1B.2 DNA replication

• when was DNA first observed?

  in the 1800s.
• why did many scientists doubt that DNA carried the genetic code?

  because it has a relatively simple chemical composition.
• who proved that DNA carries the genetic code and when?

  Watson and Crick 1953.
• what method of replication does DNA replicate by?

  semi-conservative.
• what does semi-conservative replication mean?

  each new DNA molecule consists of one strand of original parent DNA and one strand from the new molecule.
• what enzyme breaks the hydrogen bonds between bases of DNA?

  DNA helicase.
• what is the method of DNA replication?

  • DNA helicase breaks the hydrogen bonds between the bases so the double helix unwinds to form two single strands. • the single strands act as a template, free floating nucleotides are attracted to the exposed bases. • condensation reactions join the the stands, catalysed by DNA polymerase.
• whose experiment validated Watson and Cricks theory for semi-conservative replication?

  Mesleson and Stahl.
• describe Mesleson and Stahls experiment.

  • 2 samples of bacteria were left to reproduce, one in heavy nitrogen nutrient broth (15N) and one in light (14N). • when spun in a centrifuge light nitrogen settles higher than heavy.• heavy N bacteria was removed and added to the light N broth then left for one round of replication, then spun again. • if replication was conservative - the new DNA molecule would settle higher, and the original at the bottom. • if replication was conservative (what actually happened) - the new DNA molecule would settle in the middle as it contains one strand of old DNA and one of new.
• during DNA replication, which direction is the new strand made?

  5' to 3
• what is the significance of three and five prime?

  DNA polymerase is only complimentary to 3' so as nucleotides are added they go in opposite direction to the anti parallel other strand.
• What is the role of DNA polymerase in DNA replication?

  joins nucleotides to form new stands