Question

describe in detail how prokaryotic cells and eukayotic cells repair thymine dimers

Answer 1

DNA damage can occur due to mutations produced by certain agents called mutagens or mutagenic agents which can take place by both physical and chemical means. Non-ionising radiations such as UV light is one of the major sources of DNA damage. The major mutagenic effect of UV rays is the formation of thymine (pyrimidine) dimer that disturbs the DNA double helix and thus, DNA replication and transcription. There are several mechanisms by which the cells can repair this damage by removing or correcting the T-T dimers. These are as follows:

(i) Direct reversal: In this mechanism, cells eliminate the DNA damaging reactions by chemically reversing it. This mechanism does not involve a template, because the nature of damage is incurred in only one of the four bases and due to its specificity it does not involve the breakage of the phosphate-sugar backbone. The formation of T-T dimers, which resulted from the irradaition with UV lights can be corrected by photoreactivation process that directly reverses this damage by the action of enzyme photolyase. This enzyme is found mostly in prokaryotes and also in lower and higher eukaryotes, but is absent in placental mammals. Photolyase contains chromophores that can capture photons from blue light (300-500 nm wavelength). It first detect and bind to the damaged DNA site, which is the Thymine dimer, and uses the absorbed light energy to oxidize the cyclobutane ring of the thymine dimer, converting into monomers without causing any damage to the double strands. The enzyme then dissociates from the DNA and thus, the damage is repaired.

(ii) Nucleotide excision repair: This mechanism is a part of single strand damage repair which removes and replace damaged bases. Nucleotide excision repair detects damaged DNA consisting of bulky, helix-distorting damage, such as the thymine dimer formation caused by UV light. The damaged region of nucleotides are excised in this process. The distortion in the DNA caused by the formation of T-T dimers is first recognized by a protein complex followed by a pair of endonucleases that makes nicks in the DNA strand on both the sides of T-T dimer. The damaged regions are removed which are 12 nucleotides long apart and the DNA between the nicks is removed. A DNA polymerase I replaces the missing DNA by filling the gap left behind and DNA ligase seals the gap in the DNA strand. This mechanism is used by nearly all eukaryotes and prokaryotes.