Thymine dimers, formed when adjacent thymine bases in DNA are exposed to ultraviolet (UV) light, can cause significant damage to our genetic material. These lesions can interfere with DNA replication and transcription, potentially leading to mutations and skin cancer. Fortunately, our cells have evolved mechanisms to repair such damage, with excision repair playing a crucial role in removing thymine dimers.
The Process of Excision Repair for Thymine Dimers
Excision repair is a light-independent mechanism that human cells use to remove thymine dimers from DNA. Unlike photolyase, which directly reverses the dimer formation using light energy, excision repair involves a more complex process:
Recognition: Specialized proteins identify the distortion in the DNA caused by the thymine dimer.
Incision: Endonucleases make cuts on both sides of the damaged area.
Excision: The segment containing the thymine dimer is removed, creating a gap in the DNA strand.
Synthesis: DNA polymerase fills in the gap using the undamaged strand as a template.
Ligation: DNA ligase seals the newly synthesized DNA to the existing strand.
The Importance of Excision Repair in Humans
Excision repair is particularly significant for humans because we lack the photolyase enzyme found in some other organisms. This makes excision repair our primary defense against thymine dimers and other UV-induced DNA lesions. The process is highly efficient, capable of repairing about 90% of thymine dimers within a short time after exposure.
Nucleotide Excision Repair: A Specialized Mechanism
Nucleotide excision repair (NER) is a specific type of excision repair that handles thymine dimers in human cells. This system involves a complex of proteins working together to recognize and remove the damaged DNA segment. Key players in this process include:
XPA and RPA: Damage recognition proteins
XPC: Helps initiate the repair process
TFIIH: Unwinds the DNA around the damage site
XPG and XPF: Endonucleases that make incisions around the damage
The coordinated action of these proteins allows for the precise removal of thymine dimers without compromising the integrity of the surrounding DNA.