DNA Replication

Question 1

What is semi conservative replication?

Answer 1

Half of each new double helix is a strand that has come from the double helix of the original DNA molecule.

Question 2

What does DNA helicase do? (1st step of replication)

Answer 2

Unwinds the double helix and breaks the hydrogen bonds between the bases of two DNA strands.

Question 3

What is the 2nd step of replication?

Answer 3

Each original strand acts as a template. Free DNA nucleotides attach themselves to the exposed bases through complementary base pairing.

Question 4

What does DNA polymerase do? (3rd step of replication)

Answer 4

It catalyses the condensation reactions that join the free nucleotides to the new strand.

Question 5

Who talked about the structure of DNA?

Answer 5

Watson and Crick

Question 6

What are the features of DNA and why are they important for semi conservative replication?

Answer 6

The hydrogen bonds can easily break and there will be 2 strands so both can act as a template.

Question 7

Why does each DNA strand have a directional structure?

Answer 7

Because the ends of each strand being either a sugar thats attached to the 5 prime end or a hydroxyl group attacked to the 3 prime end.

Question 8

Where is the DNA polymerase complimentary to?

Answer 8

The 3 prime end of the template strand.

Question 9

Because the strands are antiparallel, what does this mean?

Answer 9

The DNA polymerase adds nucleotides in the 3 to 5 end but the new strand is bulit 5 to 3.

Question 10

What happens when the DNA is unwound?

Answer 10

One strand is continuously built while the other is built in the other direction.

Question 11

Who proved that DNA replication was semi conservative?

Answer 11

Meselson and Stahl

Question 12

What did they use for the experiment and why?

Answer 12

Two isotopes of nitrogen: one light N14 and one heavy N15. Nucleotides contain nitrogen.

Question 13

How can DNA samples with nucleotides that contain different isotopes of nitrogen be separated?

Answer 13

The heavy DNA will sink and light DNA will settle out higher up in the solution in a centrifuge.

Question 14

What did they do to get samples of heavy and light DNA?

Answer 14

Meselson and Stahl grew two cultures of bacteria in nutrient broth that contained either light nitrogen or heavy nitrogen so that it cab incorporated into their nucleotides

Question 15

The bacteria being grown with the heavy nitrogen were put into a broth containing only light nitrogen which meant what?

Answer 15

When they replicated the parent or original DNA strand, it would contain the heavy nucleotides but to make the new DNA strands they would only be able to use light nucleotides

Question 16

What should happen if the DNA replication was conservative?

Answer 16

There should be 2 bands, 1 of the original heavy DNA and 1 new lighter band for the new strand of DNA.

Question 17

What should happen if the DNA replication was semi conservative?

Answer 17

All the DNA would contain a mixture of heavy and light DNA as there would be one strand of each in the double helix and the band would settle half way between the heavy and light samples.

Question 18

Why did this continue as a light band?

Answer 18

The DNA molecules splits two strands which would have been formed with light DNA strand as the template and 2 would be a mix as they used the original heavy DNA as a template.