Question

_{Order steps 1-7, first step (1) and last step (7)}

Order the events in the repair of a GT base pair in E. coli. • The three-component complex (MutSLH) identifies the old strand by locating a methylation mark. The endonuclease then cleaves the unmethylated (new) strand. • The mismatch-binding protein complex (MutSL) scans the genome after DNA replication to find incorrect base pairs. • DNA polymerase extends the 3'OH of the unmethylated, new strand to synthesize new DNA. • Once bound to the GT base pair, the initial mismatch-binding complex (MutSL) recruits an endonuclease (MutH). • Exonucleases and helicases degrade and unwind the unmethylated, new strand. • The GT base pair is recognized and bound by the mismatch-binding complex (MutSL). • When DNA polymerase finishes filling in the ssDNA gap, DNA ligase seals up the nick. [Choose] The three-component complex (MutSLH) identifies the old strand by locating a methylation mark. The endonuclease then cleaves the methylated (new) strand. [Choose] The mismatch-binding protein complex (MutSL) scans the genome after DNA replication to find incorrect base pairs. [Choose] DNA polymerase extends the 3'OH of the unmethylated, new strand to synthesize new DNA. [Choose ] Once bound to the GT base pair, the initial mismatch-binding complex (MutSL) recruits an endonuclease (Muth). Exonucleases and helicases degrade and unwind the unmethylated, new strand. [Choose] The GT base pair is recognized and bound by the mismatch-binding complex (MutSL). [Choose] When DNA polymerase finishes filling in the ssDNA gap, DNA ligase seals up the nick. [Choose]

Answer 1

Answer: The steps for the repair are as follows

1) The mismatch binding protein complex (mutSL) scans the genome after DNA replication to find incorrect base pairs.

2) The GT base pair is recognised and bound by the mismatch binding complex.

3) Once bound to the GT base pair, the initial mismatch binding complex (MutSL)recruits an endonuclease (Mut H).

4) The three-component complex(MutSLH) identifies the old strand by locating a methylation mark.The endonuclease then cleaves the unmethylated(new) strand.

5) Exonucleases and helicases degrade and unwind the unmethylated new strand.

6) DNA polymerase extends the 3’OH of the unmethylated, new strand to synthsize new DNA.

7) When DNA polymerase finishes filling in the ssDNA gap,DNA ligase seals the gap.

Explanation: The DNA mismatch repair system works as an additional control for repairing the mismatched base pairs. The system consists of Mut S which is a homodimer ,Mut L a homodimer these together recognise the mismatch and then associate with the third component MutH which has the endonuclease activity, it cuts the newly synthesized strand. The recognition of old and new strand depends on the methylation. The old strand is methylated by Dam methyl transferase when it was synthesized and the new strand is not methylated hence the endonuclease recognises the new strand and cuts it. Then DNA Helicase unwinds the strand and ss DNA exonuclease cleaves the strand. DNA pol III synthesizes the new portion of DNA and DNA ligase joins it.

Mismatched bases are introduced mostly because of tautomerisation(a base isomer will be introduced instead of normal base) of the bases during DNA replication. Sometimes oxidised form of guanine is added instead of normal guanine in that case guanine will pair up with adenine instead of cytosine.

Methyl directed mismatch repair(MMR) corrects such wrong bases and decreases the possibility of mutations.