Introduction Enzyme involve of DNA repair Types of DNA repair direct DNA repair excision repair system mismatch repair system Conclusion Reference DNA polymerase –a class of enzyme to all synthesize 5’ to 3’ direction of nucleotides. DNA polymerase I – a class of enzyme 1st isolated by Escherichia coli, and function is removes of RNA primers ,during DNA replication. Helicase- any of a group of enzyme that unwind the two strand of DNA to facilitate DNA replication. Exonuclease – an enzyme capable of cutting phosphodiester bonds between nucleotides located at an end of a DNA strand . Endonuclease – an enzyme capable of cleaving phosphodiester bonds between nucleotide located internally in a DNA strand . DNA ligase – a enzyme that fill the gap of nucleotides.

1. By
KAUSHAL KUMAR SAHU
Assistant Professor (Ad Hoc)
Department of Biotechnology
Govt. Digvijay Autonomous P. G. College
Raj-Nandgaon ( C. G. )

2. Synopsis
• Introduction
• Enzyme involve of DNA repair
• Types of DNA repair
direct DNA repair
excision repair system
mismatch repair system
• Conclusion
• Reference

3. Introduction
 The enzymes of the DNA repair systems, which protect and maintain the
information in the genetic code
 Chemical bonds are breaking ,DNA strands are snapping ,and nucleotide bases are
flying off.
 Each cell loses more than 10,000 bases/day just from spontaneous breakdown of
DNA at body temperature.
 While ,many cells are dividing and therefore copying DNA and each copy
introduces the possibilities of error.
 Failure to repair such lesions produces permanent alterations or mutations in the
DNA.
 Damage control – normal cellular activity injur DNA through Oxidation and other
common reactions.

4. Enzyme involve in DNA repair
DNA polymerase –a class of enzyme to all synthesize 5’ to 3’ direction of
nucleotides.
DNA polymerase I – a class of enzyme 1st isolated by Escherichia coli, and
function is removes of RNA primers ,during DNA replication.
Helicase- any of a group of enzyme that unwind the two strand of DNA to
facilitate DNA replication.
Exonuclease – an enzyme capable of cutting phosphodiester bonds
between nucleotides located at an end of a DNA strand .
Endonuclease – an enzyme capable of cleaving phosphodiester bonds between
nucleotide located internally in a DNA strand .
DNA ligase – a enzyme that fill the gap of nucleotides.

5. Types of DNA repair
Direct DNA Repair-
DNA polymerase mismatch repaired -
The frequency of base pair substitution mutations in bacterial genes from
10¯⁷ and ten power minus 11 error /generation.
However DNA polymerase inserts incorrect nucleotides frequencyof10¯⁵
most of the difference between the two values is accounted for 3’ to
5’exonuclease proofreading activity of DNA polymerase in both bacteria
and eukaryotes.
When an incorrect nucleotide is inserted ,the polymerase often detects
the mismatched base pair and correct the area by “backspacing” to
remove the using nucleotide and then resuming synthesis in the forward
direction.

6. e.g. mut-D gene mutator gene of E.coli encodes the ε subunit of DNA
polymerase III ,the primary replication enzyme of E.coli .
The mut-D mutants are defective in 3’ to 5’ proofreading activity ,so many
incorrectly inserted nucleotides are left unrepaired.

7. Photolyase: repairs cyclobutane pyrimidine dimers. Uses the
energy of light to catalyze the reversal of the cyclobutane bonds,
producing intact DNA. Not very important in mammals

8. Repair of UV-induced pyrimidine dimers
• Through photoreactivation or light repair ,Uv light induced
thymine dimers are reverted directly to the original form by
exposure to near uv light in the wavelenght range from 320-
370nm
• Photoreactivation can occurs when an enzyme called photolyase
(encoded by the ‘phr’ gene) is activated by a photon of light and
split the dimers apart.
• Strains with mutation in the phr gene are defective in light repair.
• Photolyase have been found in bacteria and in simple eukaryotes
,but not in humans.

9. Repair of alkylation damage
Alkylating agents transfer alkyl groups (usually methyl or ethyl groups)
onto the base.
The mutagen methylates the oxygen in 6 carbon in guanine .
e.g. In E.coli ,the alkylation damage is repaired by an enzyme called O⁶-
methyl guanine methyl transferase,encoded by the ada gene.
The enzyme removes from the methyl group from the guanine there
by changing the base back to its original form.

