Download Chapter 7 Recombination in Bacteria and their Viruses

Chapter 7
Recombination in Bacteria and their Viruses
• Conjugation
• Transformation
• Transduction
Origin of genes of a Lactococcus lactis plasmid:
flow of genetic information in bacteria
Methods of growing bacteria in the laboratory
Bacteria
• Can be rapidly grown in large quantities
• Cells divide by binary fission
• Growth medium
– liquid
– solid, such as nutrient agar
• Colony: asexual descendents of single cell
• Prototroph: wild-type bacterium that produces colonies
on minimal medium
• Auxotroph: mutant that requires one or more nutrients in
addition to minimal medium
Selectice systems for finding bacterial mutants: mutants resistant
to an antibiotic.
Selectice systems for finding bacterial mutants: prototrophic
revertants of auxotrophic mutants.
DNA transfer in bacteria
Genetic ”exchange” in bacteria
• Conjugation
– transfer of plasmids
– transfer of genome when plasmid is
integrated
• Transduction
– bacteriophage can “pick up” fragment of
genome and deliver it to another cell
• Transformation
– uptake of DNA fragments from extracellular
medium (environment)
• All are essentially one-way transfers from donor
to recipient
Bacterial conjugation
Cells carrying the F plasmid are designated F+, and those lacking it F-. F
replicates in the cells. F promotes the synthesis of pili. F+ and F- can conjugate: F
replicates and the copy is transferred to the F- recipient, thereby converting it to
a F+ strain.
The F plasmid can carry IS
elements. These transposable
elements (which will be considered
in a following chapter) are also
present in the bacterial
chromosome. F plasmids can then
integrate into the chromosome via
homologous recombination creating
a Hfr strain.
High frequency of recombination
(Hfr) strains can transfer
chromosomal genes into Frecipient strains.
Bacterial Conjugation and Mapping by Recombination
DNA transfer in bacteria
• The F factor (sex factor) is a circular plasmid that may exist
free in the cytoplasm or integrated into the chromosome of
E. coli.
• Free F in F+ cells passes a copy of itself to F– cells in
conjugation, whereas integrated F (Hfr) transfers
chromosomal DNA.
• Bacteriophages can transduce bacterial genes from one cell
to another.
• In transformation, DNA from the environment can enter
bacterial cells and integrate into the chromosome.
• These methods of gene transfer generate partial diploids
that allow study of genes.
Bacteriophages
A phage consists of a nucleic acid
(DNA or RNA) surrounded by a
coat of protein molecules. Phages
parasitize and kill bacteria.
The lytic cycle of phage propagation
Phage plaques. Phages are spread on a bacterial "lawn". Each phage infects one
bacterium, producing ≈ 100 progeny that burst the cells and infect neighboring cells. If
this process continues, plaques are produced.
Transduction
Some phage types can (rarely) "package" bacterial DNA instead of phage-specific
DNA. These phages can transfer genes to recipient bacteria and complement a
mutant gene.
Lysogeny
Temperate phages can integrate into the host chromosome as prophages, allowing the
host cells to survive in the lysogenic state. Prophages occasionally exit the bacterial
chromosome and enter the lytic cycle.
The temperate bacteriophage lambda
Site-specific recombination takes place between the attachment on the circular l DNA
and a specific region on the E. coli chromosome between the gal and bio genes.
Recombination processes in bacteria
Marker
Hfr1
Hfr2
Hfr3
R
10
20
-
I
40
-
5
U
25
5
-
E
-
60
-
C
-
45
-
S
55
-
20
0
100
C
75
25
50
Hfr2