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.

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.

Stages of meiosis:Stages of meiosis:

Interphase

Prophase I :

Interphase

Prophase I :

• Preleptotene:• Preleptotene:

Corresponds generally to G2 of a mitotic divisionCorresponds generally to G2 of a mitotic division

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Chromosomes are duplicatedChromosomes are duplicated☺☺

Chromosomes are randomly dispersed in nucleus, no evidence of pairing

Chromosomes are randomly dispersed in nucleus, no evidence of pairing

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PreleptotenePreleptotene

• Leptotene (thin-thread stage):• Leptotene (thin-thread stage):

Although DNA synthesis has occurred, nolongitudinal doubleness is evident with light microscopy

Although DNA synthesis has occurred, nolongitudinal doubleness is evident with light microscopy

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Chromosome contraction is evidentChromosome contraction is evident☺☺

Each chromosome develops a lateral component of a nucleoprotein complex between its two chromatids

Each chromosome develops a lateral component of a nucleoprotein complex between its two chromatids

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LeptoteneLeptotene

• Zygotene (yoked-thread stage):• Zygotene (yoked-thread stage):

Synapsis or pairing of homologues is initiatedSynapsis or pairing of homologues is initiated

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Pairing is initiated at one or more sites along the length of the chromosomes and proceeds to bring the homologous pairs into alignment along their entire length

Pairing is initiated at one or more sites along the length of the chromosomes and proceeds to bring the homologous pairs into alignment along their entire length

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The lateral elements of the two chromatids combine with a central core to form the synaptonemal complex

The lateral elements of the two chromatids combine with a central core to form the synaptonemal complex

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ZygoteneZygotene

• Pachytene (thick-thread stage):• Pachytene (thick-thread stage):

The synaptonemal complex (SC) is complete with chromosome synapsis from end to end

The synaptonemal complex (SC) is complete with chromosome synapsis from end to end

The chromosomes are visibly thicker because they have paired and contracted and appear to be present in a haploid number

The chromosomes are visibly thicker because they have paired and contracted and appear to be present in a haploid number

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Each chromosome can be recognized to be two closely appressed homologuesEach chromosome can be recognized to be two closely appressed homologues

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The paired homologues are called bivalentThe paired homologues are called bivalent☺☺

PachytenePachytene

• Diplotene (double stage):• Diplotene (double stage):

The termination of pachytene coincides with the dissolution of the synaptonemal complex

The termination of pachytene coincides with the dissolution of the synaptonemal complex

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The chromosomes condense furtherThe chromosomes condense further☺☺

The sister chromatids becomes clearly evidentThe sister chromatids becomes clearly evident

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The paired chromosomes are held together at one or more points along their length

The paired chromosomes are held together at one or more points along their length

• The points of contact are called chiasma, which are the points of chromatid exchange that help preserve the bivalent structure

• As chromosomes continue to contract the chiasmata tend to move toward the ends of paired homologues

• The points of contact are called chiasma, which are the points of chromatid exchange that help preserve the bivalent structure

• As chromosomes continue to contract the chiasmata tend to move toward the ends of paired homologues

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DiploteneDiplotene

• Diakinesis (through movement):• Diakinesis (through movement):

Further contraction of the bivalents occursFurther contraction of the bivalents occurs☺☺

Bivalents become dispersed throughout the cellBivalents become dispersed throughout the cell

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Nuclear envelope breaks downNuclear envelope breaks down☺☺

DiakinesisDiakinesis

Will be continued ….