Meiosis

Process of meiosis

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Meiosis

Occurs only in diploid cells and produces haploid cells (only one chromosome from each homologous pair)

Meiosis 1 - separation of homologous chromosomes during a first meiotic division

Meiosis 2 - separation of chromatids during a second meiotic division

Differs from mitosis in a number of ways:

  • only takes place in reproductive organs 
  • involves two divisions resulting in four daughter cells 
  • chromosome arrangements in daughter cells are both different from each other and different from parent cell
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Prophase 1

  • homologous chromosomes are paired up to form bivalents
  • chromatids appear - entwined at points called chiasmata
  • chromatids may break at chiasmata and rejoin with a different chromatid, resulting in crossing over or recombination
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Metaphase 1

  • nuclear membrane breaks down and microtubules of the spindle attach to centromeres of the chromosomes
  • bivalents move to equator of the spindle
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Anaphase 1

  • pulling by spindle fibres causes the whole chromosomes to move apart towards opposite poles
  • the homologous chromosomes are separated; each chromosome still consists of two chromatids
  • ensures that one chromosome from each homologous pair will end up in a daughter cell
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Telophase 1

  • chromosomes reach opposite poles of the cell
  • nuclear membrane forms around each separate group of chromosomes; each nucleus contains the haploid number of cells
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Cytokinesis and Meiosis 2

Cytokinesis after meiosis 1 produces two daughter cells. Within each, meiosis 2 follows:

  • Prophase 2 - new spindles begin to form at right angles to old spindle
  • Metaphase 2 - chromosomes consisting of pairs of chromatids (now different because of crossing over) are arranged along the equator
  • Anaphase 2 - sister chromatids are split at the centromere and pulled to opposite poles
  • Telophase 2 - each group of separated chromosomes becomes enclosed within a nuclear envelope

Cytokinesis follows. Overall resuilt of meiosis is the production of four haploid daughter cells, each of which is genetically different from the others.

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Crossing over

Crossing over occurs as a result of chiasmata formation between the chromatids of the homologous pairs during late prophase 1.

A piece of chromatid from one chromosome swaps places with a piece of chromatid of the homologous partner.

It results in each chromosome having a different combination of alleles (recombinants) from that which occured originally. 

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Independent assortment

During metaphase 1, bivalents are arranged at random on the equator of the spindle. This means that the orientation of any one homologous pair is not dependent on the orientation of any other pair.

When the homologous chromosomes are pulled apart at anaphase 1, a chromosome of one pair is equally likely to be separated along with either member of any other homologous pair.

The process of independent assortment is very important in producing variation in the next generation

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