[2.2] DNA replication

Question 1

what are the 7 stages of semi-conservative replication?

Answer 1

1. the enzyme DNA helicase breaks the weak hydrogen bonds joining the base pairs
2. the double helix separates into its two strands and unwinds
3. each exposed polynucleotide strand acts as a template which attracts free-floating nucleotides
4. energy from ATP hydrolysis activates the free nucleotides
5. the free nucleotides form complementary base pairs with the exposed bases
6. the new nucleotides are joined into a single strand by the enzyme DNA polymerase, which forms phosphodiester bonds
7. each new DNA molecule now contains one ‘old’ strand from the original DNA molecule, and one new strand

Question 2

what is the role of the enzyme DNA helicase?

Answer 2

it breaks the weak hydrogen bonds joining the base pairs

Question 3

where is energy provided from and what does it do?

Answer 3

energy from ATP hydrolysis activates the free nucleotides

Question 4

what is the role of the enzyme DNA polymerase?

Answer 4

it joins the new nucleotides into a single strand, which forms phosphodiester bonds

Question 5

why is this called a semi-conservative model?

Answer 5

since each new DNA molecule now contains one ‘old’ strand from the original DNA molecule, and one new strand

Question 6

in which direction is new DNA always being made?

Answer 6

from the 5’ to the 3’ direction (‘ = prime)

Question 7

how is replication different in the leading and lagging strands?

Answer 7

• in the leading strand, the replication can be continuous
• in the lagging strand, it has to be done in fragments
  > DNA polymerase can only bind in one direction

Question 8

why are new strands formed in opposite directions?

Answer 8

because DNA strands are antiparallel

Question 9

why does enzyme action mean DNA polymerase can only go from 5’ to 3’? [4]

Answer 9

1. DNA polymerase has an active site with a specific shape
2. the active site of DNA polymerase is only complementary with the 3’ end of the strand
3. 5’ end is phosphate group, 3’ is hydroxyl group so the ends are different shapes
4. since nucleotides are added at the 3’ end, the new strand extends from 5’ to 3’

Question 10

description of conservative replication for 2 replications

Answer 10

• where first replication produces entirely new strand of DNA
• by second replication, 1/4 has entire parental DNA and none of the others do

(where ‘a’ is parental DNA and ‘b’ is new DNA)
- first replication: aa bb
- second replication: aa bb bb bb

Question 11

what is a description of semi-conservative replication? (proven)

Answer 11

• in first replication, 1 strand of parental DNA is present in both copies
• in second replication, 1 strand of parental DNA is present in 2 of the 4 copies
  > (where ‘a’ is parental DNA, ‘b’ is new DNA)

first replication: ab ab
second replication: ab bb ab bb

Question 12

describe dispersive replication for 2 replications

Answer 12

• first replication: both strands produced have a jumble of parental and new DNA
• second replication: all 4 strands also have a jumble of parental and new DNA

Question 13

what are the 4 stages of the meselson stahl experiment?

Answer 13

1. isolate the bacteria with DNA containing heavier nitrogen (¹⁵N)
2. centrifuge it
   > DNA settles at bottom of centrifuge tube
3. bacteria with DNA containing only heavier nitrogen allowed to replicate in broth containing only light nitrogen (¹⁴N)
4. semi-conservative replication was proven to be correct

Question 14

what would have been observed if conservative replication was correct?

Answer 14

• new DNA containing only lighter nitrogen settles at top
• original DNA containing only heavier nitrogen settles at bottom

Question 15

what was observed which proved that semi-conservative replication was correct?

Answer 15

DNA, now containing mixture of lighter and heavier DNA, settles in middle