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In: Biology

Describe the three steps in the basic mechanism for DNA repair.

Describe the three steps in the basic mechanism for DNA repair.

Solutions

Expert Solution

There are several ways of DNA repair. The basic mechanism of DNA repair consists of nucleotide excision repair, base excision repair, mismatch repair.

NUCLEOTIDE EXCISION REPAIR

It takes simultaneously with the replication process (proof
reading).Generally the original template DNA contains methylated residues (N6-methyl adenine and 5-methyl cytosine) while newly synthesized strand will not have methylated bases. So enzymes can recognise the original (correct) DNA strand.Here the mismatched base is identified and removed in accompany with a few bases around that area. The wrong base is removed by the endonuclease activity of the XPG(protein involved in nucleotide excision repair). It removes 24-32 nucleotides around the wrong base.When the endonuclease cleaves at two points, the enzyme is sometimes also called excinuclease. A small segment of DNA with correct base sequence is then synthesized by DNA polymerase beta. Later the gap or nick is sealed by DNA ligase.energy input is required for activity of ligase.

BASE EXCISION REPAIR

The cytosine, adenine and guanine bases spontaneously form uracil, hypoxanthine and xanthine respectively by depurination.which are not normal components of DNA. Special N-glycosylase removes these abnormal
bases. The sugar has no base. Apurinic endonucleases excise this abasic sugar. then proper base is added by a repair DNAP and ligase.

MISMATCH REPAIR

In this some specific proteins scan the newly synthesized DNA strands(not methylated).and mismatched are is identified and a loop is made(in E coli this recognition and looping is done by 3 proteins MutS,MutC,MutH.later that segment is removed and correct segment is synthesised with help of DNA polymerase, ssbs and ligase.


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