DNA Replication and Key Enzymes

DNA Replication Overview

• DNA replication is the process of creating two identical DNA molecules from a single original molecule.
• It's crucial for cell division and passing genetic information to daughter cells.

Key Enzymes Involved

• DNA polymerase: Adds nucleotides to the growing DNA strand, proofreads for errors.
• Helicase: Unwinds the DNA double helix, creating the replication fork.
• Primase: Synthesizes RNA primers for DNA polymerase to start replication.
• Ligase: Joins Okazaki fragments on the lagging strand.
• Single-strand binding proteins (SSBs): Prevent DNA strands from re-annealing.
• Topoisomerase: Relieves torsional strain ahead of the replication fork.

Mechanisms of Replication

• Semiconservative Replication: Each new DNA molecule has one original and one new strand. Maintains genetic fidelity.
• Leading Strand: Synthesized continuously in the 5' to 3' direction.
• Lagging Strand: Synthesized discontinuously in Okazaki fragments (5' to 3' direction, opposite of the replication fork). Joined by DNA ligase.

Replication Initiation

• Replication starts at specific sites called origins of replication.
• Eukaryotic chromosomes have multiple origins to speed up replication of large genomes.
• Initiation involves unwinding the DNA double helix and binding replication proteins.

Replication Elongation

• DNA polymerase adds nucleotides to the 3' end of the growing strand.
• The leading strand replicates continuously, the lagging strand discontinuously.
• The process continues as the replication fork moves along the DNA molecule.

Replication Termination

• Replication ends at specific termination sequences.
• These prevent replication machinery from colliding or causing errors.
• Termination ensures complete replication and accurate chromosome separation.

Errors and Repair

• DNA polymerase proofreads, minimizing errors.
• Occasional errors can still occur.
• DNA repair mechanisms correct mistakes (mismatched base pairs, damage).

Importance of Cell Cycle Regulation

• DNA replication occurs during the S phase of the cell cycle.
• Precise coordination with other cell cycle events is key for accurate replication and division.
• Errors in cell cycle control can cause DNA damage, mutations, and diseases (such as cancer).