the main points of DNA Repair

1. DNA REPAIR
BY KANCHITA TEWARI
BSC+MSC BIOTECH
8TH SEM

2. Introduction
• What does DNA have to do with cancer? Cancer occurs when cells divide in
an uncontrolled way, ignoring normal "stop" signals and producing a tumor.
This bad behaviour is caused by accumulated mutations, or permanent
sequence changes in the cells' DNA.
• Replication errors and DNA damage are actually happening in the cells of
our bodies all the time. In most cases, however, they don’t cause cancer, or
even mutations. That’s because they are usually detected and fixed by DNA
proofreading and repair mechanisms. Or, if the damage cannot be fixed, the
cell will undergo programmed cell death (apoptosis) to avoid passing on the
faulty DNA.
• Mutations happen, and get passed on to daughter cells, only when these
mechanisms fail. Cancer, in turn, develops only when multiple mutations in
division-related genes accumulate in the same cell.

3. There are mostly 3 ways
DNA can be repaired
• Proofreading
• Single stranded break repair
• Double stranded break repair

4. Proofreading
• DNA polymerases are the enzymes that build DNA in cells.
• During DNA replication (copying), most DNA polymerases can “check their
work” with each base that they add.
• This process is called proofreading.
• If the polymerase detects that a wrong (incorrectly paired) nucleotide has
been added, it will remove and replace the nucleotide right away, before
continuing with DNA synthesis

5. Single – Stranded Break Repair
• When only one of the two strands of a double helix has a defect, the other
strand can be used as a template to guide the correction of the damaged
strand.
• In order to repair damage to one of the two paired molecules of DNA,
there exist a number of excision repair mechanisms that remove the
damaged nucleotide
and replace it with an undamaged nucleotide complementary to that found in
the undamaged DNA strand.
TYPES:
 Base excision repair (BER)
 Nucleotide excision repair (NER)
 Mismatch repair

6. Base excision repair (BER)
• Damage to a single nitrogenous base by deploying enzymes called glycosylases. These
enzymes remove a single nitrogenous base to create a purin or pyrimidinic site (AP
site).
• Enzymes called AP endonucleases nick the damaged DNA backbone at the AP site.
• DNA polymerase then removes the damaged region using its 5’ to 3’ exonuclease
activity and correctly synthesizes the new strand using the complementary strand as a
template.

7. Nucleotide excision repair (NER)
Repairs damaged DNA which commonly consists of bulky, helix-distorting damage,
such as pyrimidine dimerization caused by UV light.
Damaged regions are removed in 12–24 nucleotide-long strands in a three-step
process which consists of
1) recognition of damage,
2) excision of damaged DNA both upstream and downstream of damage by
endonucleases,
3) and resynthesis of removed DNA region.

8. Mismatch repair
Systems are present in essentially all cells to correct errors that are not corrected
by proofreading.
These systems consist of at least two proteins.
One detects the mismatch, and the other recruits an endonuclease that cleaves
the newly synthesized DNA strand close to the region of damage.
In E. coli , the proteins involved are the Mut class proteins.
This is followed by removal of damaged region by
• An exonuclease,
• Resynthesize by DNA polymerase,
• And nick sealing by DNA ligase.

9. Double-stranded break repair
Some types of environmental factors, such as high-energy radiation, can cause
double-stranded breaks in DNA (splitting a chromosome in two).
Double-stranded breaks are dangerous-
Because large segments of chromosomes, and the hundreds of genes they
contain, may be lost if the break is not repaired.
Two pathways involved in the repair of double-stranded DNA breaks are the
Non-homologous end joining and
Homologous recombination pathways.

10. In non-homologous end joining
 The two broken ends of the chromosome are simply glued back together.
 This repair mechanism is “messy” and typically involves the loss, or sometimes
addition, of a few nucleotides at the cut site.
 So, non-homologous end joining tends to produce a mutation, but this is better
than the alternative (loss of an entire chromosome arm)

11. Homologous recombination
 Information from the homologous chromosome that matches the damaged one
(or from a sister chromatid, if the DNA has been copied) is used to repair the
break.
 In this process, the two homologous chromosomes come together, and the
undamaged region of the homologue or chromatid is used as a template to
replace the damaged region of the broken chromosome.
 Homologous recombination is “cleaner” than non-homologous end joining and
does not usually cause mutations

12. KEY POINTS; CONCLUSION
• Cells have a variety of mechanisms to prevent mutations, or
permanent changes in DNA sequence.
• During DNA synthesis, most DNA polymerases "check their
work," fixing the majority of mispaired bases in a process
called proofreading.
• Immediately after DNA synthesis, any remaining mispaired
bases can be detected and replaced in a process called
mismatch repair.
• If DNA gets damaged, it can be repaired by various
mechanisms, including , excision repair, and double-stranded
break repair.