DNA replication, cell cycle and mitosis

Question 1

What is the importance of DNA replication?

Answer 1

A faithful copy of the genome is needed to ensure its continuity

Question 2

Define semi conservative replication

Answer 2

Each daughter molecule has half the original DNA and half newly synthesised DNA

Question 3

What does DNA helicase do?

Answer 3

Uses energy from ATP hydrolysis to break hydrogen bonds between base pairs and unwind DNA

Question 4

What do DNA polymerases do?

Answer 4

Add nucleotides to the 3’ end of the growing chain

Question 5

What direction is DNA synthesised in?

Answer 5

5’ to 3’

Question 6

What is a replication fork?

Answer 6

Site of replication

Moves along the strand

Question 7

Why is there asymmetry in DNA replication?

Answer 7

Strands are oriented anti-parallel
Leading stand synthesised continuously
Lagging strand synthesised in short Okazaki fragments

Question 8

What are the requirements for DNA replication?

Answer 8

Template strand
Oligonucleotide primer
Supply of dNTPs

Question 9

What are nucleoside analogs?

Answer 9

Chain terminators - they replace nucleosides but have no hydroxyl group on 3’ so no further bases can be added
e.g. dideoxycytidine and azidothymidine (HIV), acyclovir (Herpes)

Question 10

What is the structure of an Okazaki fragment?

Answer 10

Short base sequence

RNA primer called DNA primase at 5’ end created by primase

Question 11

How are Okazaki fragments joined?

Answer 11

Ribonuclease removes the RNA primer using 5’ to 3’ exonuclease activity
Repair DNA polymerases replace RNA with DNA
DNA ligase joins fragments using ATP

Question 12

What is an RNA primer?

Answer 12

Short RNA fragment that is transient and allows replication to start at replication origin

Question 13

What is the purpose of single strand DNA binding proteins?

Answer 13

Prevent any localised 2° structures forming due to hydrogen bonding

Question 14

What is the purpose of a sliding clamp?

Answer 14

Ring around the DNA prevents the polymerase from falling off

Question 15

Explain how accuracy is maintained by proof reading

Answer 15

DNA polymerase checks the previous nucleotide pairing when adding a new nucleotide
If incorrect then DNA polymerase will remove it by exonuclease activity, and will add a correct replacement

Question 16

How is E.coli chromosome replicated?

Answer 16

Bi-directional replication from OriC

Two replication forks that move simultaneously in opposite directions

Question 17

How are eukaryotic chromosomes replicated?

Answer 17

They have many replication origins, 100k bp apart

Bi-directional replication along strand until forks meet

Question 18

Define the different phases of the cell cycle

Answer 18

M phase - 1hr cell division
G1 phase - 10 hr RNA and protein synthesis
S phase - 9hr DNA synthesis
G2 phase - 4hr protein synthesis and growth
(G0- cells that stop dividing)

Question 19

What are the stages of mitosis?

Answer 19

Interphase
Prophase - condense
Metaphase - align 
Anaphase - separate
Telophase - reach opposite poles
Cytokinesis - division of cytoplasm