SPEC POINT 2.11

Question 1

What type of replication does DNA undergo, and what does this mean?

Answer 1

Semi-conservative, meaning that the strands are separated and both are used as a template

Question 2

How does the process of DNA replication occur?

Answer 2

1. Enzyme helicase breaks hydrogen bonds between complementary bases.
2. DNA strands unwind, and free floating nucleotides then align themselves with the correct base pairings.
3. The enzyme DNA polymerase links the adjacent nucleotides with phosphodiester bonds in condensation reactions to form new complementary strands.
4. DNA ligase used on lagging strand.

Question 3

How do free nucleotides in the nucleus create a new strand of DNA?

Answer 3

They are nucleoside triphosphates, so when DNA polymerase comes to join them up, it cleaves off the 2 extra phosphates and uses the energy released to create phosphodiester bonds of the new sugar-phosphate backbone

Question 4

How did Meselson and Stahl’s experiment prove DNA replication was semi conservative?

Answer 4

Grew bacteria in cultures of a ‘heavy’ isotope of nitrogen (N15), and then transferred them to a medium with a lighter isotope of nitrogen (N14). DNA was centrifuged after each replication.
1st replication - single band of ‘medium’ density produced (could be dispersive or semi conservative)
2nd replication - formed 2 bands, one half heavier than other (one uses N15 as template and other N14)

Question 5

How do we know that DNA replication is semi-conservative?

Answer 5

Meselson and Stahl experiment

Used heavy and light strands of DNA to distinguish method of DNA replication

Question 6

What were the different modes of DNA replication suggested before Meselson and Stahl’s experiment?

Answer 6

Conservative - strands separate but new strands join together to form entirely new molecule and ‘old’ strands stay together
Semi-conservative - strands separate and are joined by a new strand
Dispersive - new DNA mixture of both old and new DNA

Question 7

Which direction can DNA polymerase build the new strand in?

Answer 7

ONLY in the 5’ to 3’ direction

Question 8

What strand is the leading strand, and what does this mean?

Answer 8

The original DNA strand starting at 3’, meaning it can be synthesised by DNA polymerase continuously, moving towards the replication fork

Question 9

What strand is the lagging strand, and what does this mean?

Answer 9

The other DNA strand where DNA polymerase moves AWAY from the replication fork from the 5’ end. This means DNA polymerase can only synthesise the strand in short segments called Okazaki fragments, and DNA ligase is required to join the lagging strand segments

Question 10

What is the role of DNA ligase and how does it work?

Answer 10

To connect lagging strand segments

It catalyses the formation of phosphodiester bonds to form a continuous strand