MICROBIAL MOLECULAR BIOLOGY & GENETICS
In a general sense, recombination is the process in which one or
more nucleic acids molecules are rearranged or combined to produce
a new nucleotide sequence. Usually genetic material from
two parents is combined to produce a recombinant chromosome
with a new, different genotype. Recombination results in a new
arrangement of genes or parts of genes and normally is accompanied
by a phenotypic change. Most eucaryotes exhibit a complete
sexual life cycle, including meiosis, a process of extreme importance
in generating new combinations of alleles (alternate forms
of a particular gene) through recombination. These chromosome
exchanges during meiosis result from crossing-over between homologous
chromosomes, chromosomes containing identical sequences
of genes . Until about 1945 the primary focus
in genetic analysis was on the recombination of genes in
plants and animals. The early work on recombination in higher
eucaryotes laid the foundations of classical genetics, but it was
the development of bacterial and phage genetics between about
1945 and 1965 that really stimulated a rapid advance in our understanding
of molecular genetics.
Microorganisms carry out several types of recombination. General
recombination, the most common form, usually involves a
reciprocal exchange between a pair of homologous DNA sequences.
It can occur anyplace on the chromosome, and it results
from DNA strand breakage and reunion leading to crossing-over.
General recombination is carried out by the product of rec genes such as the recA protein so important for DNA repair.
In bacterial transformation a nonreciprocal
form of general recombination takes place . A
piece of genetic material is inserted into the chromosome through
the incorporation of a single strand to form a stretch of heteroduplex
DNA. A second type of recombination, one particularly
important in the integration of virus genomes into bacterial
chromosomes, is site-specific recombination. The genetic material
is not homologous with the chromosome it joins, and generally
the enzymes responsible for this event are specific for the
particular virus and its host. A third kind of recombination, which
may be considered a type of site-specific recombination, is called
replicative recombination. It accompanies the replication of genetic
material and does not depend on sequence homology. It is
used by some genetic elements that move about the chromosome.
Although sexual reproduction with the formation of a zygote
and subsequent meiosis is not present in bacteria, recombination
can take place in several ways following horizontal gene transfer.
In this process genes are transferred from one independent, mature
organism to another. Horizontal gene transfer is quite different sometimes
persists outside the endogenote and replicates to produce a
clone of partially diploid cells. Third, the exogenote may survive,
but not replicate, so that only one cell is a partial diploid.
Finally, host cell nucleases may degrade the exogenote, a
process called host restriction.
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POSTED BY KAMAL SINGH KHADKA
Assistant Lecturer In Pokhara University, Regional College Of Science & Technology,PNC, NA,LA.