Download Transformation and Transduction File

7.3 Transformation and
Transduction
In transformation, the genotype
and possible phenotype of a
prokaryotic cell are altered by
the uptake of foreign DNA from
its surroundings. For example,
bacteria from a harmless
strain of Streptococcus
pneumoniae can be transformed
to pneumonia causing cells if
they
are placed into a medium
containing dead, broken-open
cells of the pathogenic strain.
This
transformation occurs when a
live nonpathogenic cell takes
up a piece of DNA carry the
allele for pathogenicity. The
foreign allele is then
incorporated into the cell's
chromosome,
21
Prokaryotes
replacing the existing
nonpathogenic allele- an
exchange of homologous DNA
segments. The
cell is now a recombinant: Its
chromosome contains DNA derived
from two different cells.
In transduction, bacteriophage
carry bacterial genes from one
hose cell to another;
transduction
is a type of horizontal gene
transfer. For most phages,
transduction results from
accidents that occur during the
phage reproductive cycle. A
virus that carries bacterial
DNA may not be able to
reproduce because it lacks its
own genetic material. However,
the virus may be able to attach
to another bacterium (a
recipient) and inject the piece
of
bacterial DNA acquired from the
first cell (the donor). Some of
this DNA may subsequently
replace the homologous region
of the recipient cell's
chromosome by DNA
recombination.
In such case, the recipient
cell's chromosome becomes a
combination of NA derived from
two cells; genetic
recombination has occurred.
Conjugation and Plasmids In a
process called conjugation,
genetic material is transferred
between two bacterial cells (
of same or different species)
that are temporarily joined.
The
DNA transfer is one way: One
cell donates the DNA,a nd the
other receives ti. The donor
uses sex pili to attach to the
recipient. After contacting a
recipient cell, each sex pilus
retracts, pulling the two cells
together, much like a grappling
hook. A temporary "mating
bridge" then forms between the
two cells, providing an avenue
for DNA transfer. In most
cases, the ability to form sex
pili and donate DNA during
conjugation results from the
presence of a particular piece
of DNA called F factor. The F
factor consists about 25 genes,
most required for the
production of sex pili. The F
factor can exist either as a
plasmid or
as a segment of DNA within the
bacterial chromosome
The F factor in its plasmid
form is called F plasmid. Cells
containing the F plasmid,
designated F+ cells, function
as DNA donors during