Mutations can accumulate over time and cause cancer. DNA repair mechanisms like base excision repair, nucleotide excision repair, and mismatch repair help correct mutations. However, when damage is extensive, the SOS response is triggered, activating error-prone repair and increasing mutagenesis. Multiple mutations in genes like tumor suppressors and those controlling cell proliferation can lead to cancer development.

1. WELCOME

2. MUTATION AND
DNA REPAIR
MUTATION AND CANCER ,MUTATOR AND
ANTIMUTATOR GENE , DNA REPAIR
DONE BY : ROSE MARY MARTIN

3. MUTATION AND CANCER
• Mutations happen often.
• A mutation may be beneficial, harmful, or neutral.
• This depends where in the gene the change occurs.
• Typically, the body corrects most mutations.
• A single mutation will likely not cause cancer.
• Usually, cancer occurs from multiple mutations over a lifetime.

4. What causes mutations in DNA?
• Physical or chemical agent that cause mutation in DNA
• Examples: UV light, tobacco, chemicals, x-rays
• How do mutations cause cancer?
• DNA RNA protein
• Mutated DNA mutated RNA mutated protein
• Many mutations accumulated over time can result in harmful changes in
the cells instructions
• These mutations in genes result in mutations in proteins that control the
cell cycle

5. • Cancer cells are cells gone wrong
• they no longer respond to many of the signals that control cellular
growth and death
• Cells become cancerous after mutations accumulate in the various
genes that control cell proliferation.
• most cancer cells possess 60 or more mutations
• challenge -to identify which of these mutations are responsible for
particular kinds of cancer
• Different kinds of cancers have different mutational signatures
• However-certain genes are mutated in cancer cells more often than
others

6. • tumor suppressor genes -genes that suppress cell proliferation.
• Other cancer-related mutations inactivate the tumor suppressor
genes .
• How Do Cancerous Changes Arise?
• Gene mutations accumulate over time as a result of independent
events.
• path to cancer involves multiple steps.
• cancer -a microevolutionary process.

7. • Initial mutation (initiation) alters genes
resulting in growth
• Progressive growth (influenced by tumor
promoters) creates more cells, each with a
certain probability of mutating to more
virulent state
• Rapidly growing cells more prone to
mutation than quiescent cells
• Mutant cells arise within the population of
growing cells that are able to break
through into surrounding tissues

8. Mutations that result in cancer
Germline Mutation
• hereditary in nature, since they
occur in the gametes
• occur during meiosis
• Eg:Hemophilia, Sickle cell
anemia
Somatic Mutation
• result of changes in the DNA
of somatic cells, also called
body cells
• Non-heritable
• occurs during mitosis
• Eg:development of cancer,
Coat's disease (uncontrolled
blood vessel formation in the retina
of the eyes)

9. Mutator and antimutator genes
• Mutator genes
• Mutators- which produce mutations at elevated frequencies
• Mutators contain defects in pathways that cells use to prevent
mutations
• Mutator genesinclude genes that take part in DNA synthesis, such
as the genes encoding DNA polymerase.
• Other mutator genes are involved in DNA repair.
• Defects in mutator gene are generally recessive

10. Antimutator genes
• Antimutators are mutant strains that have reduced mutation rates
• more difficult to detect and isolate than their highly visible mutator
counterparts
• The E. coli mud strain

11. DNA repair
• processes by which a cell identifies and corrects damage to the
DNA molecules that encode its genome.
• if not repaired, may affect replication and transcription, leading to
mutation or cell death
• DNA damage may arise: (i) spontaneously, (ii) environmental
exposure to mutagens, or (iii) cellular metabolism.

12. Types of DNA Damage
• Deamination :- the entire amine group (NH 2 ) may be removed
spontaneously in a hydrolytic reaction
• Depurination: -purine base (A or G) lost
• Alkylation :- an alkyl group (e.g., CH3 ) gets added to bases
• Oxidative damage :-guanine oxidizes to 8-oxo-guanine
• Replication errors: - wrong nucleotide
• Double-strand breaks :-induced by ionizing radiation, mechanical
stress on chromosomes

13. IMPORTANCE OF DNA REPAIR

14. DNA has various repair mechanisms
• Photoreactivation
• Excision Repair
• SOS Repair Mechanism
• Mismatch Repair
• Recombination Repair

15. Photoreactivation
• Ultra violet radiation causes formation of pyrimidine dimers
• Thymine dimer is most common - two adjacent thymine molecules
are chemically joined
• photo reactivation can repair this mutation.

