1: BIOLOGICAL MOLECULES - DNA REPLICATION

Question 1

Why does DNA replicate?

Answer 1

DNA copies itself before cell division so that each new cell has the full amount of DNA

Question 2

What is the name of the method of DNA replication?

Answer 2

semi-conservative replication

Question 3

What does semi-conservative replication mean?

Answer 3

1/2 the strands in each new DNA molecule are from the original DNA molecule

Question 4

What is the benefit of semi-conservative replication?

Answer 4

genetic continuity between generations of cells

Question 5

What is the process of DNA (semi-conservative) replication?

Answer 5

• DNA helicase breaks H bonds between bases on the 2 polynucleotide strands which causes the helix to unwind to form 2 single strands
• each original single strand acts as a template for a new strand. complementary base pairing means that free floating nucleotides are attracted to their complementary exposed bases
• condensation reactions join the nucleotides of the new strands together catalysed by DNA polymerase. H bonds form between bases on the original and new strands. Each new DNA molecule consists of 1 strand from the original DNA molecule and 1 new strand

Question 6

What are the 2 ends of a DNA strand called?

Answer 6

• 3’ (3 prime) end
• 5’ (5 prime) end

Question 7

In which direction is a new strand made?

Answer 7

5’ to 3’ direction

Question 8

Why are new strands made in the 5’ to 3’ direction?

Answer 8

the active site of DNA polymerase is only complementary to the 3’ end

Question 9

In what direction does DNA polymerase move down the template strand?

Answer 9

3’ to 5’ direction

Question 10

Why does DNA polymerase move in opposite directions on each template strand?

Answer 10

the strands are antiparallel

Question 11

What is the name of the small strand of DNA/RNA that marks the starting point of a new strand of DNA?

Answer 11

primer

Question 12

How does polymerase bind to the template strand?

Answer 12

via the primer

Question 13

Who proposed the theory of semi-conservative replication?

Answer 13

James Watson and Francis Crick

Question 14

Who confirmed that DNA replicated by semi-conservative replication?

Answer 14

Meselson and Stahl

Question 15

What was Meselson and Stahl’s experiment?

Answer 15

• 2 samples of bacteria were grown for many generations. 1 in a nutrient broth containing light nitrogen (N14) and 1 in a broth containing heavy nitrogen (N15). as bacteria reproduced, they took up N from the broth to make nucleotides for new DNA (so the N gradually became part of the bacteria’s DNA)
• a sample of DNA was taken from each batch of bacteria and spun in a centrifuge. the DNA from the heavy N bacteria settled lower down that the DNA from the light N bacteria because it was heavier
• bacteria grown in the heavy N broth were taken out and put in a broth containing only light N. the bacteria were left for 1 round of DNA replication and then a DNA sample was taken and spun in a centrifuge

Question 16

What would the results of Meselson and Stahl’s experiment have shown if DNA replication was conservative?

Answer 16

• the original heavy DNA would’ve stayed together and would’ve settled at the bottom
• the new light DNA would’ve settled at the top

Question 17

What did the results of Meselson and Stahl’s experiment actually show, proving that DNA replication is semi-conservative?

Answer 17

• DNA settled in the middle
• this showed that each DNA molecule contained 1 strand of the original heavy N DNA and 1 strand of the new light N DNA