Transduction is a mechanism of DNA uptake by bacteria in which
the donor DNA , consisting of fragments of the bacterial chromosome, is
introduced into a bacterial cell via a phage vector. In generalized transduction,
virtually any bacterial gene can be transferred by a lytic bacteriophage.
During packaging of the viral DNA into the capsids, some of
the host's DNA may be packed into the virus along with an incomplete
viral genome. This virus will be able to initiate infection, and therefore
introduce the original host's DNA into a new bacterial cell, but will not
be able to replicate itself or lyse the new host cell.
|Figure 7-2 Integration of bacteriophage λ
into the E. coli chromosome.
Specialized transduction is a process whereby a lysogenic bacteriophage
serves to transfer a specific gene at a high frequency. When
lysogenic bacteriophages infect host cells, their DNA is incorporated into
the host's genome by site-specific recombination, which always occurs
at a specified location and adjacent to certain genes. Through the process
of induction, the prophage genome becomes excised from the host and
undergoes a lytic cycle. Occasionally, the phage excision from the host's
genome is defective and results in the release of a viral genome that contains
part of the host's genome, in particular, those genes that are adjacent
to the phage's site of insertion.
Conjugation is a process during which genetic information is transferred
unidirectionally from a donor bacterium to a recipient through a
cytoplasmic channel between the two cells. The required cell-to-cell contact
between the donor and recipient can be achieved through sex pili,
through agglutinins (substances that promote cell clumping), or via
pheromones (chemicals that alter the behavior of other members of the
There are pieces of DNA measuring 700-20,000 bp in length that
"jump" from one region of the genome to another. In both prokaryotes
and eukaryotes, the jumping can occur from one location on the chromosome
to another or, in the case of bacteria, from the chromosome to a
plasmid, or vice versa. These "jumping genes" are called transposable
elements or transposons. They are of importance since they may turn
genes on or off when they move from one location to the control region
of another gene, and they may cause mutations if they insert themselves
within a gene. During the process, transposase causes breaks in both the
transposon itself and at sites adjacent to the target sequence in the bacterial
genome. The transposable element inserts itself by site-specific recombination
into the genome and enzymes repair the gaps.