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Crossing Over

  In the life sciences, the concept of crossing over is at the heart of the recombination of genetic information which occurs during sexual reproduction. Crossing over takes place when gametes (sperm or eggs) are produced from specialized gamete-forming cells which, in common with other cells of the organism, have two of each chromosome; when a sperm fuses with an egg the resulting cell has two of each chromosome, paired as alleles (23 pairs in humans). This cell (the zygote) can then divide many times to produce all the cells which make up the individual, all of which have paired chromosomes.

Crossing over occurs before the pairs of chromosomes are separated in the gamete-forming cells. Before the paired chromosomes are separated, they are aligned closely together, giving the DNA strands the opportunity to break and re-link. During this process, strands from opposite alleles may join with one another rather than rejoining to the chromosome they broke from. Thus, when the alleles are separated each will contain some stretches of DNA which were formerly part of the other, and the gametes formed will possess, in a single set of chromosomes, genes derived from both the alleles carried by the gamete-forming cell.

The process of crossing over increases the degree of difference between offspring and is thus the single most important source of the variation which is required for evolution. RB

See also gene; genetic linkage; meiosis.



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