BIOB11 Lecture 7

In eukaryotes, during which phase does DNA replication occur?

S phase.

Why is eukaryotic replication more regulated than bacterial replication?

It must coordinate DNA synthesis with the cell cycle.

Why does histone mRNA production increase during S phase?

Newly replicated DNA needs new nucleosomes.

Why must nucleosomes be temporarily disrupted during replication?

To allow replication fork movement.

What reassembles nucleosomes after replication?

Histone chaperones.

Why do daughter strands contain ~50% old histones?

To preserve epigenetic information.

What is a replicon?

A segment of DNA replicated from one origin.

How many origins exist in human cells?

~10,000–100,000.

What complex initiates replication at origins?

ORC (Origin Recognition Complex).

Why can’t chromosome ends be fully replicated?

Removal of final RNA primer leaves no 3’ OH.

What happens to telomeres in normal somatic cells?

They shorten with each division.

What is telomerase?

Reverse transcriptase that extends telomeres.

What does telomerase contain?

Its own RNA template.

What sequence is repeated in human telomeres?

GGGTTA (~1000 tandem repeats).

Why do stem/germ cells express more telomerase?

They divide frequently.

What disease results from reduced telomerase activity?

Dyskeratosis congenita.

What is a major risk of high telomerase activity in all cells?

Increased cancer risk.

Why is replication highly accurate?

Base pairing + proofreading + repair.

How does proofreading work?

3’→5’ exonuclease removes mismatches.

What happens if damage is not repaired before replication?

Permanent mutation.

What causes pyrimidine dimers?

UV radiation.

What causes depurination?

Thermal instability (loss of A or G).

What causes double-strand breaks?

Ionizing radiation, chemicals.

What is deamination of cytosine?

C → U.

If C deamination is unrepaired before replication, what mutation occurs?

C–G becomes T–A.

Name major DNA repair pathways.

NER, BER, MMR, NHEJ, homologous recombination.

How does MMR recognize errors?

Distortions in DNA helix.

Does MMR remove bases or nucleotides?

A stretch of nucleotides.

What does NER repair?

Bulky lesions (e.g., thymine dimers).

Why prioritize transcription-coupled NER?

Protect actively transcribed genes.

What disease results from defective NER?

Xeroderma pigmentosum (XP).

What does BER repair?

Small base alterations (e.g., deamination).

What enzyme initiates BER?

DNA glycosylase.

What enzyme removes sugar-phosphate backbone in BER?

AP endonuclease.

What happens if DSB (double stranded break) is not repaired?

Chromosome loss or fragmentation.

What is NHEJ?

Nonhomologous end joining (fast but error-prone).

What protein detects DSB in NHEJ?

Ku protein.

Are small deletions common in NHEJ repair?

Yes.

What is homologous recombination?

Accurate DSB repair using homologous template.

Which repair pathway is more accurate: NHEJ or homologous recombination?

Homologous recombination.

What template is typically used in homologous recombination?

Sister chromatid.

What is loss of heterozygosity (LOH)?

Loss of one allele due to repair using homologous chromosome.

Why is LOH dangerous?

Can unmask recessive mutations → cancer risk.

How is LOH different from meiotic crossing over?

LOH occurs in somatic repair; crossing over occurs in meiosis.

Why must DNA damage be repaired before replication?

Replication fixes damage into permanent mutation.

Why are telomeres repetitive?

Allows repeated extension without losing genes.

Why does DNA methylation matter for repair?

Deamination of 5-methylcytosine produces T–G mismatch.

Which repair pathway removes bulky distortions?

NER (Nucleotide excision repair).

Which repair pathway removes specific altered bases?

BER (Base excision repair).

What determines whether homologous recombination can occur?

Availability of homologous DNA (sister chromatid).