Interaction of basesConjugative stability of these basesPresence of waterPresence of buffersVitamin C, E, D, A, etc.Repair mechanisms of the cells
Photoreaction- repair uses an enzyme photolase in the presence of visible light. In this repair mechanism, thymine cyclobutane dimer is momomerized.
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.
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.
-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
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.
-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.
3rd: glycosylases cleave the glycosidic bonds between abnormal base and 2-deoxyribose leads to apurinic/apyrimidic sites(AP sites) with missing bases.
4th: AP endonuclease recognize AP sites, & together with phosphodiesterase excises the sugar-phospate groups at these sites.5th: DNA polymerase adds the missing nucleotide6th: DNA ligase seals the ‘nicked’ DNA
-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 DNA2nd :DNA polymerase fills the gap3rd : DNA ligase seals the nicked DNAIn E. coli : Exinuclease (encoded by genes uvr A,uvr B & uvr C), trimeric protein, made up of subunits 2 UvrA & UvrB