What the exam asks
- Identify and orient strands: Label and ends on replication-fork diagrams and distinguish the leading strand from the lagging strand based on synthesis direction relative to fork movement.
- Describe enzyme functions: State the exact molecular roles of helicase, topoisomerase, RNA polymerase (in primer synthesis), DNA polymerase, and ligase.
- Predict disruptions: Predict the immediate molecular effect of a mutation or drug that disables one of these five components.
- Explain continuity: Explain how complementary base pairing and semiconservative replication preserve genetic information across generations.
Core ideas
Semiconservative replication
DNA replication is semiconservative: the two parental strands separate, and each serves as a template for the synthesis of a new complementary strand ( pairs with , pairs with ). Each resulting double-stranded DNA molecule consists of one conserved parental strand and one newly synthesized strand. This mechanism ensures the high-fidelity continuity of hereditary information.
Directionality and the replication fork
The two strands of a DNA double helix are antiparallel ( runs opposite to ). Because can only add new nucleotides to the free hydroxyl () group on the of a growing strand, new DNA is always synthesized in the (reading the template strand from ).