16. Excision Repair
• Conserved throughout evolution, found in all prokaryotic and
eukaryotic organisms
• Three step process:
• 1. Error is recognized and enzymatically clipped out by a nuclease
that cleaves the phosphodiester bonds (uvr gene products operate
at this step)
• 2. DNA Polymerase I fills in the gap by inserting the appropriate
nucleotides
• 3. DNA Ligase seals the gap

17. Two know types of excision repair
• Base excision excision repair (BER)
• corrects damage to nitrogenous bases created by the spontaneous
hydrolysis of DNA bases as well as the hydrolysis of DNA bases
caused by agents that chemically alter them
• Nucleotide excision repair (NER)
• Repairs “bulky” lesions in DNA that alter or distort the regular
DNA double helix
• Group of genes (uvr) involved in recognizing and clipping out the
lesions in the DNA
• Repair is completed by DNA pol I and DNA ligase

18. Base excision repair
• employed to remove incorrect bases (like uracil) or damaged bases
(like 3-methyladenine)
• pathways:
• 1. Removal of the incorrect base by an appropriate DNA N-
glycosylase to create an AP site ( apurinic/apyrimidinic ).
• 2. Nicking of the damaged DNA strand by AP endonuclease
upstream of the AP site, thus creating a 3'-OH terminus adjacent to
the AP site
• 3. Extension of the 3'-OH terminus by a DNA polymerase,
accompanied by excision of the AP site

20. Enzymes involved in base excision repair
• 1. DNA glycosylase,
• 2. apurinic/apyrimidinic (AP) endonuclease
• 3. DNA polymerase
• 4. DNA ligase.

21. Nucleotide Excision repair in E.coli
• 1.UvrA and UvrB scan DNA to identify a distortion
• 2. UvrA leaves the complex ,and UvrB melts DNA locally around the distortion
• 3. UvrC forms a complex with UvrB and creates nicks to the 5’ side of the lesion
• 4. DNA helicase UvrD releases the single stranded fragment from the duplex, and
DNA Pol I and ligase repair and seal the gap

23. SOS repair
• SOS repair occurs when cells are overwhelmed by UV damage - this allows the
cell to survive but at the cost of mutagenesis.
• SOS response only triggered when other repair systems are overwhelmed by
amount of damage so that unrepaired DNA accumulates in the cell.
• About twenty genes are expressed at increased rates.
• Genes are collectively the SOS regulon.
• expression is controlled by two regulatory proteins: the LexA repressor and RecA
protein
• LexA repressor- which inhibits expression of the SOS genes during normal cell
growth
• RecA protein-which is activated by treatments that turn on the SOS response

24. SOS response
• In response to extensive genetic damage there is a regulatory system that co-
ordinates the bacterial cell response.
• This results in the increased expression of >30 genes, involved in DNA repair,
these include:
• lexA -SOS repressor
• sulA -Inhibitor of cell division
• recA - activator of SOS response
• umuC, D - an error prone bypass of thymine dimers
• recQ - recF-dependent recombinational repair
• The SOS response is regulated by two key genes: recA & lexA

25. • LexA normally represses about 18 genes
• SOS regulon includes lexA ,recA, uvrA, uvrB, uvrC, umuDC, sulA,
sulB, and ssb
• sulA and sulB, activated by SOS system, inhibit cell division in
order to increase amount of time cell has to repair damage before
replication.
• Each gene has SOS box in promoter.
• LexA binds SOS box to repress expression.
• However, LexA catalyses its own breakdown when RecA is
stimulated

26. • (a) system during normal cell
growth. The LexA protein is active
and represses synthesis of RecA (left)
and the SOS proteins (right).
• ( b) induced state caused by DNA
damage. The activated RecA protein
causes the LexA protein to cleave
itself into two pieces, which
inactivates it as a repressor.
• (c). Induced SOS state. In the absence
of active LexA, the recA and SOS
genes are expressed in large amounts.
• (d) Transition to the normal growth
state. Activation of RecA is
reversible, so that DNA repair leads
to loss of RecA activation.

28. Methyl-directed mismatch repair
• If any mismatch escapes the proof reading mechanisms it will cause
distortion of the helix.
• This methylation does not occur immediately after synthesis and
until it does the two strands are distinguishable
• Mismatch repair
• MMR system is an excision/resynthesis system that can be divided
into 4 phases:
• (i) recognition of a mismatch by MutS proteins
• (ii) recruitment of repair enzymes
• (iii) excision of the incorrect sequence,
• (iv) resynthesis by DNA polymerase using the parental strand as a
template

29. • The proteins that initiate the repair process are MutS, MutL, and
MutH
• MutS recognizes such mismatches and binds to them.
• Binding of MutL stabilizes the complex. E. coli DNA is normally
methylated at GATC sequences
• newly synthesized strand is not immediately methylated
• The MutS-MutL complex activates MutH, which locates a nearby
methyl group and nicks the newly synthesized strand opposite the
methyl group.

30. • Excision is accomplished by cooperation between the UvrD
(Helicase II) protein and a single-strand specific exonuclease of
appropriate polarity
• followed by resynthesis (Polymerase III) and ligation (DNA ligase).

31. Recombination mechanism
• recombination mechanism or retrieval mechanism called also
sister strand exchange.
• Replicating DNA molecule has four strands A, B, C and D.
• A thymine dimer is present in strand A.
• creating a gap in the newly synthesized strand B - as it cannot
form hydrogen bonds with incoming adenine bases
• short identical segment of DNA is retrieved from strand D and is
inserted into the gap of strand B.
• gap in strand D which is easily filled up by DNA polymerase
using normal strand C as a template.
• dependent on the activity of a special protein Rec A.

32. • Rec A - retrieving a portion of the complementary strand from other
side of the replication fork to fill the gap
• Rec A is a strand exchange protein.
• After filling both gaps, thymine is monomerised
• also known as daughter strand gap repair mechanism.