§ B-04 Semiconservative DNA Replication: Replication Fork Mechanics & Meselson–Stahl Experiment (Grade 10–11 · DNA Replication)
Helicase, DNA Polymerase III, 5′→3′ Synthesis, Continuous Leading Strand & Lagging Okazaki Fragments. “All known DNA polymerases can add deoxyribonucleotides exclusively to the free 3′-OH terminus of an elongating strand (5′→3′ direction), dictating continuous leading-strand and discontinuous lagging-strand synthesis.”
Theory Summary & Scientific Mechanism
Semiconservative DNA replication, demonstrated by Meselson and Stahl (1958) using ¹⁵N/¹⁴N density-gradient centrifugation, ensures that each daughter DNA duplex retains one parental template strand paired with one newly synthesized strand.
At the replication fork, hexameric DNA helicase hydrolyzes ATP to unwind the double helix while topoisomerase relieves torsional supercoiling ahead of the fork. Because DNA Polymerase III catalyzes nucleophilic attack of the 3'-OH terminus on incoming dNTPs, chain elongation proceeds exclusively in the 5' → 3' direction.
Consequently, the antiparallel duplex imposes asymmetric synthesis: the leading strand elongates continuously toward the advancing fork on the 3' → 5' template, whereas the lagging strand is synthesized discontinuously away from the fork as short Okazaki fragments initiated by RNA primers, processed by DNA Polymerase I, and sealed by DNA ligase.
Everyday Analogy
Imagine a paving crew (DNA polymerase) that can only lay asphalt in one direction (5'→ 3'). On the leading lane it follows the helicase bulldozer continuously, while on the antiparallel lagging lane it must repeatedly hop ahead and pave short backward sections (Okazaki fragments) stitched together by DNA ligase!
Interactive Question & Derivation
The two parental DNA strands are antiparallel (3'→ 5' and 5'→ 3'), whereas DNA polymerase synthesizes new DNA exclusively in the 5'→ 3' direction. Why is one daughter strand (leading) synthesized continuously while the other (lagging) is synthesized as short Okazaki fragments, and what fraction of hybrid ¹⁵N/¹⁴N duplexes appears after k = 2 generations in ¹⁴N medium in the Meselson–Stahl experiment?
- A: Both strands are synthesized continuously, and 100% of DNA molecules remain heavy after 2 generations
- B: The leading strand elongates continuously (5'→ 3') toward the replication fork while the lagging strand is synthesized discontinuously as Okazaki fragments joined by DNA ligase; after k=2 generations the hybrid fraction is 2/2² = 50%
- C: The lagging strand is synthesized in the 3'→ 5' direction without RNA primers
- undefined: After 2 generations only 1 parental strand survives, giving 25% hybrid DNA
Study Hint: Both strands grow 5'→ 3' (leading continuously, lagging via Okazaki fragments), and after k=2 generations the hybrid ¹⁵N/¹⁴N fraction is 2/2² = 50%.
Proof & Explanation: Because DNA polymerase catalyzes nucleophilic attack of the primer 3'-OH on the incoming dNTP α-phosphate, chain elongation proceeds strictly 5'→ 3'. On the 3'→ 5' template, the leading strand follows helicase continuously. On the antiparallel 5'→ 3' template, polymerase must move away from the fork, synthesizing short Okazaki fragments from RNA primers that are subsequently sealed by DNA ligase. By semiconservative replication of a heavy ¹⁵N/¹⁵N duplex in ¹⁴N medium, after k generations exactly 2 out of 2^k duplexes are ¹⁵N/¹⁴N hybrids (2/4 = 50% at k=2) and 2^k - 2 are light ¹⁴N/¹⁴N.
Key Equations & 3D Simulation Parameters
5' → 3' synthesis, N_DNA(k) = 2^k, f_¹⁵N/¹⁴N = (2)/(2^k)
- Helicase unwinding & replication fork speed vₕel: 0.4 … 2.5 rel. (default: 1.2 rel.)
- Template G≡C base-pair content (3 H-bonds per pair): 20 … 80 % (default: 50 %)
- Okazaki fragment length on lagging strand: 2 … 5 seg. (default: 3 seg.)
- Replication generation k in Meselson–Stahl (¹⁵N → ¹⁴N): 1 … 4 gen. (default: 2 gen.)