2 - DNA replication, the cell cycle and mitosis

Question 1

During DNA replication, which enzyme unwinds the DNA?

Answer 1

Topoisomerase

Question 2

During DNA replication, which enzyme separates the 2 strands by breaking the H bonds?

Answer 2

DNA helicase

Question 3

What does DNA polymerase do?

Answer 3

Adds nucleotides/dNTPs to the 3’ end of the DNA

Question 4

Name the 3 things that DNA polymerase requires to carry out its function

Answer 4

Template strand
Oligonucleotide primer
Deoxynucleotides triphosphates (dNTPs)

Question 5

What are oligonucleotides primers?

Answer 5

A fragment/sequence of RNA bases made by DNA primase (a specialised form of RNA polymerase)

Question 6

What are dNTPs?

Answer 6

Deoxynucleotide triphosphates

Nucleotides with 3 phosphates (2 bonds are used to produce energy)

Question 7

Replication begins at the _____

Answer 7

Site of origin

Question 8

The site of DNA synthesis is know as _____

Answer 8

The replication fork

Question 9

The leading strand is synthesised _____

The lagging strand is synthesised _____

Answer 9

Continuously

In Okazaki fragments

Question 10

How is the lagging strand synthesised? (Long answer question)

Answer 10

• RNA primer is added
• DNA polymerase is added and starts at the new Okazaki fragments
• When it reaches the previous primer, the primer is removed (by exonuclease activity-ribonuclease) and replaced by DNA (repair DNA polymerase)
• DNA ligase joins the new Okazaki fragment to the growing chain

Question 11

What direction does DNA synthesis occur in?

Answer 11

5’ to 3’ direction

Question 12

Where does the energy come from to drive the reaction?

Answer 12

hydrolysis pf the phosphates

Question 13

What is DNA primase?

Answer 13

a type of RNA polymerase (synthesises short fragments)

Question 14

What do ribonucleases do?

Answer 14

remove the RNA primer using 5’ to 3’ exonuclease activity

exonuclease activity - the ability of an enzyme to remove one nucleotide at a time in a polynucleotide chain

Question 15

What does repair DNA polymerase do?

Answer 15

replaces RNA with DNA

Question 16

What does DNA ligase do?

Answer 16

joins the 2 okazaki fragments together

Question 17

What does the single stranded binding protein do?

Answer 17

prevents the single-stranded DNA from locally folding

NOTE: lagging strand forms a loop so that both daughter strands can be synthesised in a coordinated matter

Question 18

What is the function of the sliding clamp?

Answer 18

makes sure the DNA polymerase is in the right place

Question 19

Describe the proofreading mechanism

Answer 19

before adding a new nucleotide, DNA polymerase checks to make sure the previous nucleotide is correct

Question 20

Explain replication of the E.coli chromosome

Answer 20

replication starts at a unique origin (OriC). Two replication forks proceed simultaneously in opposite directions. The two forks meet at the other side of the circular chromosome

Question 21

Summarise the replication of mammalian chromosomes

Answer 21

(eukaryotic chromosomes are linear and very long)There are multiple replication origins, distributed at intervals of about 100 kilobase pairs. Each replication origin gives bi-directional replication forks. Replication is finished when all the forks have met.

Question 22

What happens in the S phase of the cell cycle?

Answer 22

DNA replication

Question 23

Describe how nucleoside analogs acts as drug that targets DNA replication

Answer 23

(on a normally functioning nucleotide strand, the phosphate on a free nucleotide bing to the OH on the 3’ carbon)
nucleosides have no phosphate group to bind to, so the chain terminates

Question 24

Give an example of of nucleoside analog and what condition it is used to treat

Answer 24

zalcitabine - a drug for HIV