This document discusses several DNA repair mechanisms including photoreactivation, direct repair mechanisms like base excision repair and nucleotide excision repair, mismatch repair, recombination repair, and SOS repair. It provides details on how each repair mechanism works to identify and repair different types of DNA damage like thymine dimers, mismatches, gaps, and damage caused by environmental stress.

1. Dr Neethu Asokan

2. A change in the sequence of bases if replicated and
passed on to the next generation becomes permanent
and leads to mutation.
These however are necessary for evolution.
However nature has maintained a balance between the
mutation and repair.
Repair mechanisms are some phenomena DNA presents
for the repair of several damages like mutations, dimers,
strand breaks, deaminations and chemical modification.
Introduction

3. Pyrimidine dimers- Formation of thymine dimers is very
common in which a covalent bond (cyclobutyl ring) is
formed between adjacent thymine bases.
In order to maintain the integrity of information
contained in it, the DNA has various repair mechanisms.

4. The damage is reversed by a repair enzyme which
is called Photolyase.
This process is also called as photoreactivation.
This mechanism involves a light dependant
enzyme called DNA photolyase.
The enzyme is present in almost all cells from
bacteria to animals.
It uses energy from the absorbed light to cleave
the C-C bond of cyclobutyl ring of the thymine
dimers.
Direct Repair

6. It includes base excision repair and nucleotide
excision repair.
Base excision repair system involves an enzyme
called N-glycosylase which recognizes the
abnormal base and hydrolyes glycosidic bond
between it and sugar.
Nucleotide repair system includes three steps,
incision, excision and synthesis.
Excision Repair

8. To repair mismatched bases, the system has to
know which base is the correct one.
In E. coli, this is achieved by a special methylase
called the "Dam methylase".
The repairing process begins with the protein
MutS which binds to mismatched base pairs.
Mismatch Repair

10. In thymine dimer or other type of damage, DNA
replication cannot proceed properly.
A gap opposite to thymine dimer is left in the
newly synthesized daughter strand.
The gap is repaired by recombination mechanism
or retrieval mechanism called also sister strand
exchange.
Recombination repair

12. SOS repair is one of the DNA repair mechanisms,
which commonly refers to as “ bypass ” or “
Emergency ” repair.
This system was first introduced in the year 1975,
by a scientist named Miroslav Radman.
It is a regulatory system which comprises of many
complex inducer proteins that helps to repair the
damaged DNA, caused due to environmental
stress.
SOS repair

13. SOS system also includes a repressor protein,
namely LexA.
The RecA protein floats around the cell, which
regulates the activity of LexA protein.
The RecA regulatory protein can mediate the
repression or expression of LexA repressor.
SOS repair system is an error-prone mechanism,
as it works without a DNA template and is a
process that refers as “SOS response system”

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