Step by Step
½
Semiconservative = half old, half new
When DNA replicates, the two strands separate. Each strand serves as a template for a new complementary strand. The result: two daughter DNA molecules, each containing one original (parental) strand and one newly synthesized strand.
Memory trick: "Semi" = half. Half of the original DNA is conserved in each daughter — like photocopying a page but keeping half the original ink.
M-S
Meselson-Stahl experiment (1958) proved it
Meselson and Stahl grew bacteria in heavy nitrogen (¹⁵N), then switched to light nitrogen (¹⁴N) and let them replicate. After one round: all DNA was intermediate density (half heavy, half light) — exactly as semiconservative predicts. This ruled out conservative and dispersive models.
5→3
DNA polymerase synthesizes 5'→3' only
DNA polymerase can only add nucleotides to the 3' end of a growing strand — it always reads the template 3'→5' and builds the new strand 5'→3'. This creates an asymmetry at the replication fork.
L/L
Leading and lagging strands
The leading strand is synthesized continuously (same direction as the fork opens). The lagging strand is synthesized in short fragments called Okazaki fragments (opposite direction), which are later joined by DNA ligase. Both require an RNA primer to start.
Memory trick: Leading = continuous (smooth). Lagging = fragments (chunky Okazaki pieces joined together).
Applied Walkthrough
1
A cell prepares to divide. The double helix unwinds at the origin of replication — helicase breaks the hydrogen bonds between strands.
2
RNA primase lays down a short RNA primer on each template strand — DNA polymerase needs this to start synthesis.
3
DNA polymerase extends from the primer, synthesizing the new strand 5'→3'. Leading strand: continuous. Lagging strand: Okazaki fragments.
4
Primers are removed, gaps filled, and DNA ligase seals the fragments. Result: two identical daughter DNA molecules, each with one old and one new strand.
Exam Application
Exams test the Meselson-Stahl experiment and what it proved, the direction of synthesis (5'→3'), the difference between leading and lagging strands, and the role of Okazaki fragments. Know the enzymes: helicase (unwinds), primase (primer), DNA polymerase (synthesizes), ligase (joins fragments).
⚠ Common Trap
Students confuse semiconservative with conservative (where both strands would be new in one daughter, and both old in the other) or dispersive (where both strands would have mixed old/new segments scattered throughout). Meselson-Stahl ruled both of those out. Also: DNA polymerase cannot start a strand from scratch — it always needs an RNA primer first.
✓ Quick Self-Check
1. What does "semiconservative" mean in DNA replication?
Each daughter DNA molecule retains one original (parental) strand and one newly synthesized strand.
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2. What did Meselson and Stahl use to prove semiconservative replication?
Heavy nitrogen (¹⁵N) and light nitrogen (¹⁴N) isotope labeling — after one round of replication all DNA had intermediate density, consistent only with semiconservative replication.
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3. In which direction does DNA polymerase synthesize new DNA?
5' to 3' — it reads the template 3'→5' and builds the new strand 5'→3'.
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4. What are Okazaki fragments and on which strand do they occur?
Short DNA fragments synthesized on the lagging strand — later joined by DNA ligase into a continuous strand.
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5. Why does DNA polymerase need an RNA primer to start synthesis?
DNA polymerase can only add nucleotides to an existing 3'-OH group — it cannot initiate a new strand from scratch. The RNA primer provides that 3' end.
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