DNA Replication

Overview of DNA

• DNA: two-stranded polymer of nucleotides
• Backbone: identical sugar and phosphate groups
• Nitrogenous bases: Pairing A with T, C with G
• Coiling: Around histones, then supercoiled to form compact chromosomes
• Genetic material: Present in 23 pairs of chromosomes in every cell

Cell Division

• Constant cell division: Old cells die, new cells generated
• Female egg cells: Only cells present from birth
• DNA replication: Ensures each new cell gets a full copy of genetic material

Process of DNA Replication

• Requires about a dozen enzymes

Enzymes Involved

1. Helicase
   • Unwinds the double helix
   • Disrupts hydrogen bonds between bases
   • Creates replication fork
2. Topoisomerase
   • Breaks, untwists, and reconnects DNA ahead of the replication fork
3. Primase
   • Anneals RNA primer at specific locations to start replication
   • Primer: 5-10 nucleotides long
4. DNA Polymerase III
   • Binds to primer
   • Generates complementary strand by adding nucleotides
   • Adds nucleotides to 3' end
   • Different process for each strand (leading vs. lagging)
5. DNA Polymerase I
   • Replaces RNA nucleotides from primers with DNA nucleotides
6. Ligase
   • Joins the last nucleotide of one fragment to the first nucleotide of another

Replication of Strands

• Leading Strand
  • Continuous synthesis
  • Requires only initial primer
• Lagging Strand
  • Synthesized in Okazaki fragments (100-200 nucleotides long)
  • Each fragment requires its own primer

Summary of Steps

1. Helicase: Unwinds DNA
2. Primase: Anneals primers
3. DNA Polymerase III: Copies each strand
   • One primer needed for leading strand
   • Multiple primers for lagging strand (one per Okazaki fragment)
4. DNA Polymerase I: Replaces primers with DNA nucleotides
5. Ligase: Connects fragments

Speed and Accuracy

• About 50 base pairs per second
• DNA Polymerase is highly accurate
  • Proofreading: Corrects mistakes during replication
  • Error rate: ~1 in 10 billion base pairs
• Mismatch repair enzymes: Swap out incorrect bases

Outcome

• Two identical copies of the original DNA molecule
• Each new cell receives a copy during cell division
• Continuous replication process in living cells