DNA Replication

Question 1

Why Is DNA Replicated

Answer 1

• DNA copies itself before cell division so that each new cell has the full amount of DNA
• the method is semi-conservative replication
• which is half strands in each new DNA is from original DNA
• which means genetic continuity between generations of cells

Question 2

How Is DNA Replicated

Answer 2

• enzyme DNA helicase breaks the hydrogen bonds between bases on the 2 polynucleotide DNA strands which makes the helix unwind to form 2 single strands
• each original strand acts as a template for a new strand which is a complementary base pairing
• condensation reactions join the nucleotides of new strand together catalysed by the enzyme of DNA polymerase which means hydrogen bonds form between the bases on original and new strand
• each new DNA ,molecule contains one strand from the original DNA molecule and one new strand

Question 3

Complementary Base Pairing

Answer 3

• free-floating DNA nucleotides attracted to their complementary exposed bases on each original strand
• A-T
• C-G

Question 4

Action Of DNA Polymerase

Answer 4

• each end of DNA strand is dif in its structure
• three prime 3’
• five prime 5’
• during DNA replication active site of DNA polymerase only complementary to the 3’ end of the newly forming DNA strand
• so enzyme can only add nucleotides to the new strand at the end 3’
• it’s made in 5’ to 3’ direction and DNA polymerase moves down template strand in 3’ to 5’
• strands in DNA are antiparallel si DNA polymerase working on one of the template strands move in the opposite direction to DNA polymerase working on the other template strand

Question 5

Semi-Conservative replication

Answer 5

• meselon and stahls experiment which uses 2 isotopes of nitrogen - heavy (15N) + light (14N)
  1. 2 bact samples were grown for many generations. as bacteria reproduced it took up nitrogen from broth to help make nucleotides for new DNA so the nitrogen gradually became part of bacteria’s DNA
  2. a sample of DNA was taken from each batch of bacteria and spun in a centrifuge. DNA from heavy settled lower down the tube then DNA from light
  3. bacteria grown in heavy is taken out and put into light. this is then left for one round of DNA replication. another DNA sample is taken out and spun in the centrifuge
  4. if replication was conservative the original heavy DNA which would still be together, it would settle at the bottom and new light DNA would settle at the top
  5. if semiconservative the new bacterial DNA would contain 1 strand of old DNA containing heavy and one new strand containing light. the DNA would settle out between heavy and light.
  6. turned out DNA settled out in the middle showing DNA molecules have as mixture so it replicated in semi-conservatively