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.
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.
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.