3.1.5.2 DNA Replication

Question 1

When does the synthesis of DNA occur?

Answer 1

In mitosis during interphase

Question 2

Why do you have to replicate DNA?

Answer 2

So that each daughter cell has the normal amount of DNA

Question 3

What are the steps of the semi conservative method?

Answer 3

1. DNA helicase (an enzyme) unwinds the double helix by breaking the hydrogen bonds and splitting the helix into two strands
2. Free nucleotides which have been activated bind to the complimentary bases on the seperate strands- parental strand acts as a template for the synthesis of a new strand
3. DNA polymerase makes phosphodiester bonds and hydrogen bonds are naturally formed between the bases
4. Two identical polynucleotide chains are formed
5. This is called the semi conservative method because half the DNA is from the original DNA molecule

Question 4

What are the steps of the Meselson and Stahl experiment?

Answer 4

1. Grew E. coli in a nutrient broth containing 15N
2. E.coli took it up and used it to build their DNA
3. After many generations they had pure strands of 15N bacteria and 14N bacteria
4. They measured the density of the DNA using density gradient centrifugation (seperating molecules based on density by spinning it, allows for small changes in density)

Question 5

What were the results of the Meselson and Stahl experiment?

Answer 5

G0: 100% heavy
G1: 100% mixed, single band
G2: 50% mixed, 50% light, two strands
G3: 25% mixed, 75% light        ^
G4: 12.5% mixed, 87.5% light    ^

Question 6

How did the conservative and semi-conservative method differ?

Answer 6

The conservative method predicted that there would always be one band of heavy and one of light, without any mixing of densities