BIOL 1500 Lecture Notes - Genetic Recombination, Lysogenic Cycle, Lytic Cycle

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20 Mar 2013
Heena Loomba
Chapter 10: Genetic Recombination
10.1 Mechanism of Genetic Recombination
Genetic recombination requires two DNA molecules that differ from one another
in at least two places, a way to bring the DNA molecules into close proximity, and
a collection of enzymes to cut, exchange and paste the DNA back together
Sugar-phosphate backbone of DNA is held by strong covalent bonds
Base pairs in DNA are held by weak hydrogen bonds
Homologous: similar
recombination occurs between similar regions in DNA
Homology allows different DNA molecules to line up and recombine precisely.
Once homologous regions of DNA are paired, enzymes break a covalent bond in
each of the four sugar-phosphate backbones. The free ends of each backbone are
then exchanged and reattached to those of the other DNA molecule. Final result is
two recombined molecules.
Cutting and pasting four DNA backbones= one recombination event
10.2 Genetic Recombination in Bacteria
Genetic Recombination occurs in E.Coli
E.coli and other bacteria can be grown in minimal medium. Clones (cluster,
identical cells) of E.coli. Detectable differences related to nutrition that
occurred during changing combinations were used.
Phototrophs: strains that were able to make the necessary amino acids
Auxotrophs: mutant strains that are unable to make amino acids, the
required amino acid needed to be provided
Using mutagens (ex. X-rays), two different strains of E.coli were isolated
which carried distinctive combinations of alleles for various metabolic
Many cells of the mutant strains were mixed together and placed on
minimal medium. Colonies grew, but individually, none of the original cells
carried all of the normal alleles needed for growth
Bacterial Conjugation Brings DNA of Two Cells into Close Proximity
Genetic recombination in eukaryotes occurs in diploid cells by an exchange
of segments between pairs of chromosomes
Bacterial cells conjugate to bring their DNA together because cells contact
each other by a long tubular structure (sex pilus) and then form a
cytoplasmic bridge. During conjugation, a copy of part of the DNA of one
cell moves through the cytoplasmic bridge into the other cells. Then genetic
recombination occurs
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Heena Loomba
Chapter 10: Genetic Recombination
Conjugated starts by a bacterial cell that contains a small circle of DNA
(plasmid) and the main circular chromosomal DNA
This particular plasmid is the “fertility” or “F factor”
F factor carries many genes and a replication origin that allows a copy to be
passed on to each daughter cell during bacterial cell divisionvertical
F factor can be copied and passed directly from one cell (donor) to another
cell (recipient)
Donor cells: F+ cells
Recipient cells: F- cells
During conjugation, the F plasmid replicated using a “rolling circle”
No genetic recombination occurs between the DNA of two different cells in
such mating (recipient cell becomes F+ bit no chromosomal DNA is
transferred between the cells in this process)
Hfr Cells and Genetic Recombination
Hfr cells allow for high-frequency recombination because it transfers genes
on a bacterial chromosome to a recipient bacterium
When the F factor is integrated into the bacterial chromosome, its genes are
still active. The Hfr cells make sex pilli and can conjugate with an F- cell
In Hfr cells, the origin of transfer is near the middle of the integrated F
factor. Which is why only half of the F factor DNA is transferred at the front
of the chromosomal DNA and the other half can follow only after the rest of
the entire chromosome.
It is unusual for a recipient cell to obtain the entire F factor and become Hfr
as well. Usually the recipient cell will become a partial diploid, it will have
two copies of only those genes that came through the conjugation bridge on
the donor chromosomal DNA segment
Mapping Genes by Conjugation
The longer they allowed cells to conjugate before separation, the greater the
number of donor genes that entered the recipient and produced
Bacteria take up pieces of DNA that are released into the environment as
other cells disintegrate
Ex. In the transformation of Sreptococcus, the linear DNA fragments taken
up from disrupted virulent cells recombine with the chromosomal DNA of
the nonvirulent cells
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