High School Physics

Chromosomes during meiosis divisions – behavior and steps

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.
See also  Homologous Chromosome - definition
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.
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