1. Interaction of bases
Conjugative stability of these bases
Presence of water
Presence of buffers
Vitamin C, E, D, A, etc.
Repair mechanisms of the cells

2.  Photoreaction- repair uses an enzyme photolase in the
presence of visible light. In this repair
mechanism, thymine cyclobutane dimer is momomerized.

3.  Carried outby light-activated enzyme called DNA
photolyase.
Eg.: the damage UV light causes to DNA, pyrimidine
dimers. Dimers are formed when adjacent
pyrimidines form covalent bonds with each other.
This makes hydrogen bonding odd and it therefore
makes DNA replication impossible.
This dimers must be removed for replication to
take place.

5.  DNA photolyase bind to thymine dimers in the
dark but it cannot catalyse the cleavage of
thymine bonds
 Needs energy derived from visible light.
 Itbreaks the cross-links to restore DNA to its
original form.

7. -Involves at least 3 steps:
1st – a DNA repair endonuclease or endonuclease –
containing –enzyme complex recognizes, binds to
& excises to damage base/bases.
2nd – a DNA polymerase then fills the gap.
3rd –a DNA ligase seals the break left by the DNA
polymerase

8.  Base excision repair system remove abnormal
or chemically modified bases from DNA.
-Involves DNA glycosylases; each glycosylases
recognizes specific type of altered bases (eg.
Bromouracil)
-is a cellular mechanism that repairs damaged
DNA throughout the cell cycle. It is responsible
primarily for removing small, non-helix-
distorting base lesions from the genome.

9. -Important for removing damaged bases that
could otherwise cause mutations by mispairing
or lead to breaks in DNA during replication.
-Initiated by DNA glycosylases, which recognize
and remove specific damaged or inappropriate
bases, forming AP sites.
Steps:
1st: Deamination(eg. by nitrous oxide) of
cytosine to uracil.
2nd: Glycosylases recognizes altered base and
bind to it.

10. 3rd: glycosylases cleave the glycosidic bonds
between abnormal base and 2-deoxyribose
leads to apurinic/apyrimidic sites(AP sites)
with missing bases.

11. 4th: AP endonuclease recognize
AP sites, & together with
phosphodiesterase excises the
sugar-phospate groups at these
sites.
5th: DNA polymerase adds the
missing nucleotide
6th: DNA ligase seals the ‘nicked’
DNA

13. -repairs bigger segment of damaged DNA
- repair particularly important mechanism by which the
cell can prevent unwanted mutations by removing the
vast majority of UV-induced DNA damage (mostly in
the form of thymine dimers and 6-4-photoproducts)
Steps:
1st :exinuclease produces cuts on either side of damaged
DNA & cuts out the oligonucleotide containing the
damaged DNA
2nd :DNA polymerase fills the gap
3rd : DNA ligase seals the nicked DNA
In E. coli : Exinuclease (encoded by genes uvr A,uvr B &
uvr C), trimeric protein, made up of subunits 2
UvrA & UvrB