DNA replication

Question 1

What is semi-conservative replication?

Answer 1

• DNA copies itself before cell division so that the new cell has the full amount of DNA
• half of the strands in each new DNA molecule are from the DNA molecule
• there’s genetic continuity between generations of cells

Question 2

Semi-conservative replication (stage 1)

Answer 2

• the enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands
• the helicase unwinds to form two single strands

Question 3

Semi-conservative replication (stage 2)

Answer 3

• each original strand acts as a template for a new strand
• complementary base pairings means free floating DNA nucleotides are attracted to their exposed complementary base pairing

Question 4

Semi-conservative replication (stage 3)

Answer 4

• condensation reactions join the nucleotides of the new strands together
• the reaction is catalysed by the enzyme DNA polymerase
• hydrogen bonds form between the bases on the new and original strands

Question 5

What is the structure of a DNA strand?

Answer 5

• each end of the DNA strand is different to its structure
• one end is called the 3’ and the other is called the 5’
• in a DNA helix the strands run in opposite directions (anti-parallel)

Question 6

Who are Meselson and Stahl?

Answer 6

• they created an experiment using two isotopes of nitrogen (heavy and light) that showed DNA replication was semi-conservative not conservative
• their experiment lead other scientists carry out experiments showing semiconservative replication is the universal method for DNA replication in all living things

Question 7

semi-conservative replication experiment (stage 1)

Answer 7

• two samples of bacteria were grown
• one in a nutrient broth containing light nitrogen and one in heavy nitrogen
• as the bacteria reproduces it takes in the nitrogen help make new nucleotides for its DNA
• the nitrogen gradually becomes part of the bacterias DNA

Question 8

Semi-conservative replication experiment (stage 2)

Answer 8

• a sample of DNA was taken from each batch of bacteria and spun in a centrifuge
• the DNA from the heavy nitrogen settled lower down the centrifuge tube than the light nitrogen because its heavier

Question 9

Semi-conservative replication experiment (stage 3)

Answer 9

• the bacteria containing heavy nitrogen is removed and put into a broth of light nitrogen
• the bacteria was left for one round of DNA replication
• a sample of DNA was taken and spun in a centrifuge

Question 10

Semi-conservative replication experiment(stage 4)

Answer 10

-in conservative replication the heavy DNA would still be together and settle at the bottom with the new light nitrogen at the top

Question 11

Semi-conservative replication experiment (stage 5)

Answer 11

• the experiment showed that the DNA settled in the middle meaning the DNA molecule contained a mixture of heavy and light nitrogen
• this means the bacterial DNA had replicated semi-conservatively in the light nitrogen

Question 12

Semi-conservative replication (stage 4)

Answer 12

Each new DNA molecule contains one new strand and one original strand

Question 13

who first came up with the theory of semi-conservative DNA replication?

Answer 13

Watson and crick

Question 14

what is an isotope?

Answer 14

two different forms of the same element

Question 15

what does 3’ mean?

Answer 15

three prime

Question 16

How does DNA polymerase work?

Answer 16

• the active site of DNA polymerase is only complementary to 3’ end of the DNA strand so this is where nucleotides are added
• a new strand is made in a 5’ to 3’ direction
• DNA polymerase moves down the template strand in a 3’ to 5’ direction since the strands are anti-parallel
• DNA polymerase working on two DNA template strands moves in the opposite directions