Topic 1 - Nucleic Acids

Question 1

What is a nucleotide formed from?

Answer 1

Pentose sugar, nitrogen-containing organic base and a phosphate group.

Question 2

What bond forms between two nucleotides?

Answer 2

Phosphodiester bonds

Question 3

What reaction joins two nucleotides?

Answer 3

Condensation reaction between phosphate group of one nucleotide and the sugar of another.

Question 4

What does a chain of sugars and phosphates form?

Answer 4

A sugar-phosphate backbone

Question 5

What is the pentose sugar in DNA?

Answer 5

Deoxyribose

Question 6

What are the four possible bases of DNA?

Answer 6

1) Adenine
2) Cytosine
3) Thymine
4) Guanine

Question 7

What is the pentose sugar in RNA?

Answer 7

Ribose

Question 8

What base replaces thymine in RNA?

Answer 8

Uracil

Question 9

What bonds join two DNA polynucleotide strands?

Answer 9

Hydrogen bonds between bases (2 between A & T and 3 between G & C)

Question 10

What two things form a double helix?

Answer 10

Two antiparallel polynucleotide strands twist to form a double helix.

Question 11

When was DNA first observed?

Answer 11

DNA was first observed in the 1800s and by 1953 experiments has determined the double helix structure (Watson and Crick)

Question 12

Describe the process of DNA replication

Answer 12

1) The enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide strands. This makes the helix unwind, to form two strands.

2) Each original single strand acts as a template. Complementary base pairing means free nucleotides attracted to exposed bases.

3) Condensation reactions join the nucleotides of the new strands together, catalysed by DNA polymerase. Hydrogen bonds form between bases on original and new strands.

4) Each new DNA molecule contains one strand from the original DNA molecule and one new strand.

Question 13

What enzyme breaks the hydrogen bonds between bases on DNA strands?

Answer 13

DNA helicase

Question 14

By what process does DNA replicate?

Answer 14

Semi-conservative replication

Question 15

What enzyme catalyses the condensation reaction between new polynucleotide strands?

Answer 15

DNA Polymerase

Question 16

Who validated the theory of semi-conservative replication?

Answer 16

Meselson and Stahl

Question 17

What did Mesolson and Stahl use to prove the theory of semi-conservative replication?

Answer 17

Used heavy nitrogen 15 and light nitrogen 14.

Question 18

Describe the process that Meselson and Stahl used to prove semi-conservative replication

Answer 18

1) Two samples of bacteria were grown, one in light N and one in heavy N, as bacteria reproduce they take up nitrogen, gradually becoming part of the bacteria’s DNA.

2) A sample of DNA is taken from each bacteria and spun in a centrifuge. DNA containing heavy N settle lower down the tube because it is heavier.

3) Bacteria from heavy broth is taken out and put in the light broth contains N-14. The bacteria were left for one round of DNA replication and then another DNA sample was taken out and centrifuged.

4) If replication was conservative, original N-14 DNA would settle at the top and N-15 DNA would at the bottom.

5) If replication was semi conservative, new bacterial DNA would contain N-15 and N-14. So the DNA would settle out between N-14 and N-15.

6) DNA settled in the middle, showing DNA molecules contained a mix of N-14 and N-15, bacterial DNA had replicated semi-conservatively in the N-14.