Genetic information can move among bacteria in three ways. First, some bacteria seem able
Genetic information can move among bacteria in three ways. First, some
bacteria seem able to take up free DNA from the medium around them. If
a double crossover event occurs, this DNA can integrate into the bacterial
chromosome. This has been studied since the 1920's (Griffith's experiments
on transformation in Pneumonococcus) and, in fact, provided the system
for one of the two demonstrations that DNA is the genetic material
(Avery's experiments on transformation in the 1940s). Second, many
bacteria contain small, circular, extrachromosomal molecules of DNA called
plasmids. These plasmids often contain the genes for resistance to
antibiotics. If the plasmid contains a gene known as the F+ gene, its
bacterial host becomes capable of replicating the plasmid and passing a
copy on to a cell that lacks the F+ gene. Occasionally, some plasmids
are able to integrate into the bacterial chromosome. When this happens,
the cell can then pass a portion of the bacterial chromosome along with
the plasmid into an F- cell. For this piece of DNA to have any effect,
a double crossover event must occur. "Plasmid passing" is associated
with the production of a pilus. It _has not_ been established whether
the plasmid passes through the pilus. Third, lysogenic viruses are
capable of integrating their DNA into the bacterial chromosome. Under
certain conditions, these integrated prophages will excise themselves
and resume a viral life cycle in which they trash the bacterial cell
to make more virus. Occasionally, when the viral DNA leaves the host
chromosome it will take a small piece of host DNA with it. When this
virus enters another cell, if it integrates into the host chromosome
it will also integrate the DNA from the last host.
You can find information on these bacterial recombination processes in
any basic text on genetics. I would recommend Russell, 1992. Genetics.
Harper Collins. (Note: a picture of the pilus is on page 200).
Several comments need to be made on this:
1) In all three processes the transfer of information is one way.
2) Usually, only a very small subset of the hosts gene are transfered.
This is unlike eukaryotes, where the sexual process takes half the
diploid genome from one individual and combines it with half the
diploid genome of another. The general shuffling of genotypes that
occurs in sexual eukaryotes doesn't seem to occur in prokaryotes.
3) The demonstration of this has been made in only a few bacterial species.
Some strains of these species are able to do it and some aren't.
These are all lab rat strains. We don't know whether this occurs much
in nature. (This is an important point. If you precipitate DNA with
certain salts, calcium or magnesium salts I think, you can get a number
of different eukaryotic cell in culture to scarf it up just like
salted peanuts. This doesn't seem to happen in nature.)
Joseph Boxhorn (firstname.lastname@example.org)/|\ Self-indulgence means
Department of Biological Sciences \|/ never having to say
and Center for Great Lake Studies /|\ you're sorry.
University of Wisconsin--Milwaukee \|/
E-Mail Fredric L. Rice / The Skeptic Tank