Genetics Ch 6.4-6.5 Lo's

Exam 2: DNA Replication, Recombination, Heteroduplex & Gene Conversion, Reciprocal Recombination

What model of DNA replication was confirmed by the Meselson–Stahl experiment?

Semiconservative replication — each daughter DNA contains one parental strand and one newly synthesized strand.

What experiment proved semiconservative replication?

Meselson–Stahl experiment using 15N/14N density gradient centrifugation.

What does DNA polymerase require to synthesize DNA?

• All four dNTPs

• Single‑stranded template

• Primer with free 3′‑OH

Which strand is synthesized continuously?

Leading strand (toward replication fork).

Which strand is synthesized discontinuously?

Lagging strand as Okazaki fragments.

What enzyme unwinds DNA?

Helicase.

What enzyme synthesizes RNA primers?

Primase.

What enzyme joins Okazaki fragments?

DNA ligase.

What enzyme relieves supercoiling?

Topoisomerase

What protein initiates meiotic recombination?

Spo11, which creates a double‑strand break.

What happens during resection?

Exonuclease degrades 5′ ends, leaving 3′ single‑stranded tails.

What protein mediates strand invasion?

Dmc1, assisted by other proteins

What structure forms when the invading strand displaces the original strand?

D‑loop, stabilized by RPA.

What structure forms after DNA synthesis and ligation in recombination?

Double Holliday junction

What is branch migration?

Movement of Holliday junctions that extends heteroduplex regions.

What enzyme resolves Holliday junctions?

Resolvase, followed by ligase.

What are the two possible outcomes of a recombination event?

• Crossover

• Non‑crossover (still produces heteroduplex)

What is a heteroduplex?

A DNA region where one strand comes from each nonsister chromatid.

How does heteroduplex formation lead to gene conversion?

Mismatched bases in the heteroduplex are repaired, sometimes changing one allele to match the other → gene conversion.

What allele ratio indicates gene conversion?

3:1 instead of normal 2:2 in tetrads.

Does gene conversion require crossing‑over?

No — it can occur in non‑crossover pathways.

What evidence shows recombination is usually reciprocal?

Tetrad analysis: equal numbers of recombinant classes in T and NPD tetrads.