2. SAFEGUARDING THE GENOME:
If DNA is the master copy of instructions for an organism, then it is important not to make mistakes
when copying the DNA to pass on to new cells.
All DNA suffers damage over time, from exposure to ultraviolet and other radiation, from various
chemicals in the environment, point mutations…
Maintaining the integrity of the cell's "blueprint" is of vital importance and this is reflected in the
numerous mechanisms that exist to repair mistakes and damage in DNA
DNA damage is a change in the basic structure of DNA that is not itself replicated when the DNA is
What causes damage to DNA?
a. Radiation (e.g., UV rays in sunlight and in tanning booths, or ionizing radiation)
b. Exposure to damaging chemicals, such as nitrosamines or polycyclic aromatic hydrocarbons
c. Chemical reactions within the cell (such as the deamination of cytosine to give uracil, or the
methylation of guanine to produce methylguanine).
3. DNA REPAIR:
◦ A collection of processes by which a cell identifies and corrects damage to the DNA
molecules that encode its genome.
◦ DNA repair process is constantly active as it responds to damage in the DNA structure.
◦ Different DNA repair pathways includes—base excision repair (BER), nucleotide excision
repair (NER), mismatch repair (MMR), homologous recombination (HR) and non-
homologous end joining (NHEJ)—are active throughout different stages of the cell cycle,
allowing the cells to repair the DNA damage.
4. SOS response
• The SOS response was discovered and named by Miroslav Radman in 1975.
• A global response to DNA damage in which the cell cycle is arrested and DNA
repair and mutagenesis is induced.
• It’s a part of DNA repair system; synthesizes DNA repair enzymes.
• Named for standard SOS distress signals (stands for “Save Our Soul”), the term “SOS repair”
refers to a cellular response to UV damage
Damage done by UV rays: two adjacent pyrimidine bases in the DNA will be cross-
linked to form cyclobutane pyrimidine dimers or CPDs
UV exposure can also lead to the formation of another type of lesion, known as a (6-4)
photoproduct or 6-4PP
5. ELEMENTS: AN SOS SYSTEM IS COMPOSED OF THE
Repressor gene: encoded by the “LexA” gene - causes the inactivation of inducer
proteins. The repressor binds to the operator and causes inactivation or repression
of the SOS system.
Regulator gene: encoded by the “RecA” gene - to activate the repressed SOS
system by inhibiting the binding of LexA to the SOS operator.
Inducer/structural genes: They are encoded by SOS-box genes that can activate
the inducer proteins relative to the type of DNA damage.
-Includes- uvrA, umuC, umuD…
6. SOS proteins Encoded by Functions
Regulatory protein RecA gene It functions as regulator of SOS system
Repressor protein LexA gene It functions as inhibitor of SOS system
Inducer proteins uvrA gene Repairs short patch nucleotide damage, cross-
links and long patch nucleotide damage
umuC gene It functions to bypass the lesion site of DNA and
leads to mutagenesis
umuD gene It functions to bypass the lesion site of DNA and
leads to mutagenesis
7. In normal DNA, a bacterial cell does not need DNA repair genes to be
activated. Thus, there should be some controller that must control the
expression of such genes. LexA acts as a repressor protein by binding to a
20-bp consensus sequence , the SOS box. The binding will repress the activity
of SOS genes.
But in damaged DNA, the inactivation of LexA repressor becomes necessary
to induce the expression of SOS genes. RecA acts as an activator of SOS
genes in the SOS system, which causes proteolysis of the repressor protein
and allows the SOS genes expression into different DNA repairing inducer
8. Mechanism of SOS Repair
The mechanism of SOS repair is a complex cellular process mediated by the organism
itself. It includes the following steps:
1.In case of excessive DNA damage, stress conditions etc., a cell responds by activating
signal or RecA protein. It floats in the vicinity of the cell in search of any damage in the
2.A RecA protein specifically binds to the single stranded DNA. On binding with the single
stranded DNA fragments, RecA forms a filament-like structure around the DNA.
3.Then, a LexA repressor comes in contact with the nucleoprotein filament assembled by
the RecA protein. When RecA interacts with the repressor protein, it gets converted
to RecA protease.
4.The formation of RecA protease causes autocatalytic proteolysis of LexA repressor
protein. Thus, a LexA protein cannot bind with the SOS operator.
5.Inactivation of LexA protein activates the inducer proteins that repair the DNA damage
but alters the DNA sequence.
6.After DNA repair, the RecA protein loses its efficiency to cause proteolysis, and the LexA
protein will again bind to the SOS operator or switch off the SOS system.
11. Nucleotide Excision Repair (NER) :
After UV irradiation, the amount of Lex A
repressor decreases nearly 10 -fold in a few
The SOS genes, however, are not all induced at
the same time and to the same level.
The first genes to be induced are endonuclease
uvr A, uvr B, and uvr C, that catalyze nucleotide
excision repair (NER).
These endonuclease enzymes excises or cleaves
the damaged nucleotides from ds DNA.
The excised region undergoes polymerization
and then ligation.
12. Translesion DNA synthesis:
Translesion DNA synthesis (TLS) is the process by which cells copy DNA
containing unrepaired damage that blocks progression of the
TLS is a DNA damage tolerance process that allows the DNA
replication machinery to replicate past DNA lesions such
as thymine dimers.
Occurs when other repairs are not efficient.
When NER doesn’t fix the damage umu C and umu D genes synthesizes
DNA polymerase V ( that has larger active sites which can facilitate
the insertion of bases opposite damaged nucleotides)
DNA polymerase V replaces the normal replicative polymerase (i.e DNA
polymerase III) and continues the replication across the lesion and
prevents cell from having unreplicated chromosome at the cost of some