10. Excision repair of DNA damage
• Depending on the damage excision may involve a single base
or nucleotide or two or more nucleotides .
• Each excision repair system involves a mechanism to
recognize the specific DNA damage it repairs.

11. Base excision repair
• damaged single bases or nucleotides are most commonly repaired
by removing the base or the nucleotide involves and then inserting
the correct base or nucleotide.
• In base excision repair a repair glycosylase enzyme removes the
damage base from the DNA by cleaving the bond between the base
and deoxyribose sugar.
• Other enzyme can cleave the sugar phosphate backbone before and
after the now base less sugar ,releasing the sugar and leaving a gap
in the DNA chain.
• The gap is filled by with the correct nucleotide by a repair DNA
polymerase and DNA ligase.

12. (a) A DNA glycosylase recognizes a
damaged base and cleaves between the
base and deoxyribose in the backbone.
(b) An AP endonuclease cleaves the
phosphodiester backbone near the AP
site.
(c) DNA polymerase I initiates repair
synthesis from the free 3’ OH at the
nick, removing a portion of the damaged
strand (with its 5’→3’ exonuclease
activity) and replacing it with undamaged
DNA.
(d) The nick remaining after DNA
polymerase I has dissociated is sealed by
DNA ligase.
AP= apurinic or apyrimidinic
Base excision repair system

13. Nucleotide excision repair
In 1964 two groups of scientists –R.P.Boyce and P.HowardFlanders and R. Setlow
and W. Carrier isolated mutants of E.coli that after uv irradiation showed higher
than normal rate of induced mutation in the dark. these uv sensitive mutants were
called Uvr A mutants (Uvr for uv repair).
The uvrA mutants can repair thymine dimers only with the input of light meaning
may have a normal photoreactivation repair system.
However uvrA (wild type) E.coli can repair thymine dimers in dark .because the
normal photoreactive repair system can not operate in the dark the investigators
hypothesized that there must be another light independent repair system.
The called the system dark repair system or excision repair system ,now typically
referred to as the nucleotide excision repair system.
The NER system involves four protein UvrA UvrB and UvrC and UvrD ,encoded by
the genes uvrA uvrB uvrC and uvrD .

14. A complex of two UvrA proteins and one UvrB protein slides along the DNA when
a complex recognized a pyrimidine dimer or another serious distortion in the DNA
the UvrA subunits dissociate and UvrC protein binds to UvrB protein at the lesion.
And 12 genes encode protein involved in nucleotide excision repair

15. (a) Two endonucleases (excision endonucleases) bind DNA at the site of bulky lesion.
(b) One cleaves the 5’ side and the other cleaves the 3’ side of the lesion, and the
DNA segment is removed by a helices.
(c) DNA polymerase fills in the gap and
UvrA recognizes
bulky lesions
UvrB and
UvrC make cuts
Structural distortion = signal
Nucleotide excision repair

16. MutH - Binds 7-meGATC
MutS - Binds mismatch
MutL - links MutH and MutS
The mutation is in the new strand!
-CH3 marks the parental strand!
Mismatch repair

17. Methyl directed mismatch repair
• Despite proofreading by DNA polymerase a number of mismatched base pairs
remain uncorrected after replication has been completed .
• Many mismatched base pairs left after DNA replication can be corrected by methyl
directed mismatch repair.
• This system recognizes mismatched base pairs excise the in corrected bases and
then carries out repair synthesis.
• In E.coli product has three genes- mutS ,mutL and mutH are involved in the initial
stages of mismatch repair .
• The Muts gene encoded protein MutS binds to the mismatch .
• Then the repair system determines which bases in the correct one (the base on the
parental DNA strand) and which is the errorness one (the base on the new
stranded).

18. DNA repair causes defect

19. Conclusion
If the mutation occurs in a cell destined to become a gamete
,the genetic alteration may be passed to the next generation.
• Extreme change in cell's environment (heat, UV, radiation)
activates genes that code DNA repair enzymes
• Xeroderma pigmentosum is inherited disease defect in seven
different nucleotide excision repair protein like XPA ,XPB,XPC,
XPD,XPE,XPF and XPG

20. References
Cell And molecular biology by Gerald Karp 6thedi.
http://www.2modern.com/index.asp?PageAction=VIE
WP ROD&ProdID=985
http://www.senescence.info/WS.jpg
http://www.funpecrp.com.br/gmr/year2003/v
ol1- 2/imagens/sim0001fig1.jpg