This variability among the organismsoccurs through the ability of DNA to undergo genetic rearrangements resulting in a little change in gene combination. Rearrangement of DNA occurs through genetic recombination

TYPES

Many kinds of recombination occur in microorganisms.

These are classified basically into the following three groups:

(i) General recombination,

(ii) Non-reciprocail recombination, and

(iii) Site specific recombination.

(i) General Recombination

General recombination occurs only between the complementary strands of two homologous DNA molecules. Smith (1989) reviewed the homologous recombination in prokaryotes. General recombination in E. coli is guided by base pairing interactions between the complementary strands of two homologous DNA molecules.

Double helix of two DNA molecules breaks and the two broken ends join to their opposite partners to reunite to form double helix. The site of exchange can occur anywhere in the homologous nucleotide sequence where a strand of one DNA molecule becomes base paired to the second strand to yield heteroduplex just between two double helices

(ii) Non-reciprocal Recombination (Gene Conversion):

The fundamental law of genetics is that the two partners contribute the equal amount of genes to the offsprings. It means that the offsprings inherit the half complete set of genes from the male and half from the female. One diploid cell undergoes meiosis producing four haploid cells; therefore, the number of genes contributed by male gets halved and so the genes of female.

In higher animals like man it is not possible to analyse these genes taking a single cell. However, in certain organisms such as fungi it is possible to recover and analyse all the four daughter cells produced from a single cell through meiosis

(iii) Site-Specific Recombination:

Site specific recombination alters the relative position of nucleotide sequences in chromosome. The base pairing reaction depends on protein mediated recognition of the two DNA sequences that will combine. Very long homologous sequence is not required.

Unlike general recombination, site specific recombination is guided by a recombination enzyme that recognises specific nucleotide sequences present on one of both recombining DNA molecules. Base pairing is not involved, however, if occurs the heteroduplex joint is only a few base pair long.