IBDP - Meiosis

Before the cell as a whole can divide, first the nucleus must divide, either by mitosis or meiosis. Meiosis produces four daughter nuclei, each with half the number of chromosomes as the parent cell.

• In sexual reproduction, two gametes fuse to give rise to new offspring. If each gamete had a full set of chromosomes then the cell that they produce would have double the number. 
• In humans, the diploid number of chromosomes is 46, which means that this cell would have 92 chromosomes. 
• This doubling would continue at each generation. It follows that, in order to maintain a constant number of chromosomes in the adults of species, the number of chromosomes must be halved at some stage in the life cycle. This halving occurs as a result of meiosis.

1. In the first division the homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of these chromatids may be exchanged by a process called crossing over. By the end of this stage the homologous pairs have separated, with one chromosome from each pair going into one of the two daughter cells.
2. In the second meiotic division the chromatids move apart. At the end of meiosis 2, four cells have been formed. In humans, each of these cells contains 23 chromatids.

Meiosis also produces genetic variation among the offspring, allowing an organism to adapt and survive in a changing world. This variation is brought about by:

• Independent segregation of homologous chromosomes
• Recombination of homologous chromosomes by crossing over

During meiosis 1, each chromosome lines up alongside its homologous partner. In humans, this means that there will be 23 homologous pairs lying side by side, arranged randomly.

• One of each pair will pass to each daughter cell. Which one of the pair goes into the daughter cell, and with which one of any of the other pairs, depends on how the pairs are lined up in the parent cell. 
• Since the pairs are lined up at random, the combination of chromosomes that goes into the daughter cell at meiosis 1 is also random. This is called independent segregation.
• The random distribution and consequent independent assortment of these chromosomes therefore produces new genetic combinations.

During meiosis 1 each chromosome lines up alongside its homologous partner then:

• The chromatids of each pair become twisted around one another
• During this twisting process, tensions are created and portions of the chromatids break off
• These broken portions then rejoin with the chromatids of its homologous partner
• Usually it is the equivalent portions of homologous chromosomes that are exchanged
• In this way new genetic combinations are produced

The chromatids cross over one another many times and so this process is known as crossing over. The broken-off portions of chromatid recombine with another chromatid, so this process is called recombination.

The effect of this recombination by crossing over on the cells produced is shown above, all four cells will have a different genetc composition.

This is the end of the topic

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Photo references:

1. http://ahsmediacenter.pbworks.com/w/page/2029816/XYY%20Supermale%20Syndrome%3A%20Meiosis%203
2. https://ib.bioninja.com.au/higher-level/topic-10-genetics-and-evolu/101-meiosis/random-assortment.html
3. https://ib.bioninja.com.au/higher-level/topic-10-genetics-and-evolu/101-meiosis/crossing-over.html