1.5.5 Semi-Conservative Replication

Process of Semi-Conservative Replication

1. Unwinding the Double Helix

• DNA helicase, an enzyme, unwinds the double helix by breaking the hydrogen bonds between the complementary bases (adenine pairs with thymine, cytosine pairs with guanine).

2. Formation of the Replication Fork

• The separation of the DNA strands creates a replication fork, exposing the bases for pairing.

3. Complementary Base Pairing

• Free nucleotides in the nucleus align with their complementary bases on the exposed strands.
• Adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G) through hydrogen bonding.

4. Synthesis of New Strands

• The enzyme DNA polymerase catalyses the formation of phosphodiester bonds between adjacent nucleotides, synthesising the new strand.
• Replication occurs in the 5' to 3' direction for the new strand. The leading strand is synthesised continuously, while the lagging strand forms in short segments called Okazaki fragments, later joined by DNA ligase.

5. Completion

• Two identical DNA molecules are produced, each containing one original strand and one new strand.

Evidence for Semi-Conservative Replication

• Meselson and Stahl Experiment
  • Conducted using bacteria grown in isotopes of nitrogen (15N and 14N).
  • DNA extracted after one and two generations showed intermediate and light densities in a density gradient centrifugation, consistent with semi-conservative replication.