Origins of life

• Structure of lecture
  • Review of DNA structure
  • Details of DNA replication
  • Meselson-Stahl experiment
  • PCR (polymerase chain reaction)

Review of DNA structure

• DNA replication
  • Using a template strand, a new strand is synthesized by DNA polymerase
  • New strand grows in the 5' -> 3' direction

• Some Details of DNA synthesis
  • template and new strands
  • DNA polymerase
    • moves 3' -> 5' on template strand
  • replication forks
  • Okazaki fragments

• More details on lagging strand
  • RNA primers (starters)
  • DNA ligase (glue)

• Summary
  • New strand grows 5' -> 3'
  • polymerase moves 3' -> 5' on template
  • leading vs lagging strand
  • replication forks
  • RNA primers

What is the mode of DNA replication?

• The Meselson-Stahl experiment
  • Showed that DNA is replicated semi-conservatively

• Key to Meselson-Stahl experiment
  • DNA containing Òheavy nitrogenÓ (15N) can be distinquished form DNA containing Òlight nitrogen (14N) by CsCl density-gradient centrifugation.

• Applications of DNA replication -- PCR
  • Polymerase chain reaction
  • Allows one to obtain very large amounts of DNA in a specific region, even starting with a single DNA molecule

• How many cycles?
  • After 10 cycles, 2^ 10 = 1024
  • 20 cycles, 2^ 20 = 1,048,576
  • 30 Cycles , 2^ 30 = 1,073,741,824