DNA Synthesis

Question 1

Describe replication origins

Answer 1

• DNA réplication initiated at specific sites on DNA called replication origins
• Replication origins recognised by an initiation complex
• DNA at origin unwinds to form a replication bubble to allow access to the replication machinery

Question 2

Give an overview of the bacterial cell cycle

Answer 2

• S phase
• M phase
• divides once every 20-30 mins

Question 3

Give an overview of the mammalian cell cycle

Answer 3

• G1 phase: pairing DNA, replication of proteins
• S phase: DNA replication (semi conservative)
• G2: prepare for mitosis
• M phase: cell divides

Question 4

State some differences between bacterial and mammalian cell cycles

Answer 4

• B: 20-30 mins long
• M: 16-24 hours long
• B: one replication origin
• M: multiple replication origins

Question 5

What are the main types of polymerases?

Answer 5

Bacterial polymerases:
- I, II, III for repair

Eukaryotic polymerases:

• α replication
• β replication
• γ mitochondrion
• δ replication (causes elongation)
• ε replication (causes elongation)

Question 6

State the properties of DNA Polymerase

Answer 6

• acts in 5’ to 3’ direction
• utilises thé AT and GC bp to make new strand
• has a proof reading / editing function

Question 7

What does DNA Polymerase require to function?

Answer 7

• DNA template
• Primers
• 4 dNTPs and Mg2+

Question 8

How does replication occur?

Answer 8

• DNA unwinds the helix by breaking H bonds which creates negative supercoils
• topoisomerase II relieves the stress and tension created by -ve supercoils by adding +ve supercoils
• single stranded binding proteins bind to DNA which helps keep the strands apart so they don’t reanneal and bells protect them from degradation by nucleases
• a primer is laid down by RNA primate at 5’ end of both strands
• the leading streams only needs one primer and the synthesis is continuous
• the lagging strands more than one primer as the synthesis is discontinuous bc it copies away from the replication form as it opens
• okazaki fragments are joined by DNA ligase

Question 9

Naw the other enzymes involved in DNA synthesis and their function

Answer 9

• Helicase: separates base pairs producing single strands
• Topoisomerase: removes the supercoils of the helix
• Primase: lays down RNA so DNA Pol knows where to start replicating
• DNA Binding Proteins: stabilise thé single stranded DNA and prevent reannealing
• Replicative DNA Pol: copies parental strand
• Repair DNA Pol: repair the fragments and takes out RNA so it will be a full DNA strand
• DNA Ligase: joins okazaki fragments together

Question 10

What are the procedures to ensure no mistakes in replication?

Answer 10

1) BP: ensures accurate copying
2) Pol has it’s ‘editing’ feature
3) Mismatch repair system: enzymes corrects most of the error

Question 11

What is the error rate of DNA Pol?

Answer 11

1 in 10^8

Question 12

Why are DNA replication inhibitors important?

Answer 12

act as antibacterial, antitumour and antiviral agents

Question 13

State examples of antibacterials

Answer 13

• ciprofloxacin

- novabiocin

Question 14

State some antitumour drugs

Answer 14

• doxorubicin

- mitoxantrone