Biological Molecules: DNA replication

Question 1

What are the three models for DNA replication?

Answer 1

Semi conservative
Conservative
Dispersive

Question 2

Explain the idea of semi-conservative replication:

Answer 2

• During DNA replication the two strands come apart
• Two new complementary strands are formed

Question 3

Explain the idea of conservative replication:

Answer 3

• During DNA replication the original DNA molecules remains intact
• Another completely new DNA molecule is formed

Question 4

Explain the idea of dispersive DNA replication:

Answer 4

• During DNA replication the original molecule breaks down into chunks
• New chunks are formed and form a double helix with the original ones

Question 5

Explain the process of DNA replication:

Answer 5

1) Enzyme DNA helicase breaks hydrogen bonds between bases on the two polynucleotide strands- makes helix unwind to form two single strands.
2) Each single strand acts as a template strand. DNA nucleotides are attracted to exposed complementary bases
3) Condensation reaction joins nucleotides of new strand together catalysed by DNA polymerase. Hydrogen bonds form between bases on the new and original strands.
4) The DNA molecule formed has one new strand and one old strand.

Question 6

Name two enzymes involved in the semi-conservative replication of DNA:

Answer 6

DNA Polymerase
DNA Helicase

Question 7

What is the role of DNA polymerase?

Answer 7

Catalyses the condensation reaction that joins nucleotides together.

Question 8

What is the role of DNA helicase?

Answer 8

Breaks hydrogen bonds between bases on the two polynucleotide strands.

Question 9

Why do the DNA polymerases working on each of the template strands in DNA move in opposite directions?

Answer 9

The strands in the double helix are antiparallel.

Question 10

What did Meselson and Stahl’s experiment show?

Answer 10

That DNA is replicated using the semi=conservative method.

Question 11

What two chemical isotopes were used in Meselson and Stahl’s experiment?

Answer 11

(Heavy) Nitrogen 15
(Light) Nitrogen 14

Question 12

Nitrogen 15 is _____.

Answer 12

Heavy

Question 13

Nitrogen 14 is _____.

Answer 13

Light

Question 14

Explain Meselson and Stahl’s experiment:

Answer 14

1) Two bacteria samples were grown- one in a N15 broth, and one in a N14 broth.
2) As the bacteria reproduced, they took up nitrogen from the broth to produce nucleotides
3) Sample of DNA was taken from each bacteria and spun in a centrifuge
4) DNA from N15 sunk, DNA from N14 settled higher
5) N15 bacteria were put in a N14 broth and spun again.
6) New DNA sample contains one N15 strand and one N14 strand

Question 15

In Meselson and Stahl’s experiment, what would have happened to the DNA is replication was conservative?

Answer 15

Original heavy DNA would settle at the bottom
New light DNA would settle at the top