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Chapter 9
Genetics of
Bacteria and
Their
Viruses:
Transduction
and phage
genetics
Jones and Bartlett Publishers © 2005
Transduction
• Lytic cycle – the phage attaches to a
bacterium then injects its nucleic acid.
More copies of the phage are made by the
infected bacterium, then the cell lyses to
release the new phage particles.
• Phage that only have a lytic cycle are called
virulent.
• Phage research has helped scientists
understand prokaryotic genetics.
• Phage are 1/100 to 1/500th the size of a
bacterium.
• They can be lytic or lysogenic.
• Lytic = virulent.
• Lysogenic = prophage.
• Temperate phage = do both.
Life cycle of a bacteriophage
Phage structure
Life cycle of bacteriophage T4
Assembly of
bacteriophage T4
Life cycle of the
Lambda phage
Generalized transduction of bacterial genes
mediated by bacteriophage P1
When a
bacteriophage can
transfer any bacterial
gene, the process is
called generalized
transduction
Transduction can be used for mapping genes at a higher
resolution than is possible by conjugation mapping
Bacteriophage genetics
• Phage progeny generally are identical to
their parent (except for mutations).
• If two or more phage particles infect a
single bacterial cell simultaneously, new
phage genotypes can arise.
• This is different from eukaryotic
recombination in two ways:
• The number of participating DNA molecules differs from
cell to cell.
• Reciprocal recombinants are not always recovered in equal
frequencies.
Infection of a bacterial lawn by a lytic bacteriophage
creates a clear spot (plaque) due to the lysis of bacteria
Results of a number of phage crosses can be combined
to generate the complete genetic map of a
bacteriophage chromosome
The genetic map of bacteriophage T4
Bacteriophage T4
chromosome is
approximately
167,000 base pairs in
size. The
chromosome is
linear but the
genetic map is
circular
Bacteriophage T4 chromosome is circularly
permuted and terminally redundant
Mapping of bacteriophage T4 deletion mutants in rapid
lysis genes (rII mutants) using overlapping deletions
The logic of mapping of rII deletion mutants by
crossing to a set of ordered overlapping
deletion mutants
A high resolution genetic map of the rIIA and
rIIB genes showing 2 hot spots of mutation
Each small square in
this map represents an
independently isolated
mutation. A very large
number of mutations
occur at 2 sites, one in
rIIA gene and one in
rIIB gene.
A genetic map of the bacteriophage 
showing the genes and the transcripts
Insertion of bacteriophage  into the
bacterial chromosome and silencing of the
lytic genes creates a lysogenic cell
Upon infection, the sticky ends of the bacteriophage
 DNA join to make a circular molecule
Cleavage sites and
reciprocal recombination
Overview of  integration into the
bacterial chromosome
Site-specific recombination is involved in the
insertion of bacteriophage  DNA to create a lysogen
Maps of the lytic virus and the prophage
are circular permutations of each other
Abnormal excision of the  prophage transfers
bacterial genes to the excised phage and
leaves behind phage genes
The “d” in the
name of the
excised phages
(dgal or dbio)
stands for
defective. Phage
genes were lost
during the pick
up of the host
genes.