DNA replication (semi conservative)

    Cards (11)

    • Define semi-conservative
      One DNA strand comes from original DNA & other half is a newly synthesised strand
    • Steps of DNA replication (steps 1&2)
      1. DNA helicase unwinds the double helix by breaking hydrogen bonds between the complementary bases in the polynucleotide strand
      2. New DNA nucleotides are attracted to exposed bases on template strands & attach to the correct base through complementary base pairing
    • Steps of DNA replication (steps 3 & 4)
      3. Condensation reactions join the nucleotides together catalysed by the enzyme DNA polymerase. Hydrogen bonds form
      4. Each new DNA molecule contains one strand from the original DNA molecule and one new strand
    • What is different about the ends of DNA? What are they called?
      • Each end of DNA is slightly different in its structure
      • one end is called the 3‘ (prime) end and the other 5’ (prime). They are antiparallel
    • Why is it helpful to have antiparallel strands?
      • The active site of DNA polymerase is only complimentary to the 3' end, so it can only add nucleotides to that end.
      • This means the new strand is made in a 5' to 3' direction and DNA polymerase moves into a 3' to 5' direction.
    • Who provided evidence for semi-conservative replication?
      Meselson and Stahl
    • What would have the results of the experiment by Meselson and stahl shown IF it was conservative replication?
      It would have shown that the original DNA strands would have stayed together.
      The new DNA molecule would contain two new strands
    • Describe the experiment that Meselson and Stahl carried out? (step 1&2)
      1. Two samples of bacteria DNA is grown in two isotopes, one in light nitrogen (14N) & one is grown in heavy nitrogen (15N)
      2. A sample of DNA was taken from each and was spun in a centrifuge. DNA from the heavy nitrogen sunk and DNA from the light nitrogen settled higher up.
    • Describe the experiment that Meselson and Stahl carried out? (Step 3&4)
      3. The bacterial DNA grown in light and heavy nitrogen is grown together for one generation. They took 14N and 15N sample and mixed them again and spun them in a centrifuge
      4. The DNA settled out in the middle showing the bacteria contained a mixture of heavy and light nitrogen. The DNA had semi conservatively replicated in the light nitrogen
    • Describe how a phosphodiester bond forms between two nucleotides within a DNA molecule.
      1. Condensation reaction occurs
      2. Between phosphate and deoxyribose
      3. Catalysed by DNA polymerase
    • Use your knowledge of semi-conservative replication of DNA to suggest:
      1. the role of the single-stranded DNA fragments
      2. Role of DNA nucleotides
      • Acts as a template and determines order of bases and nucleotides
      • DNA nucleotides form complementary base pairs
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