Question

Fill in the blanks with the following options. You will not use every answer.

DAM, MutL, MutH, MutS, DNA polymerase I, DNA polymerase II, DNA polymerase III, DNA polymerase IV

a) Initially binds to damaged DNA: __________

b) The matchmaker - brings together the two other proteins ___________

c) The endonuclease that cuts the DNA at the damaged site ___________

d) Polymerizes DNA into the gap excised by the endonuclease ____________

e) This is the protein responsible for the Mut proteins knowing which strand is the old one ______________

Answer 1

a) Initially binds to damaged DNA: ____MutS______

b) The matchmaker - brings together the two other proteins __MutL and MutH_________

c) The endonuclease that cuts the DNA at the damaged site _____MutH______

d) Polymerizes DNA into the gap excised by the endonuclease ____DNA Polymerase 111________

e) This is the protein responsible for the Mut proteins knowing which strand is the old one __MutH____________

DNA mismatch occur during DNA replication. DNA polymerase 3 catalyses the incorporation of new complementary bases to the new DNA strand. However, it is possible for DNA polymerase 3 to incorporate a non complementary base to the new strand. This is known as mismatch, which could result in faulty protein being coded. So the mismatch need to be repaired.

The first step in mismatch repair pathway in E.coli is the recognition of the mismatch by a protein MutS homodimer. MutS recognizes the mismatch and binds to it. Then the MutL homodimer binds to MutS. Meanwhile, up to a kilobase pair away, the MutH protein recognizes a GATC site and binds to it. MutH is a weak endonuclease which will create a backbone nick or incision between Guanine and Adinine residues where it binds( GATC) in the newly synthesised strand. When activated MutH is able to differenciate between newly synthesised strand and the template strand by lack of methylation markers on the new strand. This is the reason why this mismatch repair pathway is also called metal directed mismatch repair. Inorder for the MutH to become fully activated it needs to bind with MutL. This causes the DNA to bend. The newly formed backbone incision allows an exonulease to enter and begin excising nucleotides from the new strand and it strat excising till the mismatched nucleotide and then stops excising just beyond the mismatch pair. Now the DNA polymerase 3 starts synthesizing nucleotides in the excised part, inserting the correct complementary bases where the mismatch war occured. At the final step, DNA ligase ,ligate the backbone insition or nick. By this the mismatch repair process gets completed.