Question

Topic : The mechanism of DNA repair (write a review on it more than 3000 words)

Answer 1

There are 3 types of DNA repair mechanisms.

1)NUCLEOTIDE EXCISION REPAIR (NER)

->this type of repair prevents unwanted mutation by removing many of UV induced DNA damage (mainly thymine dimer).

*about 9 major proteins involved in NER in Mammalian cells and named according to the diseases associated with the deficiencies in those proteins.these are,XPA, XPB, XPC,XPD, XPE, XPF and XPG.these names are derived
from xeroderma pigmentosum and CSA and CSB
are enzymes deficient in Cockayne syndrome.

*NER takes place simultaneously with the replication process (proof reading). The original template DNA contains methylated residues (N6-methyl adenine and
5-methyl cytosine).The newly synthesized strand will not have methylated bases. So enzymes can recognise the original (correct) DNA strand. The mismatched base is identified and removed along with a few bases around that area. The wrong base is removed by the endonuclease activity of the XPG.It removes 24-32 nucleotides around the wrong base.As 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. Then the gap or nick is sealed by DNA ligase. The ligase requires energy input for its activity.

2)MISMATCH REPAIR

->Specific proteins scan the newly synthesized DNA strand (new strand is identified by not being methylated). The mismatched area is identified, a loop is made.in E.coli, this recognition and looping is done by three proteins, MutS,MutC and MutH. Then that segment is removed. Finally correct segment is synthesized by the help of DNA polymerase, SSBs,and ligase.

3)BASE EXCISION REPAIR

->by depurination cytosine, adenine and guanine bases spontaneously form uracil, hypoxanthine and xanthine respectively. These are not normal constituents of DNA. Specific N-glycosylases can remove these abnormal
bases. The sugar has no base. Apurinic endonucleases excise this abasic sugar. Proper base is then added by a repair DNAP and ligase.