Gene Repair Systems

Question 1

Name 5 DNA repair systems.

Answer 1

1. Proof reading during replication
2. Post-replication repair
3. Base excision repair
4. Nucleotide excision repair
5. Double strand break repair

Question 2

The proteins/enzymes involved in post-replication repair are:

Answer 2

RecA protein
DNA polymerase
DNA ligase

Question 3

How does the RecA protein repair DNA post replication?

Answer 3

It uses recombination and takes the correct sequence from the parental strand and inserting into the gap opposite the lesion.

Question 4

What is Homologous Recombination Repair? What repair systems is homologous recombination a feature of?

Answer 4

The use of an undamaged complementary region of a DNA strand recombined into a damaged DNA strand. It is a feature of post replication repair and double stranded break repairs.

Question 5

Replication may be paused when lesions are reached caused by:

Answer 5

Thymine dimers

Question 6

The RecA protein uses what to repair a DNA strand?

Answer 6

homologous recombination

Question 7

How does the RecA protein use recombination to repair thymine dimers?

Answer 7

It takes the correct sequence from the parental strand and inserts it into the gap opposite the lesion. DNA polymerase and ligase then fill the new gap formed by the RecA protein.

Question 8

When does Base Excision repair occur?

Answer 8

It corrects DNA that contains a damaged DNA throughout the cell cycle.

Question 9

The enzymes involved in Base Excision repair are?

Answer 9

DNA glycosylase
AP endonuclease
DNA polymerase
DNA ligase

Question 10

What is the role of the DNA glycosylase in base excision repair?

Answer 10

It recognises the altered base and cuts the bond between the base and sugar. There are different DNA glycosylases for specific bases.

Question 11

What is the role of the AP endonuclease in Base Excision Repair?

Answer 11

It recognises the sugar with the missing base and cuts the phosphodiester backbone surrounding the baseless sugar.

Question 12

The three steps of base excision repair are:

Answer 12

1. Altered base recognised and base removed by DNA glycosylase.
2. Baseless sugar removed by AP endonuclease.
3. Gap is filled by DNA polymerase and DNA ligase.

Question 13

Nucleotide Excision Repair is used to repair…

Answer 13

bulky lesions in DNA that alters the double helix, including thymine dimers caused by UV, are repaired by…

Question 14

How many nucleotides are clipped out around both sides of a lesion during nucleotide excision repair in a prokaryote and eukaryote?

Answer 14

13 and 28

Question 15

What are the steps of nucleotide incision repair?

Answer 15

1. Recognition of damaged DNA by proteins encoded on several different genes.
2. Other repair genes encode helicases and nucleases that form an excision repair complex that removes the fragment.
3. Resulting fragment filled by DNA polymerase and DNA ligase.

Question 16

Xeroderma Pigmentosum is caused by…

Answer 16

mutation of the Nucleotide Excision Repair enzymes.

Question 17

What type of genetic disorder is Xeroderma Pigmentosum?

Answer 17

Autosomal recessive

Question 18

The mutations causing Xeroderma pigmentosum affect the ability of a cell to..

Answer 18

to repair damage cause by UV (thymine dimer lesions), causing skin cancers.

Question 19

Double strand breaks can be caused by:

Answer 19

exposure to ionising radiation, leading to chromosomal rearrangements.

Question 20

A pathway involved in double strand break repair is…

Answer 20

homologous recombination.

Question 21

Homologous double stranded break repair occurs during?

Answer 21

During late S phase, or G2. When DNA replication has occurred and sister chromatids are available.

Question 22

What is required for homologous double stranded break repair?

Answer 22

Sister chromatids.

Question 23

What is the process of homologous double stranded break repair?

Answer 23

1. recognition of break by an enzyme that trims the 5’ ends to leave overhanging (sticky) 3’ ends.
2. Overhanging end searches for complementary sequence on the sister chromatids, invades the homolgous DNA duplex and aligns at the complementary sequences.
3. DNA synthesis proceeds from the 3’ overhanging ends using the undamaged DNA as templates

Question 24

What Is the Ames Test?

Answer 24

A test of the mutagenicity of different compounds.

Question 25

What mutations of bacteria ‘salmonella typhimurium’ does the Ames test exploit to test mutagenicity?

Answer 25

1. histidine operon
2. genes involved in DNA repair
3. genes involved in protective lipopolysaccharide barrier.