Contrast the various types of DNA repair mechanisms known to counteract the effects of UV radiation. What is the role of visible light in repairing UV-induced mutations?

Ultraviolet (UV) radiation is a type of electromagnetic radiation that mainly comes from the sun. It has a wavelength shorter than visible light but longer than X-rays. UV radiation can cause damage to the DNA within our cells, primarily by forming covalent bonds between adjacent pyrimidine bases, such as thymine and cytosine. This results in the formation of cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs), which can lead to errors in DNA replication and transcription if left unrepaired, ultimately leading to mutations and an increased risk of skin cancer.

There are several known DNA repair mechanisms that help counteract the effects of UV radiation on our DNA. These include: i) Nucleotide Excision Repair (NER): NER is a versatile and highly conserved mechanism that can remove a wide range of DNA lesions, including CPDs and 6-4PPs. In this process, a small segment of the damaged DNA strand is excised by endonucleases, and the resulting gap is filled by DNA polymerase and ligase enzymes, using the undamaged complementary strand as a template. ii) Base Excision Repair (BER): BER is responsible for detecting and repairing small base lesions that do not severely distort the DNA helix. In this mechanism, a specialized DNA glycosylase enzyme first recognizes and removes the damaged base, generating an apurinic/apyrimidinic (AP) site. This site is then processed by AP endonucleases, DNA polymerase, and ligase enzymes, similar to the NER mechanism. iii) Direct Repair: Some DNA lesions can be directly reversed without the need for excision and resynthesis. For example, photolyase enzymes can repair CPDs by using visible light energy to break the cyclobutane ring, returning the pyrimidine bases to their original state. This process is called photoreactivation.

Visible light plays an essential role in the direct repair of UV-induced mutations, specifically in the process of photoreactivation. Photoreactivation is mediated by photolyase enzymes, which require visible light to function. When photolyases bind to CPDs, they absorb visible light, particularly blue light wavelengths, causing a photochemical reaction that transfers an electron to the cyclobutane ring. This electron transfer cleaves the covalent bonds between the pyrimidine bases, thereby reversing the dimerization and restoring the original DNA sequence. In summary, there are several DNA repair mechanisms, such as NER, BER, and direct repair, that counteract the effects of UV radiation. Visible light plays a vital role in the direct repair mechanism called photoreactivation, by providing the energy required for photolyase enzymes to repair CPDs.