In: Biology
Describe the three steps in the basic mechanism for DNA repair.
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.