DNA replication

Question 1

How do features of DNA aid DNA replication

Answer 1

• DNA is stable
• hydrogen bonds are weak and strands can be separated relatively easily
• base-pairing rules guide the synthesis of complementary strands

Question 2

What is needed for DNA replication to occur

Answer 2

DNA replication is semi conservative, daughter DNA has 1 original and 1 new strand

dNTPs are needed, a template for copying, a 3’ end to add nucleotides to, an enzyme for polymerisation and an energy source

Question 3

How is DNA separated

Answer 3

DNA helicase: opens DNA helix by breaking hydrogen bonds between bases

Topoisomerase: relieves the strain on DNA when unwinding by causing breaks and then resealing

Single-stranded DNA binding proteins: binds to separated strands preventing reannealing

to allow DNA polymerase to start replicating DNA primase makes an RNA primer forming a 3’ OH for DNA polymerase

Question 4

How does DNA replication occur on the lagging strand

Answer 4

• DNA primase synthesises short RNA primers at the replication fork
• DNA polymerase iii extends from the 3’ end of the primer making Okazaki fragments
• DNA polymerase i removes RNA primers and fills in the gaps
• DNA ligase joins the nicks on DNA

A sliding clamp helps DNA polymerase stay attached to the template

Question 5

How quick is DNA replication

Answer 5

around 1000 nucleotides per second are replicated in prokaryotes with an error rate of 10-8 to 10-10

DNA replication is slower in eukaryotes

Question 6

What are the sources of error in DNA replication

Answer 6

• frameshift: extra nucleotides are inserted or omitted
  (if DNA polymerase dissociates and re-associates with the template or DNA loops out causing strand slippage)
• substitutions: an incorrect nucleotide is incorporated

Question 7

How can errors in DNA replication be fixed

Answer 7

DNA polymerase has 3’-5’ exonuclease activity that can remove the wrongly incorporated base. DNA replication pauses to correct the mistake
- this is spotted as the position of the bases is not 1.1 nm

Question 8

How is a prokaryotic chromosome replicated

Answer 8

• circular bacterial chromosome has a single origin of replicaiton (ori)
• a replication bubble opens with two replication forks in opposite directions

Question 9

How is a chromosome replicated in eukaryotes

Answer 9

• there are multiple origins of replication per chromosome
• multiple methods increase efficiency
• takes around 7 hours to replicate the entire human genome
• polymerisation is 2kb/min