9: DNA Packaging, Damage, Repair

What are histones?

Proteins that help package DNA into nucleosomes.

What is a nucleosome?

A histone complex with about 200 base pairs of DNA.

What is depurination?

Loss of purine bases (A/G) from DNA.

What is deamination?

Removal of an amino group from DNA bases (C, A, G).

What are and what causes pyrimidine dimers?

Covalent bonds between adjacent pyrimidine bases, UV radiation.

What are mutagens?

Agents that cause DNA damage (chemical or radiation).

What is base excision repair?

Removes damaged bases and fills the gap.

What is single-strand break (SSB) repair?

Repairs single-strand breaks using proteins like PARP-1. PARP-1 inhibitors used as RT sensitizors.

What is double-strand break (DSB) repair?

Repair of both strands of DNA using NHEJ or HR.

What is non-homologous end joining (NHEJ)?

DSB repair that joins DNA ends, error-prone.

What is homologous recombination (HR)?

DSB repair using a sister chromatid, error-free.

How does chromatin structure regulate DSB repair?

Chromatin state influences access to DNA for repair.

How does the cell cycle regulate DSB repair?

HR occurs in S/G2 phase, timer model; NHEJ in G1 phase.