The Reproductive System: Part A - Next2Eden

CentriolepairsInterphase cell2n = 4NuclearenvelopeChromatinInterphase eventsAs in mitosis, meiosis is precededby DNA replication and otherpreparations for cell division.MEIOSIS ICrossoverSpindleSisterchromatidsNuclear envelopefragments latein prophase ICentromereProphase IProphase events occur, as in mitosis. Additionally,synapsis occurs: Homologous chromosomes cometogether along their length to form tetrads. Duringsynapsis, the “arms” of homologous chromatids wraparound each other, forming several crossovers. Thenonsister chromatids trade segments at points ofcrossover. Crossover is followed through the diagramsbelow.Metaphase IThe tetrads align randomly on the spindleequator in preparation for anaphase.TetradDyadChromosomesuncoilNuclearenvelopesre-formCleavagefurrowAnaphase IUnlike anaphase of mitosis, the centromeres do notseparate during anaphase I of meiosis, so the sisterchromatids (dyads) remain firmly attached. However, thehomologous chromosomes do separate from each otherand the dyads move toward opposite poles of the cell.Telophase IThe nuclear membranes re-form around thechromosomal masses, the spindle breaks down, andthe chromatin reappears as telophase and cytokinesisare completed. The 2 daughter cells (now haploid)enter a second interphase-like period, calledinterkinesis, before meiosis II occurs. There is nosecond replication of DNA before meiosis II.Copyright © 2010 Pearson Education, Inc. Figure 27.6 (1 of 2)