The mechanics of meiosis

Meiosis consists of two successive divisions without an intervening DNA replication. Each of the divisions looks similar to a mitotic division, though the first involves a separation of chromosomes which is unlike mitosis (the second is simply a mitotic division).

The two divisions are called Meiosis I and Meiosis II. In each of these divisions the intervals within are designated with a I or II (e.g, Prophase I and Prophase II).

Meiosis I

Meiosis I begins as Mitosis, with the disassembly of the nuclear membrane and condensation of the chromosomes (Prophase I).

The duplicated chromosomes (each with two sister chromatids) attach to the spindle and move to the metaphase plate (Metaphase I).

Note that each pair of duplicated chromosomes is still paired. They therefore align with each other at the metaphase plate. The centromeric regions of each duplicated chromosome is fused into one structure which is attached to one of the spindle poles by microtubules.

The spindle pulls the duplicated chromosomes to the poles (Anaphase I). At this stage the maternal and paternal chromosomes are separated from each other. This is the separation which is random, giving rise to the large number of possible segregation patterns. The chromosomes dissociate from the spindle (Telophase I).

Meiosis II

This occurs after cell division-two cells are now involved. The two resulting cells then proceed directly to the next division with the assembly of new spindles in each cell, and attachment of the still condensed chromosomes to it (Prophase II).

Note that each sister chromatid must attach to a different spindle in preparation for division of the sister chromatids from each other.

The chromosomes line up on the metaphase plate (Metaphase II).

Since each of the duplicated chromosomes in these cells are unique they can not pair with each other. Each chromosome must line up by itself on the plate. This is a mitotic division, but unlike normal mitosis the cells are diploid and will give haploid progeny (mitosis in diploid somatic tissue occurs by a tetraploid producing two diploid cells).

Each pair of sister chromatids then separates and moves to opposite poles of the cell (Anaphase II). The chromosomes decondense and the nuclear membrane reforms (Telophase II).

The result is the production of four haploid cells each containing a single copy of each chromosome.

DNA recombination and meiosis

There is another level at which the collection of genes in the genome are randomized-DNA recombination.

During prophase I the chromosomes pair with each other so that the maternal and paternal chromosomes are in close contact. Enzymes in the cell recognize regions of identical sequence in the chromosomes. They catalyze the breakage and rejoining of DNA strands to exchange sequences between non-sister chromatids. This changes the linkage of genes on the chromosome.

Remember that the two homologous chromosomes are not identical in every way. They carry sequence differences between the genome of the mother and the father. These differences are called alleles. The father's chromosome will have a set of alleles on each chromosome many of which will be different from those on the mother's chromosome. Recombination puts some of the mother's alleles and some of the father's alleles on the same DNA molecule

So, since in Anaphase I the paternal and maternal chromosomes separate to the two poles of the cell, what is the effect of recombination on segregation of maternal and paternal alleles? How does this influence the amount of variability in the products of meiosis?

The take-home-esson for meiosis is that random segregation of chromosomes at Anaphase I and the shuffling of sequences by recombination during Prophase I provides an immense variability in the products of meiosis.

The segregation provides over 8 million possible ways chromosomes can segregate into the gametes. Recombination greatly increases this number since each chromosome probably undergoes at least one recombination per chromosome arm per meiotic prophase. In combination with the great diversity of alleles present in a population, this random assortment of alleles provides for an immense number of possible gametes. Fusion of two such gametes creates a new individual who is almost certainly unique genetically