DNA and Cell Division

• Structure of DNA?


  2 strands of polynucleotides that run antiparallel to each other, connected by hydrogen bonds between complementary nitrogenous bases forming a double helix shape
• DNA replication is semi-conservative, meaning that of the new DNA one strand is the original parent strand that acted as a template whilst the other strand is newly synthesised DNA
• Enzymes involved in DNA replication?

  DNA helicase

  DNA polymerase
• what is the role of DNA helicase in DNA replication?

  responsible for breaking hydrogen bonds between bases to allow for DNA polymerase to attach
• What is the role of DNA polymerase in DNA replication?

  DNA polymerase catalyses the formation of phosphodiester bonds between nucleotides and does this along the strand
• Process of DNA replication?

  1. DNA helicase breaks the hydrogen bonds between bases between the 2 strands
  2. Each original parent strand acts as a template strand and Free-floating Nucleotides are attracted to exposed complementary bases (which ensures accuracy as the newly synthesised strand will be completely complimentary to the original strand)
  3. DNA polymerase attaches to the end of the strand and catalyses the formation of phosphodiester bonds between the nucleotides forming another strand
  4. DNA polymerase goes along the strand and then detaches and hydrogen bonds reform
• Components of a nucleotide?

  Deoxribose sugar (pentose)

  nitrogenous bases ( adenine, guanine, cytosine, thymine )

  phosphate group
• Differences between RNA and DNA

  RNA is single stranded

  RNA only contains an adenine base

  RNA has a ribose sugar
• DNA helicase is responsible for breaking hydrogen bonds between DNA strands for DNA replication
• What are the steps in DNA extraction/purification?

  1. Grind the sample to break down cell walls
  2. Add detergent to remove cell and nuclear membranes
  3. Add protease to separate the DNA from histones
  4. Add ethanol to precipitate the DNA
  5. Extract the DNA using a Glass rod
• Meselson and Stahl's Experiment

  1. Bacteria grown in broth with heavy (15N) nitrogen isotope

  2. DNA contains nitrogen in bases

  3. Bacteria replicate using nitrogen from broth to make new DNA nucleotides

  4. Culture of bacteria has DNA with only heavy (15N) nitrogen

  5. DNA from 15N culture extracted and spun in centrifuge

  6. DNA with heavy nitrogen settles at bottom of centrifuge tube

  7. Bacteria with 15N DNA added to broth with lighter 14N nitrogen

  8. DNA extracted and spun in centrifuge after one round of replication
• Conservative DNA replication

  Original template DNA molecules only contain heavier nitrogen, settle at bottom

  New DNA molecules only contain lighter nitrogen, settle at top
• Semi-conservative DNA replication

  DNA molecules contain mixture of heavy 15N and light 14N nitrogen, settle in middle of tube
• Meselson and Stahl confirmed bacterial DNA underwent semi-conservative replication