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Meiosis

Meiosis. a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete". reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished.

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Meiosis

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  1. Meiosis a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete". reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished.

  2. Stages of meiosis: • Interphase • Prophase I : • Preleptotene: ☺ Corresponds generally to G2 of a mitoticdivision ☺ Chromosomes are duplicated ☺ Chromosomes are randomlydispersed innucleus, no evidence of pairing

  3. Preleptotene

  4. Leptotene (thin-thread stage): ☺ Although DNA synthesis has occurred, no longitudinal doubleness is evident with light microscopy ☺ Chromosome contraction is evident ☺ Each chromosome develops a lateralcomponent of a nucleoprotein complexbetween its two chromatids

  5. Leptotene

  6. Zygotene (yoked-thread stage): ☺ Synapsis or pairing of homologues isinitiated ☺ Pairing is initiated at one or more sites alongthe length of the chromosomes and proceedsto bring the homologous pairs into alignmentalong their entire length ☺ The lateral elements of the twochromatidscombine with a central core to form thesynaptonemal complex

  7. Zygotene

  8. Pachytene (thick-thread stage): ☺ The synaptonemal complex (SC) iscomplete with chromosome synapsis fromend to end ☺ The chromosomes are visibly thickerbecause they have paired and contracted andappear to be present in a haploid number ☺ Each chromosome can be recognized to betwo closely appressed homologues ☺ The paired homologues are called bivalent

  9. Pachytene

  10. Diplotene (double stage): ☺ The termination of pachytene coincides withthe dissolution of the synaptonemal complex ☺ The chromosomes condense further ☺ The sister chromatids becomes clearly evident

  11. ☺ The paired chromosomes are held together atone or more points along their length • The points of contact are called chiasma,which are the points of chromatid exchangethat help preserve the bivalent structure • As chromosomes continue to contract thechiasmata tend to move toward the ends ofpaired homologues

  12. Diplotene

  13. Diakinesis (through movement): ☺ Further contraction of the bivalents occurs ☺ Bivalents become dispersed throughout thecell ☺ Nuclear envelope breaks down

  14. Diakinesis Will be continued ….

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