MB - DNA Replication II

Question 1

What is involved in Supercoil removal?

Answer 1

Specific enzymes that introduce or remove supercoils

Question 2

What are the 2 types of topoisomerases?

Answer 2

Type I cleave backbone of one strand, allowing duplex rotation and loss of -ve supercoils

Type II cleave backbone of both strands, using ATP and introduce a -ve supercoil

Question 3

What is the topoisomerase type I mechanism? (3)

Answer 3

1) Phosphodiester bond is transferred to Tyr residue on
enzyme, breaking one DNA strand

2) The unbroken strand is passed through the gap

3) The phosphodiester bond is transferred back to DNA, reforming the backbone on the other side

(Lk increase by 1)

Question 4

What is the topoisomerase type II mechanism? (3)

Answer 4

• Use a “two gate” mechanism for carrying out topological
  changes in DNA
• The enzyme requires ATP hydrolysis for its reaction
• ATP hydrolysis is used for for conformational changes of the enzyme, and is not directly involved in DNA breakage or resealing

(Lk decreases by 2)

Question 5

What runs faster in agarose gel? (Supercoiled or Linear DNA)

Answer 5

Supercoiled DNA

Question 6

What are Cantanenes?

Answer 6

When circular DNA molecules are replicated, the two daughter rings are interlocked and form a catanene

Question 7

What topoisomerase is involved in decantanation?

Answer 7

Topoisomerase IV

Question 8

How are Okazaki fragments joined?

Answer 8

By using DNA Ligase

Question 9

Can DNA Ligase mutants join lagging strand fragments?

Answer 9

No

Question 10

Are Okazaki fragments completely DNA

Answer 10

No, Using DNase tiny fragments are left over (RNA primer)

Question 11

What solves the problem of circular DNA molecules during DNA replication?

Answer 11

Bidirectional Synthesis