Lecture 14: DNA repair

Question 1

What are the types of DNA repair mechanisms?

Answer 1

1. Mismatch repair
2. Single-strand base excision repair
3. Nucleotide excision repair
4. Double-stranded break repair

Question 2

Mismatch repair is seen in both E.Coli and Humans. True/False?

Answer 2

FALSE. We do NOT have this repair mechanism

Question 3

What is a possible cause for Mismatch Repair?

Answer 3

An error during replication that was not corrected by polymerase

Question 4

How do we tell which strand/base is the WRONG one, in order to fix it? (Mismatch repair)

Answer 4

In the OLD strand, the ADENINE base is METHYLATED (CH3 attached), while the new strand is unmethylated

Question 5

Describe the process of mismatch repair

Answer 5

1. Mark the wrong section with a ‘cut’
2. Exonuclease removes the nucleotide
3. DNA polymerase adds the correct nucleotide
4. DNA ligase glues

Question 6

What is DEAMINATION?

Answer 6

Loss of an amine group (NH2), causing mutation

Question 7

When you lose an amine group (deamination) of Cytosine, what does it become?

Answer 7

URACIL

Question 8

What is DEPURINATION?

Answer 8

When the whole base is removed from the backbone

Question 9

List the types of “single base” damage

Answer 9

1. Deamination
2. Depurination
3. Incorrect methylation
4. Oxidative damage

Question 10

What is the difference between mismatch repair and base-excision repair?

Answer 10

mismatch repair is when the matching of bases goes wrong. base-excision is when the WHOLE base is wrong; for example having uracil instead of cytosine… (?)

Question 11

B.E.P: what enzymes are involved to rid URACIL?

Answer 11

DNA glycosylase: cuts out the uracil base
AP endonuclease: cuts the phospate backbone
DNA polymerase 1: uses 5’ to 3’ exonuclease activity
DNA ligase: glues

Question 12

What does DNA glycosylase do?

Answer 12

Involved in BEP. Cuts out the URACIL base

Question 13

What does AP endonuclease do?

Answer 13

Involved in BEP. Cuts the phosphate backbone

Question 14

What are Thymine dimers?

Answer 14

Instead of base paring, they form covalent bonds with each other. i.e Two Thymines. Causes KINKS

Question 15

How are thymine dimers repaired?

Answer 15

Nucleotide excision repair

Question 16

Name the components involved in NUCLEOTIDE EXCISION REPAIR

Answer 16

1. EXCInuclease
2. DNA helicase
3. DNA polymerase 1
4. DNA ligase

Question 17

Describe the mechanism of nucleotide excision repair

Answer 17

1. The DNA lesion (part that has suffered damage) is recognised
2. EXCInuclease: cuts
3. Helicase: unwinds
4. Polymerase 1: repairs
5. Ligase: glues

Question 18

How is DOUBLE-STRANDED BREAKS repaired?

Answer 18

Cuts out the regions on both strands, joins

Question 19

Why is deletion of nucleotides in double-stranded repair not a big deal?

Answer 19

Because most of our DNA is non-coding for genes

Question 20

What is the purpose of uracil glycosylase and why is it important?

Answer 20

Prevents mutagenesis by eliminating uracil from DNA molecules by cleaving the N-glycosylic bond and initiating the base-excision repair pathway

Question 21

Why can’t thymine dimers be removed by base pair excision?

Answer 21

The kink (covalent bonds) in the DNA caused by the dimer prevents DNA polymerase 1 from removing affected nucleotides

Question 22

Why is double stranded DNA break dangerous?

Answer 22

It could lead to the potential break down of the chromosome by protective mechanism (attacking the bare ends)

Question 23

What is apoptosis?

Answer 23

Programmed cell death

Question 24

For a child to inherit a mutation, where in the body must that mutation occur?

Answer 24

In germ cells