Question

answer the following regarding DNA repair mechanisms

What is "direct repair"? Give a couple of examples of the enzymes involved. Which one requires light and what kind of mutation does it repair? Which one repairs bases with alkyl groups attached? Which base is involved?

What is “nucleotide-excision repair”? What kinds of mutations can it repair? In prokaryotes, what four key proteins are involved and how did they get their names? What two enzymes are involved?

What is “mismatch repair”? What does it do? This system corrects mistakes that have been missed by the normal correction system that occurs during DNA replication. Which enzyme normally corrects these mistakes during replication?

What source of DNA is usually used in “homologous recombination repair” to fix the mutated strand of DNA? During which parts of the cell cycle can this type of repair occur? HRR can also occasionally occur at other times, but the homologous regions may not be identical. Why might that be the case?

What happens during “nonhomolgous end joining”? During which part of the process might some of the sequence be lost? When during the cell cycle can this type of repair occur?

Answer 1

1. Direct repair - when some lesion in DNA occurs sometime direct reversal of the damage is done is called direct repair. It is one of the efficient way of repair but occurs very less frequently.

Some examples od enzymes involved in Direct reapir are - CPD photolyase and O⁶-alkylguanine-DNA alkyltransferase.

O⁶-alkylguanine-DNA alkyltransferase repairs the bases with alkyl groups attached.

Guanine base is involved in this repair.

2. In nucleotide excision repair the damage sequence is recognised and removed by excision and repair by synthesizing the based using other strand as template.

Any changes in the base pair of DNA can be repair by nucleotide excision repair.

In prokaryotes the four key proteins involved are UvrA, UvrB, UvrC and helicase. They get their name when mutation in these genes found to be very sensitive in exposure to UV light and cannot repair properly.

The two enzymes involved are DNA polymerase I and ligase.

3 Missmatch repair- when the based pair synthesized during replication is missmatch with the template, such base is removed by excision and repair by using the template to restore the parent sequence.

missmatch repair correct the wrong base added by polymerase during a replication.

The enzyme which normally corrects these mistakes during replication is DNA polymerase.

4. Homologous DNA present in the nucleus is used in homologous recombination reapir to fix the mutated strand of DNA.

During the cell cycle G2 and S cycle this repair is used.