Genetics Chapter 12

Why must DNA replication be fast?

Because every cell division requires a complete copy of DNA. For example, E. coli divides every 20 minutes and must replicate 4.6 million base pairs at ~1,000 nucleotides per second.

Why must DNA replication be accurate?

Errors can be harmful. DNA replication is highly precise, with only one error per billion nucleotides. In humans, this results in ~12 errors per cell division.

What are the three models of DNA replication?

Conservative, Dispersive, and Semiconservative.

Which model of DNA replication is correct?

Semiconservative, proven by Meselson & Stahl in 1958.

What is the Meselson & Stahl experiment?

An experiment using nitrogen isotopes to distinguish old and new DNA strands, confirming semiconservative replication.

What are the three modes of replication?

Theta replication (circular DNA), Rolling circle replication (viruses), and Linear replication (eukaryotes).

What is theta replication?

A replication mode resembling the Greek letter theta (θ), used by bacteria and plasmids.

What is rolling circle replication?

A mode used by viruses to produce multiple copies of DNA in a production-line fashion.

What is needed for DNA synthesis?

ssDNA template, dNTPs, RNA primers, and enzymes like DNA polymerase.

What direction does DNA synthesis occur in?

5′ to 3′ direction.

What are the stages of DNA replication?

Initiation, Unwinding, Elongation, Termination.

What does helicase do?

Breaks hydrogen bonds between DNA strands to unwind the helix.

What does gyrase do?

Reduces torsional strain by making double-stranded breaks and resealing them.

What does primase do?

Synthesizes short RNA primers to initiate DNA synthesis.

What is the leading strand?

The strand synthesized continuously in the 5′ to 3′ direction.

What is the lagging strand?

The strand synthesized discontinuously in fragments called Okazaki fragments.

What does DNA polymerase III do?

Synthesizes the new DNA strand.

What does DNA polymerase I do?

Replaces RNA primers with DNA.

What does DNA ligase do?

Seals the nicks between DNA fragments.

How is replication terminated in bacteria?

When replication forks meet and termination proteins block further synthesis.

How is replication fidelity achieved?

Through base pairing, proofreading, and mismatch repair.

What is proofreading?

DNA polymerase removes incorrect bases using its 3′ to 5′ exonuclease activity and inserts the correct ones.

What is mismatch repair?

A post-replication process that replaces mismatched bases using methylation to identify the old strand.

How do eukaryotic cells manage multiple replication origins?

By licensing each origin with replication licensing factors that prevent reuse within the same cycle.

What happens to nucleosomes during replication?

They are dissolved before replication and reassembled afterward using a mix of old and new histones.

What is the end-replication problem?

Removal of RNA primers at the 5′ ends leaves gaps that cannot be filled, leading to chromosome shortening.

What is telomerase?

An RNA-protein complex that extends the 3′ ends of chromosomes using its RNA template.

Where is telomerase active?

In rapidly dividing cells like germ cells, embryonic cells, and stem cells.

What happens in cells lacking telomerase?

Telomeres shorten with each division, leading to aging and cell death.

How is telomerase linked to cancer?

Cancer cells often have active telomerase, allowing them to divide indefinitely.

How is telomere length linked to health?

Shortened telomeres are associated with aging and stress; lengthening may occur in space travel but may increase cancer risk