Re: WHAT IS CONJUGATION IN BACETRIA, AND HOW DOES IT MAINTAIN GENETIC VARIATION

In a word, bacterial conjugation is bacterial sex.  We normally think about 
bacteria growing and then dividing into two progeny cells.  In this 
scenario, genes are only past from parent to offspring, what we call 
vertical gene transfer or heredity.  There are however several ways in which 
genes can be transferred between cells of the same generation, what we call 
horizontal or lateral gene transfer.  We don't see this in humans.  It would 
be like two of your classmates exchanging genetic information, perhaps 
causing the recipient to now have red hair like the donor.  Bacteria can 
transmit genes to their neighbors, without having sex or any offspring.  A 
commonplace example is the spread of antibiotic resistance genes.  More 
species of bacteria are becoming resistant to more antibiotics because genes 
are being passed to them from resistant bacteria.

I hope that you can see that if a bacteria continues to reproduce simply by 
binary fission (grow and divide), that the only genetic variation possible 
is through mutation.  This is a powerful force to be sure.  On the other 
hand, I hope that you can also see that by exchanging genes with its 
neighbor, the organism can obtain new and different traits and abilities.  
Recombination of chromosomal genes from two organisms also drives genetic 
variation.  Higher animals do this through sexual reproduction which mixes 
the genes from both parents.  Bacteria do this in other ways.

There are several methods of lateral gene transfer in bacteria.  In 
transformation, naked DNA is released into the environment, likely from a 
dead donor cell.  Another cell, the recipient, then takes up this DNA and at 
some low frequency integrates it into its chromosome.  Thus, a gene has 
moved from the donor to the recipient.  The transfer can be mediated by a 
virus in a process called transduction.  Here, a viral infection progresses 
through the lytic cycle.  The virus enters the host cell and takes over the 
cell's machinery.  The virus will naturally destroy the host cell's genome 
and cut it into pieces.  When new virus particles are being assembled, some 
of the host cell chromosomal DNA may be incorporated into the capsid of the 
virus.  When these viruses then attack a second host cell, they carry these 
chromosomal genes with them.  Usually these viruses are defective and can't 
kill the new host.  However, the bacterial chromosomal gene they carry can 
become intergrated into the new host's chromosome.  Thus, a gene has moved 
from donor to recipient.

In conjugation, two bacterial cells come together, make an attachment, and 
DNA can be exchanged.  Some bacterial cells contain a plasmid known as the 
fertility factor.  We call these cells F+.  Those without the factor are F-.  
The F+ plasmid contains the genes needed for mating with an F- cell.  The F+ 
cell will generate a long thin tube, called a pilus, that will form a tunnel 
between the donor F+ cell and the recipient F- cell.  A copy of the F+ 
plasmid will move through the pilus.  The F- cell thus becomes F+.  Genes 
have been transferred from one cell to another.  The F+ plasmid can also 
carry genes in addition to the fertility genes.  In some cases, the F+ 
plasmid will integrate into the host cell chromosome, creating an Hfr cell 
(high frequency of recombination).  During conjugation, an Hfr cell can 
transmit copies of some of the chromosomal DNA (adjacent to where the F+ 
plasmid has intergrated) to the F- recipient.  Now chromosomal genes and not 
just the F+ plasmid genes have been transferred.

The role of lateral gene transfer in maintaining and contributing to genetic 
diversity is an area of active scientific investigation.  I would have you 
ask yourself these questions.  What type of genes can be exchanged between 
organisms?  Are there some functions that are so integrated with other 
functions that they cannot be exchanged?  Let's say a human ribosomal 
protein gene was on a naked piece of DNA in the soil.  A bacteria then takes 
up this DNA and intergrates it into its chromosome.  What would result?  The 
bacterial ribosomes are so different from eucaryotic ribosomes that is it 
likely that this human protein would not be able to work in a bacterial 
ribosome.  Get my point.  There are limits to genetic exchange.  Early in 
evolution it is likely that functions like protein synthesis were not as 
integrated.  Thus, genetic exchange of these genes was still valuable.  
However, in today's world, many functions are so integrated that exchange 
between distantly related organisms is no longer fruitful.  This is why 16S 
rDNA makes a good model for determining phylogenetic relationships.  It was 
integrated long ago.  On the other hand, tRNA synthetases are not as good at 
predicting phylogeny and hence were locked in (or not even completely 
integrated) at a later date.  Less integrated functions are even worse 
indicators of evolutionary relatedness.  For these genes, lateral gene 
transfer is still driving their evolution.

A bit long-winded today.  But I hope this helps.

Cheers,

Mark.