lecture 4 - DNA structure and replication

Question 1

what are nucleic acids?

Answer 1

• deoxyribonucleic acid (DNA)

- ribonucleic acid (RNA)

Question 2

describe DNA

Answer 2

• stores information for the synthesis of specific proteins
• directs RNA synthesis
• directs protein syntheis through RNA

Question 3

what are the possible methods of DNA replication?

Answer 3

semiconservative, conservative and dispersive

Question 4

know the concepts and how the meselson-stahl experiment works

Answer 4

C14 and carbon 15

Question 5

how does bacterial DNA replication (schematic) work?

Answer 5

• replication starts at the origin

- and proceeds in BOTH DIRECTIONS until the entire chromosome has been copied.

Question 6

how do you unwind the DNA double helix without creating knots? (HINT KEY ENZYMES AND PROTEINS USED)

Answer 6

> helicase
- untwists the DNA helix to give single stranded DNA, but increases coiling ahead of the replication fork
topoisomerase
- ‘fixes’ the increase coiling in the DNA template by transiently nicking both strands and allowing the two strands to rotate around each other.
single-stranded binding protein
-stabilises the single strand template.

Question 7

how is new DNA synthesised?

Answer 7

• uses nucleotide building blocks
• ALWAYS proceeds in 5’ to 3’ direction
  + energy for polymerisation is supplied by incoming triphosphate nucleotide
• DNA polymerase catalyse DNA synthesis

Question 8

how does synthesis work on the leading strand?

Answer 8

• to start DNA synthesis the enzyme DNA primase synthesises an RNA primer
• the RNA primer is then extended by DNA polymerase III
• the RNA primers are removed by the action of the 5’ exonuclease activity of DNA polymerase I, with DNA synthesised in their place at the same time.

Question 9

describe DNA synthesis on the lagging strand

Answer 9

• the basic mechanism of DNA synthesis is the same (5’-3’) BUT:
• new RNA primers have to be made frequently to keep DNA synthesis going
• this creates lots of DNA fragments (OKAZAKI fragments)
• DNA polymerase I removes the RNA primers
• DNA ligase joins the Okazaki fragments to create a continuous DNA strand

Question 10

what is the mechanism involved in DNA synthesis to correct mistakes?

Answer 10

• DNA polyermases have exonuclease activities
• enables proofreading and editing
• Mismatched pair - DNA polymerases correct mismatched bases
• Nucleotide excision repair - requires the enzyme nuclease to remove bulky, damaged DNA components

Question 11

what leads to mutations?

Answer 11

if mismatched/damaged bases are not removed

Question 12

for non-circular DNA, where do you start/stop replication?

Answer 12

at multiple origins along the chromosome

Question 13

how do eukaryotic chromosomes remain full length?

Answer 13

• the ends of chromosomes are called ‘telomeres’
• telomeres usually do not contain coding regions
• the last part of linear DNA strand cannot be synthesised during ‘normal’ DNA replication.
• telomerase is an enzyme that creates an extension to the unreplicated end of the DNA strand.
• DNA primase and DNA polymerase III can then synthesis the lacking piece of DNA

Question 14

What is telomerase?

Answer 14

> An enzyme that creates an extension to the end of the un replicated end of the DNA strand.
DNA primase and polymerase III can then synthesise the lacking piece of DNA