DNA Replication 1B

Question 1

Which enzyme breaks the hydrogen bonds between bases on the two polynucleotide stands

Answer 1

DNA helicase

Question 2

What happens to the DNA strands when DNA helicase has broken the hydrogen bonds

Answer 2

This causes the DNA to unwind to form two single strands

Question 3

Each original strand acts as a ——– for the new strand

Answer 3

template

Question 4

What are free nucleotides attracted to

Answer 4

complementary bases

Question 5

What type of reaction joins the nucleotides of the new strands together

Answer 5

condensation reaction

Question 6

What enzyme catalyses the condensation reaction which joins the nucleotides of the new strands together

Answer 6

DNA polymerase

Question 7

Why is this model called semi-conservative

Answer 7

because each new DNA molecules is made up of one new strand and one old strand

Question 8

Evidence for semi conservative model

Answer 8

1. Two bacteria samples grown, one in a broth containing light nitrogen and one containing heavy nitrogen as the bacteria reproduced the took the nitrogen from the broth so the nitrogen became part of the DNA.
2. Sample of DNA taken from each batch of bacteria and spun in a centrifuge DNA from the heavy nitrogen settled at the bottom - light nitrogen settled near top
3. Then bacteria grown in heavy nitrogen was taken out and put into a broth containing light nitrogen and left for one round of DNA replication and then spun again. If DNA was conservative heavy DNA would settle at bottom light at top. If replication was semi conservative - one strand containing heavy nitrogen and one strand containing light nitrogen so DNA would be settled out.

Question 9

Why does DNA polymerase work in the opposite direction

Answer 9

Antiparallel active site of polymerase only complementary to 3’ end of the newly formed DNA strand so the enzyme can only add nucleotides to the 3’ this means that the new strand is made in the 5’ to 3’ direction and polymerase moves down the template strand in the 3’ to 5’ direction because the strands are antiparallel