DNA Replication

Question 1

Briefly describe the structural features of DNA replication

Answer 1

Formation of phosphodiester bonds between 3OH + 5’ Phosphate group
Nucleotides added to 3’ OH end of chain so synthesised 5’-3’

Question 2

Describe initiation of DNA replication

Answer 2

Initiating protein binds to origin of replication (ori)
Multiple ori along eukaryotic chromosome, each bubbles along axis of chromosome (Prokaryotes have 1 ori, Euk have 10,000 in humans)

Question 3

What direction does replication occur in?

Answer 3

Both directions from Ori > Bi-directional

Question 4

Describe the process of DNA replication

Answer 4

DNA helicase/Initiating proteins open double helix, primer added to 3’OH end
When nucleotides added, DNA polymerase I removes RNA, replaces with DNA
Missing bonds filled by DNA ligase joining 5’P-3’OH

Question 5

What is a primer?

Answer 5

Enzyme - Synthesises RNA primer

Question 6

what is DNA ligase?

Answer 6

Enzyme - Joins gaps in DNA backbone

Question 7

Which DNA polymerases are found in E.Coli?

Answer 7

DNA poly III - Responsible for replication

DNA poly I - Responsible for repair/removing primers

Question 8

Which polymerases are found in eukaryotes?

Answer 8

DNA poly alpha, delta, epsilon - In chromosome replication

Question 9

How do prokaryotes divide?

Answer 9

By binary fission using chromosome replication and cell growth, forms 2 new cells with identical DNA

Question 10

Describe end-replication in eukaryotes?

Answer 10

Eukaryotes have telomeres which repeat sequences at end of chromosomes

Question 11

Describe telomerase

Answer 11

Uses RNA template to complete ends of chromosomes

Question 12

What is the function of DNA helicase?

Answer 12

Breaks H-Bonds btw CBP>ATP needed to move along DNA and to form 2 template strands

Question 13

How is a double helix stabilised?

Answer 13

Single strand binding proteins bind single stranded DNA (ssDNA) made by helicases
Straighten/stabilise ssDNA working together with DNA polymerase

Question 14

What is a polymerisation reaction?

Answer 14

A condensation reaction

Question 15

Why do extra phosphate groups carry energy?

Answer 15

For forming covalent bonds

Question 16

What are free nucleotides in the nucleus called?

Answer 16

Deoxynucleoside triosphosphate

Question 17

What is the function of DNA polymerase I?

Answer 17

To proof read the strands

If error occurs, translocation of enzyme to next position along template is inhibited, so opportunity for correction

Question 18

What happens to incorrectly paired nucleotides during proofreading?

Answer 18

Nucleotides removed through exonuclease activity

Cell has other DNA repair mechanisms

Question 19

What are topoisomerases?

Answer 19

Prevent DNA tangling during replication - Reversible nucleases that break phosphodiester bonds

Question 20

Describe type I topoisomerases

Answer 20

Make nicks in one DNA strand, relieve supercoiling

Question 21

Describe type II topoisomerases

Answer 21

Make nicks in both strands, relieve supercoiling/untangle linked DNA helices

Question 22

What happens in the absence of topoisomerase?

Answer 22

DNA can’t rapidly rotate and torsional stress builds up

Question 23

What is DNA supercoiling caused by?

Answer 23

Torsional stress