CH 5 THE GENETICS OF BACTERIA AND THEIR VIRUSES
Oct 13/11- pg 181-
- Bacteria often carry extra DNA elements called plasmids that are smaller than the main
bacterial genome
- Bacteria can be parasitized by viruses called bacteriophages which can have DNA or RNA as
genetic material
5.1 Working with
Microorganisms - Liquid culture of bacteria is spread onto an agar plate in the process of plating, this forms a
clump of bacteria called a colony with members of a colony that have a single ancestor as a cell
clone
Categories of mutants
Prototrophic 1. Wild type bacteria are prototrophic, meaning that they can grow on minimal medium, a
substrate containing only inorganic salts
Auxotrophic - From this culture, auxotrophic mutants can be obtained, these are cells that won’t grow unless
the medium contains one or more specific cellular building blocks such as adenine, threonine or
biotin.
2. Another type of mutant can’t use a specific energy source whereby the wild type can. (I.e.
lac+ can use lactose and lac- can’t)
Resistant mutants 3. Wild types are susceptible to an inhibitor, such as antibiotic streptomycin, resistant
mutants can divide and form colonies in the presence of the inhibitor.
Genetic markers - These types of mutants are genetic markers to keep track of genomes and cells in experiments.
5.2 Bacterial Conjugation
Discovery of Conjugation - Discovered by Lederberg and Tatum who studied 2 strains of E.coli with different auxotrophic
mutations.
o Strain A: met- bio- thr+ leu+ thi+
o Strain B: met+ bio+ thr- leu- thi-
- The strains were mixed together and plated on minimal medium where none of the auxotrophs
could grow
- A small minority of cells were found to grow as prototrophs (wild type) and grew but the
controls had no growth
- Some form of recombination of genes had taken place.
Could it be that the cells of 2 strains leak substances that the other cells can absorb and use for growing?
- Davis constructed a U-tube where the arms were separated by a filter that can’t let bacteria to
pass but dissolved substances passed through
- Prototrophic cells were searched for but none were found.
- Physical contact is needed for these wild type cells to form.
Conjugation - The physical union of bacterial cells is conjugation
Discovery of the Fertility
Factor (F)
- Hayes discovered that one parent seemed to transfer some or all of its genome into the other
cell – donor into the recipient which is different from eukaryotes that transfer genomes
equally.
- Donor ability is itself a hereditary state, imposed by a fertility factor
F+ - F+ can donate
F- - F- can’t donate
Process of conjugation:
1. The F plasmid directs the synthesis of a pili, projections that initiate contact with a
recipient and pulls it closer
Rolling circle replication 2. F DNA makes a single stranded copy of itself using rolling circle replication where the
copy is reeled out through a pore into the recipient, where it becomes a double strand. 3. A copy of F remains in the donor and the other appears in the recipient.
Hfr Strains - Hfr strains have the F plasmid integrated into bacterial genome
- Sforza discovered an F+ strain with 2 unusual properties:
1. When crossed with F- strains, the new strain produced 1000 times as many recombinants
as a normal F+ strain
2. In Hfr x F- crosses, none of the F- parents were converted to F+ or into Hfr. This is
different because when crossed F+ x F-, infectious transfer of F results in a large
proportion of the F- parents being converted into F+.
- During conjugation, the F factor inserted in the chromosome efficiently drives part or all of that
chromosome into the F- cell, the chromosomal fragment can then engage in recombination with
the recipient chromosome = production of a lot of recombinants
- The replication of the chromosome ensures a complete chromosome for the donor cell after
mating, creating a recombinant cell. If no recombination, transferred fragments of DNA are lost
in cell division.
Linear transmission of HFR genes from a fixed point
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