Benzoapyrene the cancercausing agent in cigarette smoke is a

Benzo[a]pyrene, the cancer-causing agent in cigarette smoke, is a powerful mutagen. Benzo[a]pyrene itself is relatively harmless, but it is metabolized in the liver to produce active molecules that react covalently with DNA. In an experiment, benzo[a]pyrene is incubated with a mixture of liver enzymes to form its genotoxic metabolites. These metabolites are added to E. coli cells that have a mutation in a gene encoding an enzyme of the serine-synthesizing pathway (i.e., the cells are serine auxotrophs, requiring serine for growth). When the treated cells are grown on serine-containing medium, the results show that the benzo[a]pyrene metabolites kill cells in a dose-dependent manner. When treated and untreated serine-auxotrophic cells are plated separately on serine-free media, the cells treated with benzo[a]pyrene metabolites show a 10- to 100-fold increase in survivors compared with untreated cells. Explain these results.Compare and contrast mismatch repair in E. coli and eukaryotes.

List two reasons translesion sysnthesis tends to be mutagenic.

Describe, in detail, nucleotide excision repair. Include especially ways the damage can be recognized.

Solution

Consider a Benzopyrene react with liver enzyme and turns into powerful mutagen \'AD\'.

In this experiment Ecoli cells are Auxotropic to serine So, in untreated and serine free media the E.coli started to
die. Because serine needs for the growth of this microorganism.

In Treated cells (AD) mutagen with serine free medium cells - undergo mutation in enzyme encoding in serine synthesizing gene and reverse back the enzyme activity. Hence cells started to produce serine for its growth.

Mismatch repair in prokaryotes

MutS recognises mismatch in the DNA region and bind to them. MutL binds with MutS and stabillizes the complex.
Ecoli DNA methylated at GATC region in old strand but not in the new strand.This complex activates the MutH it nicks the newly synthesized strand opposite to methyl group. Then DNA polymerase and DNA ligase join the nicked region.

Mismatch repair in euk

Instead of MutS here it has MSH2 binds the mismatch region. MLH1 binds with MSH2 and Pms2. This complex moves in either direction. The newly synthesised strand in eukaryotes identified by the presence of gaps between Okazaki fragments on the lagging strand and free 3-terminus on the leading strand. Exonuclease 1 excise the mismatch strand and PCNA identified the nick region and DNA polymerase fills the gap.

Translesion synthesis
AP site highly error prone are bypasses by TLS polymerase and leads to deletions or base substuting errors in genome and cause mutation.

Nucleotide excision repair.
UV light cause a Thymine dimer in the DNA sequence are repired by this process. The protein responsibe
for the process UvrABC exinuclease and UvrD helicase. Due to thymine dimer the structure of the DNA changed.
This structural distrotions are identified by UvrA2 and UvrB protein. Then UvrA2 removed from this site and UvrC binds with UvrB. his complex nicks at both 5\' and 3\' of the damaged site. Both complexed removed and
DNA polymerase and DNA ligase fills the nicked area.