Genetic recombination
Genetic recombination is a general category for a number of processes involving DNA studied in classical genetics, population genetics, molecular biology, and evolutionary biology.
Crossing over
Main article: Chromosomal crossover
The most important and well-studied of these processes is the crossing over of two chromosomes during meiosis. After chromosomal replication, the four available chromatids are in tight formation with one another. During this time, homologous sites on two chromatids can mesh with one another, and may exchange genetic information. Immediately after replication, the tetrad formed by replication contains two pairs of two identical chromatids; after crossing over, each of the four chromatids carries a unique set of genetic information.
Enzymes known as recombinases catalyze the reactions that allow for crossover to occur. A recombinase will create a nick in one strand of a DNA double helix, allowing that strand to pull apart from its complementary strand and anneal to the double helix on the opposite chromatid. A second nick allows the second double helix to pull apart and anneal to the first, forming a structure known as a cross-strand exchange or a Holliday junction. The Holliday junction is a tetrahedral structure which can be 'pulled' by other recombinases, moving it along the four-stranded structure it creates. When the process is reversed, the two double helices have swapped DNA.
In most eukaryotes, a cell carries two copies of each gene, each referred to as an allele. Each parent passes on one allele to each offspring. Even without recombination, each gamete contains a random assortment of chromatids, choosing randomly from each pair of chromatids available. With recombination, however, the gamete can receive a (mostly) random assortment of individual genes, as each chromosome may contain genetic information from two different chromatids.
Crossover recombination can occur between any two double helices of DNA which are very close in sequence and come into contact with one another. Thus, crossover may occur between Alu repeats on the same chromatid, or between similar sequences on two completely different chromosomes. These processes are called unbalanced recombination. Unbalanced recombination is fairly rare compared to normal recombination, but severe problems can arise if a gamete containing unbalanced recombinants becomes part of a zygote. Offspring with severe unbalances rarely live through birth.
In conservative site-specific recombination, a mobile DNA element is inserted into a strand of DNA by means similar to that seen in crossover. A segment of DNA on the mobile element matches exactly with a segment of DNA on the target, allowing enzymes called integrases to insert the rest of the mobile element into the target.
Another form of site-specific recombination, transpositional recombination does not require an identical strand of DNA in the mobile element to match with the target DNA. Instead, the integrases involved introduce nicks in both the mobile element and the target DNA, allowing the mobile DNA to enter the sequence. The nicks are then removed by ligases.
Chemistry of crossover
Consequences of crossover
Problems of crossover
Other types of recombination
Conservative site-specific recombination
Transpositional recombination
References