Chromosomes during meiosis divisions

In this post, we will discuss the behavior of homologous chromosomes during meiosis. Also, we will briefly discuss a few important features of meiosis and how chromosomes behave in each step.

For gametes to be formed, special cells in the sex organs of the organism divide by a process known as meiosis.
When a cell divides in this manner, there are three key outcomes:
– it produces four ‘daughter’ cells
– these daughter cells have only half the number of chromosomes of the original cell;
– the daughter cells show genetic variation

A cell does not normally divide to produce four cells – it produces two. Therefore, meiosis must entail two divisions. We call these Meioses I and Meiosis II.

An overview of meiosis - Chromosomes during meiosis divisions - behavior and steps — **figure 1: An overview of meiosis**

The behavior of homologous chromosomes during meiosis

Let’s see how just one pair of homologous chromosomes behaves through the two divisions of Meiosis.

At the start of the process, each chromosome is a double structure; it is made of two chromatids held together by a centromere. This is because the DNA in each chromosome replicated prior to meiosis commencing.
Before any division takes place, chromatids from different chromosomes in the homologous pair undergo ‘crossing over’. In this process, they exchange sections of DNA. [figure 2]
After Crossing Over, Meiosis I follows, and the two chromosomes that make up the pair are separated into different cells.
In Meiosis II, the two chromatids that make up each chromosome are separated into separate cells. Because of crossing over, none of these chromatids are the same.
There is genetic variation in the daughter cells, which also have only half the original chromosome number – they are said to have the Haploid number of chromosomes, unlike the parent cell which had the Diploid number of chromosomes.

figure 2: Crossing Over - Chromosomes during meiosis divisions - behavior and steps — figure 2: Crossing Over

Chromosomes in Meiosis – step by step events

During meiosis, the following things happen to the chromosomes:

The chromosomes duplicate – the DNA in each chromosome makes an exact copy of itself and histones associate with it to make another chromosome. The original and the copy remain attached by a centromere and are called not chromosomes but chromatids.
The chromosomes ‘condense’ – when chromosomes are not involved in cell division, they are very long and thin and all the genes can be active. However, they cannot be moved around a cell in this form. Hence, during cell division to move around a cell they condense to become much shorter and fatter.
The chromosomes of a homologous pair (each one by now duplicated) ‘find’ each other (this is called synapsis) and form a bivalent.
Whilst associated in the bivalent, chromatids from different chromosomes undergo crossing over. These chromatids from different chromosomes are called non-sister chromatids. The chromatids that make up one chromosome are sister chromatids.
In this process, the chromatids exchange equivalent sections of DNA, and all four chromatids in the homologous pair are genetically different – as
shown in figure 2.
The chromosomes (or chromatids) are moved around the cell by fibres that make up a spindle.
This is achieved by the spindle fibres contracting and pulling the chromosomes/chromatids. In the two divisions of meiosis, the chromosomes attach to the spindles differently so that:
– in meiosis I, whole chromosomes are moved and the chromosomes that make up a homologous pair are separated
– in meiosis II, the chromatids that make up each chromosome are separated.