Recombination

2-13-2008

Recombination

2-13-2008 Recombination

1.Overview of Recombination

2.Requirements of Recombination

3.Functions of Recombination

4.Mechanism of Recombination and Holiday structureformation. Enzymes involved

5.Chi-sites-function

6.Examples of consequences of recombination and geneconversion: hin recombinase and Salmonella flagella,

Four Requirements for Recombination

1.Identical or very similar DNA sequences in the cross over region.What would happen to the structure of the chromosome if this werenot the case?

2.Complementary base pairing between double stranded DNAmolecules. This ensures that recombination will occur only at thesesequences. What would happen if this did not happen?

3.Recombination enzymes-the machinery of the recombinationcomplex.

4.Heteroduplex formation-complementary base pairing between twoDNA molecules in a synapse. This process occurs in all organismscapable of undergoing some kind of genetic exchange. Replicationto fill in any gaps.

Induction of Recombination

•Phage infection

•DNA damage, SOS response: uv irradiation,DNA damaging compounds, thymidinestarvaton

•Stalled replication forks-variety of reasons

•Conjugal plasmid introduction-F and others

• ?

Figure 10.1

Figure 10.3

Single strand invasion model

Figure 10.4

Double strand break model

http://www.cfkeep.org/html/snapshot.php?id=41670106,

Dr. Richard Losick, Harvard

Figure 10.5

Definition of a Chi Site:

Chi-sites were discovered by the lambda phage biologists. A mutantof lambda, called a red-gam mutant that lacks its own recombinationsystems, was used to infect wild type E.coli. Red-gam mutants areunable to replicate DNA normally.

-small plaques were observed, but large plaques were also seen.

-the only way for the phage to propagate is for recombination to occurwith the chromosome at a high frequency

-lambda DNA isolated from these plaques and sequenced

-a sequence was identified 5’GCTGGTGG3’ that is not in wild typelambda

-the sequence came from the E.coli chromosome

What is the frequency of a 8-base pair sequence?

4-base: 44 256

5-base: 45 1024

6-base: 46 4096

8-base: 48 65536

Frequency of Chi-sites in the E.coli chromosome: 1 every 4-5kb