DNA replication

• The semiconservative model of DNA replication

  Keep Half of original DNA to help replicate
• 1. DNA gyrase

  relieves tension in the double helix
• 2. helicase 


  hydrolyses hydrogen bonds between nitrogenous base pairs producing 2 single stranded templates
• 3. single stranded binding proteins

  prevent re-annealing of the single stranded template (sticking back together)
• 4. primase

  adds RNA nucleotides to create a primer so DNA pol III can combine RNA primer
• 5. DNA polymerase 3

  will read template strand nitrogenous bases in the 3-5 prime direction end

  recruit the complimentary DNA nucleotides according to nitrogenous bases pairing in the 5-3 prime direction end
• leading strand 


  built continuously
• lagging strand 


  build discontinuously in fragments called okozaki fragments
• 6. DNA polymerase 1

  enzyme that will remove the RNA primer when it is no longer required and replaces it with DNA nucleotides
• 7. DNA ligase

  joins together Okazaki fragments on DNA backbone by facilitating the formation of phosphodiester bond
• chargaff

  provided that DNA contained nitrogenous bases pairs
• chargaffs experiment

  1. DNA- uniform in composition and structure. was bigger than a tetra nucleotide but was thought to just be a polymerisation of one. they didn’t think DNA could hold genetics
  2. role of holding genetics was given to protein but was a dogma even though limited data. William Astbury reported a x-ray crystallographic photograph which concluded the order of nucleotides must be irregular
  3. chargaff made a technique to analyse small amounts of DNA he sued different substances to prove that A and T had similar concentration and so did G and C this concluded they worked in pairs
• Hershey and chase

  tried to prove if it was DNA of Protein that carried the genetic code
• hershey and chase experiment 1

  DNA contained phosphorus but no sulfur

  • they infected bacteria with phosphurus/DNA which had been radioactively marked. once injected they noticed that it still glowed using autoradiography proving that DNA holds genetic material
• Hershey and chase experiment 2

  proteins contained sulfur but no phosphorus

  • they infected bacteria with sulfur/protein which had been radioactively marked. the protein was not injected into the bacteria as it didn’t glow proving that the protein did not hold genetic material