2.11 DNA Replication

Question 1

Why does DNA replication occur?

Answer 1

DNA must be copied before cell division so that each new daughter cell has the full amount of DNA
this means there is genetic continuity between generations of cells (the daughter cells inherit all the genes from their parent cells

Question 2

Describe the process of DNA replication:

Answer 2

• the enzyme DNA helicase breaks the hydrogen bonds between the bases on the the two polynucleotide DNA strands - the DNA double helix unwinds
• each original strand of DNA forms the template for a new strand
• due to complementary base pairing, free DNA nucleotides line up along side the template strand, and hydrogen bonds form between the complementary bases of the template strand and the free nucleotides
• condensation reactions occur between the previously free nucleotides, and so the new nucleotides are joined by phosphodiester bonds - this is catalysed by DNA polymerase
• there are now two DNA molecules - each new DNA molecule contains one strand from the original DNA and one new strand formed by free nucleotides - this is known as semi-conservative replication

Question 3

Why is DNA replication semi-conservative?

Answer 3

each new DNA molecule formed during replication contains one strand from the original DNA and one new strand formed by free nucleotides

Question 4

What research has been done to support semi-conservative DNA replication?

Answer 4

research by Meselson and Stahl

Question 5

Describe Meselson and Stahl’s experiment:

Answer 5

1. E-coli was grown in a medium containing a heavy nitrogen isotope - 15N. this would make their DNA denser as the DNA bases would contain heavy nitrogen
2. they then moved the bacteria into a medium containing a lighter nitrogen isotope - 14N. this meant that all new nucleotides in replicated DNA would be lighter, whereas the nucleotides in the original DNA would be denser
3. they allowed the bacteria to divide once, so DNA was only replicated once
4. they then extracted the DNA and centrifuged it after dissolving the DNA in a density gradient solution
5. they then let the bacteria divide twice (DNA replicate twice) and centrifuged it again
6. if the DNA was made of dense 15N nitrogen only, it would settle at the bottom of the tube. if the DNA was made of a mixture of dense 15N nitrogen and light 14N nitrogen (hybrid), it would settle in the middle of the tube. if the DNA was made of light 14N nitrogen only, it would settle at the top of the tube
7. after 1 replication: only one layer was formed in the middle of the tube, indicating all the DNA was hybrid
   after 2 replications: half the DNA settled in the middle of the tube, and the other half at the top of the tube, indicating 50% of DNA was hybrid and 50% was light

Question 6

What were the three possible ways DNA could replicate?

Answer 6

• semi-conservative (one original strand and one new strand)
• conservative (original DNA strands stay together with two new strands forming replicated DNA)
• fragmentary (nucleotides all mixed and reformed)

Question 7

How did Meselson and Stahl’s experiment prove that DNA replicated semi-conservatively?

Answer 7

• after the first replication, all the DNA contained both heavy and light isotopes of nitrogen, so the DNA could not have replicated conservatively, otherwise half the DNA would be light and half would be heavy
• after the second replication, half the DNA contained both heavy and light isotopes of nitrogen and the half the DNA only contained the light isotopes of nitrogen. so DNA could not have undergone fragmentary replication, otherwise all the DNA would remain hybrid and light DNA would not have been formed
• semi-conservative replication is the only one that makes sense, as after the first replication, all the DNA molecules would have one original strand and one new strand, hence there is only hybrid DNA. after the second replication, half the DNA molecules would still have the original heavy strand and the other half would only have strands made from new DNA, hence half the DNA being hybrid and half the DNA being light only