Maintenance and use of genetic information

Question 1

What are the stages of the cell cycle?

Answer 1

Gap 1, DNA synthesis, Gap 2, mitosis

G1, S, G2, M

Question 2

How does DNA replication initiate?

Answer 2

Initiation proteins acts at the replication origin to create a pair of replication forks; so DNA polymerase now has access to single strands.
Immediate reformation of double helix prevented by single strand binding proteins
Topoisomerase enzymes prevent supercoiling by breaking a single phosphodiester bond which allows the fork to relax.

Question 3

What do topoisomerases do?

Answer 3

Prevent supercoiling of DNA in replication by breaking a phosphodiester bond

Question 4

What direction does DNA polymerase work in?

Answer 4

5’ to 3’

binds to 3’ end and first places the 5’

Question 5

What are Okazaki fragments?

Answer 5

Discontinuous lengths of DNA complementary to the lagging strand in DNA replication

Question 6

Why are RNA primers used to initiate DNA synthesis?

Answer 6

DNA polymerase cannot initiate synthesis, but RNA polymerase can so a primer is placed and then continued by DNA polymerase from 5’ to 3’

Question 7

What do exonucleases do in DNA replication?

Answer 7

Removes RNA primer, leaving a gap with a free 3’ end.

Question 8

What does DNA ligase do in DNA replication

Answer 8

After RNA primer removed and gap filled in (from 5’ - 3’) the missing phosphodiester bond is filled in then by DNA ligase

Question 9

What is the importance of the lagging strand loop?

Answer 9

Allows synthesis of DNA on each strand coordinated even though topologically travelling in opposite directions

Question 10

What does helices do in DNA replication?

Answer 10

Unwinds DNA strands

Question 11

What does primase do in DNA replication?

Answer 11

Marks RNA primer so DNA synthesis can begin

Question 12

What is the end replication paradox?

Answer 12

RNA primers removed and gaps between DNA fragments filled in but terminal RNA primer is not between two DNA fragments so once removed the gap cannot be filled in causing the loss of genetic material with every replication

Question 13

How is loss of genetic material prevented in the end replication paradox

Answer 13

Telomeres on either end of the DNA (repeats of TTAGGG)

Question 14

Types of point mutation?

Answer 14

Silent - base changed codes for same AA (degenerate) no different in protein
Nonsense - AA replaced by STOP codon
Missense - one AA replaced by another AA

Question 15

`What are indels and what can they do?

Answer 15

Small scale insertions/deletions
Causes frameshift if doesn’t occur in triad
Won’t do anything if it occurs in introns

Question 16

What is the genetic change that causes Huntington’s disease?

Answer 16

Increase in CAG repeats at HD locus; from 28-36

Causes a build up of huntingtin

Question 17

What is the genetic change that causes Cri du Chat syndrome?

Answer 17

Deletion of the end of chromosome 5

Question 18

What is the genetic change that causes chronic myelogenous leukaemia?

Answer 18

Translocation of pieces of chromosomes 9 & 22

Question 19

What does reverse slippage cause?

Answer 19

The gain of a repeat of a codon

Question 20

What does forward slippage cause?

Answer 20

The loss of a repeat of a codon

Question 21

How can UV light affect DNA synthesis

Answer 21

Causes adjacent thymine nucleotides to form thymine dimers preventing replication

Question 22

What is nucleotide excision repair?

Answer 22

Strands of DNA with a bad base has section removed and retranscribed by helicase and nuclease

Question 23

What is nucleosome sliding?

Answer 23

Adjacent nucleosomes slide apart exposing strand of DNA between them so it is available for transcription; requires ATP