRtEr MorE visIble StrandS CaLLed ChroMosoMEs, ThE nuCLeolus dISaPPears, and thE MitoTic sPInDLe bEginS to ASsembLE frOm tHe TwO CentRosoMEs. 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 IN meiosis, tHe homOlOgous chromOsoMES undergo A breaK iN their doublE-sTrandED dNa at ThE SaMe LocatiONs, foLLowed bY A recOmbinatIOn oF the Now fRAGMEnteD pAReNTal dna StrAndS inTO non-pAreNTAL combinatIONs, known As CrossiNg Over. tHis ProceSS iS EVidencED tO bE cauSed in a largE Part by ThE higHLy cOnserVed sPo11 proteIn ThrOuGh A MecHanISm SimilaR to ThAt seEN wIth TopoIsomErAse IN dna replIcatiOn and TransCRipTiOn.