Ultraviolet Radiation Damage and Repair

What is the consequence of DNA absorbing UV radiation at 254 nm?

It forms new covalent bonds between adjacent pyrimidines, leading to pyrimidine dimers.

What are the covalent bonds formed between adjacent pyrimidines called?

Pyrimidine dimers.

Which is the most common type of pyrimidine dimer?

Thymine dimers.

What are the two main repair mechanisms that bacteria like Escherichia coli have for DNA damage caused by UV radiation?

Light repair (photoreactivation) and excision repair (dark repair).

What is the role of DNA photolyase in light repair?

It is activated by visible light and monomerizes the dimer by reversing the original reaction.

What does DNA ligase do during DNA repair?

It closes the gap between the last nucleotide of the new segment and the first nucleotide of the old DNA.

What enzyme is involved in excision repair that detects thymine dimers?

UvrABC endonuclease.

What does UvrABC endonuclease do?

It breaks two bonds around the thymine dimer, facilitating the removal of the damaged DNA.

What enzyme removes the 14-nucleotide fragment during excision repair?

Helicase (UvrD, also known as helicase II).

Why can long or intense exposures to UV radiation be lethal to cells?

They produce more damage than the cell can repair.