DNA Replication

Accuracy

• Amazing process : 6 billion base pairs in human genome.
• Equivalent to 1000 books each with 1000 pages of 12 font text.
• Error rate < 1 error per billion base pairs.

Replication

• About 50 base pairs (eukaryotic cells) per second are replicated.
• A single DNA helix is split into two strands.
• A new DNA strand is made to complete each strand of old DNA.
• This is called semi-conservative replication. New DNA helix is ½ of the old DNA helix.
• Replication begins at replication origin. Eukaryote DNA have many origins.
• Origin forms a bubble as helicase opens the helix.
• Each end of the bubble called a replication fork.
• DNA polymerases travels along strand and makes complementary strand.
• DNA polymerase moves along DNA template in 3' to 5' direction.
• The new DNA builds from 5' to 3' (the complement remember).
• Leading strand made directly.
• Lagging strand assembled in segments (Okazaki fragments) by ligase.
• Single-strand binding proteins keep the single DNA strand stable.

Repair

• Initial errors are 1 in 10,000 base pairs.
• DNA polymerases do proof reading.
• Complementary nature of DNA allows such high accuracy.
• Many repair enzymes correct, usually by excising and using complementary strand.
• Example:
  • UV can cause two adjacent thymine bases to bond covalently.
  • The two complementary adenine are unbonded and the helix has a kink.