7.5 DNA Replication

Question 1

In semi-conservative replication, DNA replicates in this method one strand of the old molecule…

Answer 1

Is kept intact an the other strand is newly made

Question 2

(Semi-conservative 1). The DNA molecule ‘unzips’…

Answer 2

The hydrogen bonds between the complementary bases break

New nucleotides align alongside their complementary base

Question 3

(Semi-conservative 2) an enzyme called…

Answer 3

DNA polymerase joins these new nucleotides together

Question 4

(Semi conservative 3) Each new strand is…

Answer 4

Complementary to the old strand and that each of the new molecules formers is identical

Question 5

(Semi-conservative 4) DNA replication occurs during…

Answer 5

Interphase before the cell divides by mitosis

Question 6

Semi-conservative model -

Answer 6

Watson and Crick - each DNA helix has one old and one new strand

Question 7

Conservative model -

Answer 7

One helix that has 2 old strands and one helix that has 2 new strands

Question 8

Dispersive model -

Answer 8

The parental strand is the template, but the helices that result are made up of old and new strands that are dispersed (at random)

Question 9

Meselson-Stahl experiment purpose:

Answer 9

To determine which model of DNA replication was correct

Question 10

The experiment 1)

Answer 10

Grew e.coli in the presence of 15NH2Cl (it’s sole nitrogen base)

• 15N is an isotope of 14N
• anything that is made in the cell and contains nitrogen will have 15N incorporated into it

Question 11

Experiment 2)

Answer 11

After a time the bacteria were transferred to modia where the sole nitrogen source was contained 14N

Question 12

Experiment 3)

Answer 12

From this point, all newly synthesised products will contain 14N; old things will contain 15N

Question 13

Experiment 4)

Answer 13

After a time, the DNA was harvested from the bacteria and subjected to centrifugation

Question 14

Experiment 5)

Answer 14

15N denser forms a band lower down in the centrifuge tube (generation 1)

Question 15

Experiment 6)

Answer 15

After the bacteria had divided twice on the 14N medium, another DNA sample was extracted and centrifuges as before (generation 2)

Question 16

Experiment 7)

Answer 16

This was repeated again when the bacteria had divided three times on the 14N medium (generation 3)

Question 17

Results showed that DNA replicates self-conservatively:

Answer 17

• The DNA from gen.1 forms a band, Midway between the position of ‘light’ and ‘heavy’ DNA - shows the molecules have one ‘heavy’ and one ‘light’ strand
• The DNA from gen.2 shows a ‘midway’ and a ‘light’ band