Question 1

What makes DNA replication semiconservative?

Accepted Answer

Each daughter DNA molecule receives one of the two original parental strands and one newly synthesised complementary strand. The original strands are separated and retained rather than kept together in one molecule.

Question 2

Why is the lagging strand discontinuous?

Accepted Answer

DNA polymerase can add nucleotides only to a free 3'-OH and therefore synthesises DNA in the 5' to 3' direction. Because the two templates are antiparallel, synthesis on the lagging template proceeds away from the advancing fork as short Okazaki fragments.

Question 3

What is the role of DNA ligase after primer replacement?

Accepted Answer

RNA primers are removed, the resulting gaps are filled with DNA nucleotides, and DNA ligase seals the remaining nicks. It forms phosphodiester links in the sugar-phosphate backbone to join adjacent Okazaki fragments.

Question 4

What did the first generation show in the Meselson-Stahl experiment?

Accepted Answer

E. coli cells grown with heavy 15N nitrogen were transferred to a light 14N medium. After one generation, every DNA molecule contained one old heavy strand and one new light strand, so the sample formed one intermediate-density band.

Question 5

Why did the second generation show two bands?

Accepted Answer

Hybrid 15N/14N DNA molecules continued to form, while some molecules contained two newly synthesised 14N strands. These produced an intermediate band and a light band, with the denser intermediate band positioned below the light band.

DNA Replication Fork — Semiconservative Replication, Meselson–Stahl & Okazaki Fragments

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