Monday, January 26, 2009

Various Mechanisms of Genetic Recombination

Genetic recombination is the transfer of DNA from one organism to another. The transferred donor DNA may then be integrated into the recipient's nucleoid by various mechanisms.

Mechanisms of genetic recombination include:

1) Transformation: DNA fragments (usually about 20 genes long) from a dead degraded bacterium bind to DNA-binding proteins on the surface of a competent recipient bacterium. Nuclease enzymes then cut the bound DNA into fragments. One strand is destroyed and the other penetrates the recipient bacterium. This DNA fragment from the donor is then exchanged for a piece of the recipient's DNA by means of Rec A proteins.

2) Transduction: Transfer of fragments of DNA from one bacterium to another bacterium by a bacteriophage.

(a) Generalized transduction: During the replication of a lytic phage, the capsid sometimes assembles around a small fragment of bacterial DNA. When this phage infects another bacterium, it injects the fragment of donor bacterial DNA into the recipient where it can be exchanged for a piece of the recipient's DNA. Plasmids, such as the penicillinase plasmid of Staphylococcus aureus, may also be carried in a similar manner.

(b) Specialized transduction: This may occur occasionally during the lysogenic life cycle of a temperate bacteriophage. During spontaneous induction, a small piece of bacterial DNA may sometimes be exchanged for a piece of phage genome (that remains in the nucleoid). This piece of bacterial DNA replicates as a part of the phage genome and is put into each phage capsid. The phages are released, adsorbed into recipient bacteria, and injected into the donor bacterium DNA/phage DNA complex and into the recipient bacterium where it inserts into its nucleoid.

3) Bacterial conjugation: Transfer of DNA from a living donor bacterium to a recipient bacterium. In gram-negative bacteria, a sex pilus produced by the donor bacterium binds to the recipient. The sex pilus then retracts, bringing the two bacteria into contact. In gram-positive bacteria sticky surface molecules are produced that bring the two bacteria into contact. DNA is then transferred from the donor to the recipient.

Tags: Bio Technology, Bio Genetics, Genetic Recombination

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