Sept 11

Question 1

What does UV light damage to E. coli DNA do?

Answer 1

First it stops DNA replication, and triggers the activation of RecA, a recombinase, that promotes DNA strand exchange and repair.
LexA is transcription repressor, it prevents RNA polu from recognizing promotor for 3 genes for DNA repair after UV light (UvrA, UmuC, UmuD). RecA induces proteolysis of LexA, so now DNA repair starts. RecA also induces proteolysis of repressor monomers, so now they can’t dimerize and won’t bind to OR1 and OR2, so no repressor synthesis.

Question 2

How does Cro bind and work?

Answer 2

It is only a negative regulator. It binds independently, no cooperative binding. It binds to OR and affinity is OR3>OR2=OPR1.. When bound to OR3, Promotor RM is blocked from RNA poly so C1 off. Also PR now open so more Cro synthesis. If bound to OR3+OR2 or OR3+OR1, it now blocks RNA poly to PR, now shut down it’s own gene expression (NO Cro).

Question 3

Once prophage part of host, it is immune to lysis by more phages introduced later on, why?

Answer 3

The prophage produces C1 and C2, which will bind to any phage and prevent induction of lytic pathway.

Question 4

In DNA which is the glycosidic and phosphodiester bonds?

Answer 4

Glycosidic is sugar to base, phsophodiester is phosphate to sugar