3.1.5.2 DNA replication

Question 1

What is the function of semi-conservative replication?

Answer 1

Semi-conservative replication occurs to make an exact copy of DNA, a copy of DNA is required before cell division can occur.

Question 2

Explain how semi-conservative replication occurs

Answer 2

• Before DNA replication occurs the two strands need to separated, so an enzyme called DNA helicase is added, which overcomes the hydrogen bonds between the complimentary bases, causing the two strands to unwind and separate.
• These strands are used as a template to ensure that DNA is replicated accurately.
• Free nucleotides (nucleotides which have been synthesised by the cell) line up opposite their complimentary bases
• DNA polymerase catalyses the formation of phospodiester bonds between C5 of the phosphate group and C3 of the deoxyribose sugars of an adjacent nucleotide.
• Hydrogen bonds between the complimentary bases reform, causing the strands to form

Question 3

Explain how Meselson and Stahl provided evidence for the theory of semi-conservative replication

Answer 3

• All DNA bases contain nitrogen. There are two isotopes of nitrogen (N!4 and N15)
  Meselson and Stahl grew bacteria in N14, therefore their nucleotides contain N14.
• The bacteria were then transferred to a medium of N15.
• When DNA replication occurred, the free nucleotides were made from N15. - Each new molecule of DNA formed therefore contained one strand of N14, and one strand of N15.
• After the second division, 75% of the DNA was made of N15, therefore proving the theory of semi conservative replication.
• They used centrifugation to determine the proportions of N14 and N15, as N15 is heavier than N14.

Question 4

During replication, the two DNA strands separate and each
acts as a template for the production of a new strand. As
new DNA strands are produced, nucleotides can
only be added in the 5’ to 3’ direction. Use your knowledge
of enzyme action and DNA replication to explain why new
nucleotides can only be added in a 5’ to 3’ direction.

Answer 4

DNA polymerase;
- is specific;
- Only complementary with / binds to 5’ end (of strand);
Reject hydrogen bonds / base pairing
- Shapes of 5’ end and 3’ end are different shapes