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Variability: Understanding Crossover


The main consequence of meiosis, no doubt, is the emergence of diversity among individuals that are produced in the sexual reproduction of the species.

The relationship between meiosis and variability is mainly based on the occurrence of crossing-over.

Crossing is a phenomenon involving homologous chromatids. It consists of the breaking of these chromatids at certain points, followed by an exchange of corresponding pieces between them.

Exchanges cause new gene sequences to appear along the chromosomes. Thus, if on a chromosome there are several genes combined in a certain sequence, after the occurrence of crossing the combination may no longer be the same. So when thinking about crossing, it is common to analyze what would happen, for example, about the combination of allele genes THE and The and B and B in the pair of homologues illustrated in the figure.

In this combination the gene THE and B are on the same chromosome while The and B are on the homologous chromosome. If the distance of THE and B considerable, there is a high chance of an exchange. And if that happens, a new gene combination may emerge.

The combinations THEb e TheB They are new. These are genetic recombinations that contribute to the generation of greater variability in cells resulting from meiosis. If we think of the existence of three genes linked in the same chromosome (A, b and C, for example), the possibilities of crossings will depend on the distance between the genes - if they are distant, the variability produced will be much greater.

Another process leading to the emergence of variability in meiosis is the independent segregation of chromosomes. Assuming that a cell with two pairs of homologous chromosomes (A and a, B and b), if divided by meiosis, the four resulting cells at the end of division may have the following chromosomal constitution: (a and b), (a and B), (A and b) and (A and B).

The genetic variability that exists between organisms of different species is very important for the occurrence of biological evolution. It is on this variability that natural selection acts, favoring the survival of individuals with genetic characteristics adapted to the environment. The greater the variability generated in meiosis through gene recombination allowed by crossing-over, the greater the chances for selective action of the medium.

In meiosis, the variation in the amount of DNA can be represented as in the graph, starting, for example, from a cell that has a 2C amount of DNA in G1.