Meiosis

In response to Charlotte and Emma or Emily, or possibly both, a little clarification of meiosis.

I think  it is probably the language that becomes confusing here, since there are chromosomes, chromatids and bivalents all floating around in the mix.

Unlike mitosis, there is no cell cycle. This is because after mitosis each daughter continues in a cycle like the parent cell of growth, DNA and organelle replication etc. Gametes end up with  a haploid chromosome number, so they do not divide again. However, it is worth considering the cell cycle in terms of the cell just prior to the gamete (primary spermatocytes and primary oocytes) since it is during interphase that the DNA is replicated. Remember that you can’t actually see that the DNA strand (and therefore each chromosome) has replicated until the supercoiling during prophase. Nevertheless, the replication occurs during the cell cycle so that each chromosome strand now consists of two sister chromatids.

Refer to it as a chromosome when single strand, a chromosome comprising two sister chromatids after DNA replication.

The first stage of meiosis is similar to mitosis (PMAT) but are numbered, e.g. prophase 1, metaphase 1. During P1 the chromosomes condense and become visible. This is when they take on the characteristic ‘X’ shape with the centromere holding the two sister chromatids together. Each of the homologous chromosomes (e.g. pair number 22, or pair number 9) pair up near the other (remember each of the pair of chromosomes is made up of two sister chromatids, so there are now 4 strands of DNA involved). When the homologous pairs are adjacent they are referred to as bivalents.

The bivalents then undergo crossing over, or chiasmata (singular chiasma) where portions of genetic material are swapped between non-sister chromatids within a bivalent. The 23rd pair of chromosomes only show a small amount of crossover. 

in M1, the bivalents line up opposite each other at the cell equator, and are pulled apart by spindle fibres in A1. This is the key step to remembering the difference between mitosis and meiosis stages: in meiosis the bivalents are opposite each other, in mitosis the chromosomes all line up across the equator. Remember that in each bivalent, there is a chromosome (made of two sister chromatids that have undergone chiasmata) from the mother and one from the father. It is random as to which side of the equator the maternal and paternal chromosomes line up, another source of variation.

After T1, the second phase of meiosis occurs (P2, M2, A2, T2), to produce the 4 haploid gamete cells. The second part od meiosis proceeds in pretty much the same way as mitosis, but with only half the number of chromosomes present.