Question

Which of the following is the type of DNA repair in which thymine dimers are directly broken down by the enzyme photolyase? a. direct repair b. nucleotide excision repair c. mismatch repair d. proofreading

Short answer

a. direct repair

Step-by-step solution

Understanding Thymine Dimers

Thymine dimers form when DNA is exposed to ultraviolet (UV) light, causing two adjacent thymine bases to bond together. This bonding can result in DNA mutations if not repaired.

Identifying the Repair Enzyme

Photolyase is an enzyme that specifically repairs thymine dimers by absorbing light and using the energy to break the bonds between the dimers, restoring the DNA to its original state.

Selecting the Correct Type of DNA Repair

The type of DNA repair involving photolyase directly breaking down thymine dimers is known as direct repair, as it directly reverses the damage without removing any part of the DNA strand.

Key concepts

Understanding Thymine Dimers

Thymine dimers are a form of DNA damage that is particularly common after DNA is exposed to ultraviolet (UV) light, such as sunlight. In this process, adjacent thymine bases on a DNA strand become abnormally bonded, creating a dimer. This can distort the DNA molecule, causing problems during DNA replication and transcription, potentially leading to mutations and cell malfunction.

Often, thymine dimers are associated with skin diseases, including cancer, because skin cells are frequently exposed to UV light. When these dimers form, they must be repaired for the DNA to function correctly. If this repair does not occur, the result can be errors in the genetic code and harmful mutations. Therefore, understanding the mechanism by which cells repair thymine dimers is crucial in comprehending how organisms prevent genetic damage and maintain genomic integrity.

The Role of Photolyase in Repair

Photolyase is a remarkable enzyme that specializes in repairing thymine dimers, thereby safeguarding the integrity of the DNA. Photolyase has a unique mechanism of action, which relies on light energy. The enzyme absorbs photons, typically from visible light, and uses this energy to catalyze the cleavage of the bonds between the thymine bases, effectively reversing the damage without the need for cutting out segments of DNA.

Photolyase is not present in all organisms. Notably, humans and other placental mammals lack this enzyme and rely on other repair mechanisms, like nucleotide excision repair. But in organisms where photolyase is present, it plays a vital role in allowing cells to recover from UV-induced damage rapidly and efficiently. It basically serves as a molecular sunscreen, preventing mutations that could lead to cancer.

Direct Repair Mechanism

Direct repair is a simplified and effective DNA repair mechanism. Unlike other repair processes that require removing and replacing damaged sections of DNA, direct repair targets the specific abnormality and fixes it, leaving the rest of the DNA strand intact. This specificity significantly reduces the risk of errors during the repair process.

One of the best examples of direct repair is the photolyase-mediated restoration of thymine dimers. This process embodies the direct repair mechanism because it involves the direct reversal of the DNA damage—breaking the abnormal bonds without removing any base pairs. Although not all organisms have the direct repair mechanisms, those who do benefit from a quicker and less error-prone DNA repair system.