Question

GENETICS

Exposure to UV light results in thymine dimers between adjacent thymines in a single DNA strand. Describe how repair of this damage might lead to a mutation.

Answer 1

Exposure of UV light induces the formation of pyrimidine dimers in which two adjacent pyrimidines i.e thymines which are joined by the cyclobutane ring structures.

During the formation of thymine dimer, the two adjacent thymine nucleotides were linked covalently and if these are unrepaired it may lead to stoppage of DNA replication and transcription at that point.

Proof reading : DNA polymerase helps in correcting the mismatch pairs and reconcile its work but sometimes it slips the mismatch pair as sometimes it cannot fit into active site correctly that proceeds and replicate the dimer incorrectly and leads to formation of frameshift mutation or point mutation.

Nucleotide excision repair pathway : There is mechanism of repairing the above thymine dimer lesions known as nucleotide excision repair or dark repair where enzymes remove the nucleotide pyrimidine thymine dimer and replace them with the correct nucleotides.

Photoreactivation repair pathway : There is other mechanism known as light repair i.e. direct repair of thymine dimers through the process of photoreactivation i.e in presence of visible light.

Photolyase is the enzyme that recognises the distortion in the DNA helix which is caused by thymine dimer formation and binds to it.

In presence of visible light this enzyme changes the conformation that breaks the thymine dimer and make it match with the correct base pair.

Error prone and Recombinant repair pathway : They also utilises some enzymes to repair the DNA damage but sometimes they may damage the important genes and leads to the mutation.

Several proteins are required for the repair of double strand breaks caused by the thymine dimer and deficiency of these proteins causes hereditary diseases. These leads to the mutation.