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CHAPTER 8
Bacterial and Viral Genetic Systems
GENE TRANSFER IN BACTERIA
Bacteria can transfer genes from one strain to another by three different mechanisms
In all three mechanisms, one cell - the donor - provides the genetic material for transfer,
while a second cell - the recipient - receives the material.
Conjugation - one bacterial cell transfers DNA to another cell by direct cell-to-cell
contact
Transformation - a bacterial cell acquires DNA from the environment and incorporates
this DNA into its own chromosome
Transduction - certain bacterial viruses can pick up a piece of DNA from one bacterial
cell and inject it into another, where it can be incorporated into the chromosome
The recipients of a gene transfer are known as transformants, transconjugants, or
transductants, depending on the mechanism of DNA transfer
All bacterial gene transfer is asymmetrical
Most recipients receive 3% or less of a donor’s DNA; some transconjugants contain
a greater percentage of donor material
The amount of donor DNA entering the recipient is small relative to the size of the
recipient’s chromosome, and the recipient retains most of its own DNA
Discovery of the fertility factor (F)
Strains that carry F can donate, and are designated F+
Strains that lack F cannot donate and are recipients. These strains are designated F-
F is an example of a plasmid that can replicate in the cytoplasm independently of the
host chromosome
Hfr strains
High frequency of recombination
On crossing an Hfr strain with F-, the Hfr strain produces 1000 times as many
recombinants for genetic markers as did a normal F+ strain
In Hfr x F- crosses, virtually none of the F- parents were converted into F+ or
into Hfr
In F+ x F- crosses, transfer of F results in a large proportion of the F- parents
being converted into F+
Hfr strain results from the integration of the F factor into the chromosome
During conjugation between an Hfr and a F- cell a part of the chromosome is
transferred with F
Random breakage interrupts the transfer before the entire chromosome is transferred
The chromosomal fragment can then recombine with the recipient chromosome
Bacterial transformation
The conversion of one genotype into another by the introduction of exogenous DNA is termed
transformation
Transformation was discovered in Streptococcus pneumoniae in 1928 by Frederick Griffith
In 1944, Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty demonstrated that the
“transforming principle” was DNA
The field of bacterial viruses is a fine playground
for serious children who ask ambitious questions
- Max Delbruck