During meiosis 4 haploid cells are produced, all of which are genetically different. Two process occur during meiosis that help to provide genetic variation within the gametes. These processes are called crossing over and independent assortment.
Crossing Over
Meiosis I involves a process called crossing over which takes place during prophase I in order to increase the genetic variation of the new cells. This is done by exchanging genetic information between non-sister chromatids of homologous pairs of chromosomes.
One chromosome in each pair is inherited from the mother and the other is from the father. During interphase the chromosomes duplicate so that each chromosome has two sister chromatids (each chromosome in the pair has two sister chromatids).
Homologous pairs align during prophase I of meiosis so that corresponding genes match up. The diagram below shows a homologous pair of chromosomes, one dark blue and one light blue (each with two sister chromatids).
The letters on the chromosomes represent genes. The capital letters represent the same genes as the lowercase letters, they are just different forms of the same gene – these are called alleles.
Two of the chromatids twist around each other and a section of DNA from each, at the same position, breaks off and then recombines with the other chromatid. This results in recombinant chromosomes (chromosomes that are a combination of maternal and paternal components).
Many crossovers can take place for each pair. For simplicity the diagram below shows only one cross over. As you can see the break takes place between the E and F genes (or e and f on the light blue chromosome) resulting in the chromosomes exchanging genetic information – in this case the F gene.
The gametes produced are all genetically different as shown in the following diagram:
Independent Assortment
During metaphase I of meiosis pairs of homologous chromosomes line up along the centre of the cell with each other. However, when the cell separates in meiosis I the two cells produced don’t necessarily have all maternal chromosomes or all paternal chromosomes.
This is because the orientation of the chromosomes is random when they line up. Therefore, a mix of maternal and paternal chromosomes is usually found in gametes.
In the diagram below the dark red and blue chromosomes are maternal and the light red and blue chromosomes are paternal. For simplicity, crossing over isn’t shown in the diagrams.
A couple of different combinations are shown. On the left the orientation of the chromosomes gives a mix of maternal and paternal chromosomes, while on the right the maternal chromosomes align on the same side of the cell. This gives rise to gametes with different sets of chromosomes in each case.
This happens every time meiosis occurs and each time the combination is random – which can result in many differences between gametes. As humans have 23 pairs of chromosomes the number of possible combinations is 223 which is equal to 8,388,608 possibilities.
As crossing over also takes place, the number of possible gametes that are genetically different is even larger. This makes it very unlikely that two gametes will be the same.
Genetic Variation
Both crossing over and independent assortment help to increases the genetic variation between gametes. This lowers the chances of genetic errors being passed on to offspring, allows adaptation and helps to ensure the survival of the population.
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