Genetic recombination during chromosomal crossing over
A process occurring during meiosis wherein two chromosomes pair up and exchange segments of their genetic material. This occurs at the stage when chromatids of homologous chromosomes pair up during synapsis, forming X-structure (chiasma). The chromatids break into segments (of matching regions), which are then exchanged with one another.Crossing over is important because it results in new combinations of genes that are different from either parent, contributing to genetic diversity.
A second source of genetic variation during meiosis is the exchange of genetic material between the maternal and paternal chromosomes, a process called crossing over or recombination. Recombination occurs when two molecules of DNA exchange pieces of their genetic material with each other. One of the most notable examples of recombination takes place during meiosis (specifically, during prophase-I), when homologous chromosomes line up in pairs and swap segments of DNA. This process, also known as crossing over, creates gametes that contain new combinations of genes, which helps maximize the genetic diversity of any offspring that result from the eventual union of two gametes during sexual reproduction. It might be safely said that the crossing over process is more complex than anything man has ever designed.
While the homologous chromosomes are paired together in prophase-I, pieces of one chromosome may be exchanged with the identical portion of the other chromosome. This means that the resulting chromosomes are not entirely maternal or paternal, but rather a mixture of both. In humans, crossing over occurs about 2–3 times per chromosome pair, between non-sister chromatids only (not between sister chromatids).
Genetic diversity or variation occurs because certain physical characteristics, like skin color, are variable; this variability is the result of alternate DNA sequences that code for the same physical characteristic. These sequences are commonly referred to as alleles. The various alleles associated with a specific trait are only slightly different from one another, and they are always found at the same location (or locus) within an organism's DNA. For example, no matter whether a person has fair skin, brown skin, or dusky skin, the alleles for skin color are found in the same area of the same chromosome in all humans. The unique combination of alleles that all sexually reproducing organisms receive from their parents is the direct result of recombination during meiosis.
Thus, the important feature of meiosis is the exchange of chromosome pieces which occurs in the first division of this process (prophase-I), called recombination or crossing over. Recombination is a very significant source of genetic variation between individuals of sexually reproducing species, and the driving force for the process of natural selection.