DNA Replication

1. Location

DNA replication takes place in the nucleus of a cell, as well as in the organelles mitochondria and chloroplasts.

2. Most important enzyme

DNA polymerase is the key enzyme involved in DNA replication.

3. DNA replication in prokaryotes

• Step 1: Helicase, an enzyme, breaks the hydrogen bonds between the two strands of DNA, creating a Y-shaped replication fork.

• Step 2: DNA polymerase uses the base pairing rules (A-T, G-C) to synthesize a new strand.

• Role of RNA polymerase: RNA polymerase synthesizes a primer RNA sequence, providing a 3′-OH end for DNA polymerase to initiate new strand synthesis.

• Direction of synthesis: DNA polymerase synthesizes the new strand in the 5′ to 3′ direction (new nucleotides are added to the 3′-OH end).

• Direction of synthesis of the two new strands:

• One strand is synthesized continuously (leading strand, 5′ -> 3′).

• The other strand is synthesized discontinuously (lagging strand) in short fragments called Okazaki fragments. Okazaki fragments are joined together by the enzyme ligase, creating a continuous strand with the direction 3′ -> 5′.

4. Semiconservative principle

Each parental DNA molecule produces two identical daughter DNA molecules. In each daughter molecule, one strand is inherited from the parent molecule, and the other strand is newly synthesized.

5. Differences in eukaryotes

• Due to their larger genomes, eukaryotes have multiple origins of replication, while prokaryotes only have one.

• The enzyme system in eukaryotes is more complex than in prokaryotes.