6.1.5—Semi-conservative DNA replication

Semi-conservative DNA replication produces two DNA molecules from one original molecule. Each product contains one original template strand and one newly synthesised complementary strand.

1. Separate the templates: Hydrogen bonds between complementary bases break, so the double helix opens and the two original antiparallel strands are exposed. Each original strand remains intact and acts as a template.
2. Match free nucleotides: Free DNA nucleotides align with exposed template bases by complementary pairing: A with T and C with G. This selects the sequence of each new strand.
3. Extend new strands: DNA polymerase joins adjacent nucleotides by forming phosphodiester bonds. It can extend a new strand only in the 5′→3′ direction, so the two templates are copied differently.
4. Account for leading and lagging synthesis: The leading strand is made continuously in the direction of the replication fork. The lagging strand is made as short Okazaki fragments because polymerase still works only 5′→3′; DNA ligase joins the fragments into a continuous strand.
5. Check the outcome: The original molecule has become two DNA molecules, each with one old strand and one new complementary strand: each product = 1 original strand + 1 new strand.

Complementary base pairing copies the information, while the fixed 5′→3′ direction of DNA polymerase explains why one new strand is continuous and the other is assembled in fragments. Ligase completes the lagging-strand backbone; the semi-conservative result follows because each original strand is retained as a template in one product.

Semi-conservative means that each complete DNA product keeps one whole original strand; it does not mean random pieces of old DNA are mixed into both strands. Hydrogen bonds open between the templates, whereas phosphodiester bonds form the new backbones. Staff-only visual brief retained in the Topic Blueprint: show one replication fork, antiparallel templates, continuous leading synthesis, Okazaki fragments and ligase joining; no image is generated here.