DNA repair and damage

Question 1

What is a telomere?

Answer 1

Repetitive sequence of nucleotides at the end of each chromosome - TTAGGG
Acts as a buffer - the telomere sequence is degraded rather than genes

Question 2

How many time does the telomere sequence repeat?

Answer 2

Telomere sequence repeats 2500 times

Question 3

How many bases do somatic cells loose each day?

Answer 3

Somatic cells lose 26 bases daily from telomere

Question 4

What is the result of shortened telomeres?

Answer 4

Shortened telomeres = cell degredation and death

Question 5

Where are telomeres found?

Answer 5

Telomeres are only found in eukaryotes as they have linear chromosomes (ends which can be degraded)

Question 6

How do telomeres prevent degradation?

Answer 6

Telomeres prevent a DNA protein complex from TRF2 and telomere binding proteins

Question 7

What is telomerase? Where is it found?

Answer 7

Found in germ, stem and malignent cells - NOT FOUND IN SOMATIC CELLS
Telomerase maintains the chromosome by adding TTAGGG repeats to the end of the chromosome.
DNA Pol and RNA primase are needed to add the telomerase repeats.

Question 8

What are the different forms of DNA damage? Are they endogenous or exogenous

Answer 8

Endogenous damage - replication errors and cell metabolism

Exogenous damage - UV light, ionizing radiation, chemical exposure

Question 9

How do inspection repair enzymes repair DNA?

Answer 9

Inspection repair enzymes:
Detect damage
Use undamaged nucleotide on complementary strand as a template for replacing damaged nucleotides

Question 10

What proofreads DNA?

Answer 10

DNA polymerase proofreads DNA

Question 11

What is meant by NER? Describe the steps

Answer 11

Nucleoside Excision Repair:

1. DNA pol recognizes mistake in non-methylated daughter strand
2. 3’ to 5’ exonuclease removes error nucleotides
3. DNA polymerase aligns and joins correct nucleotides together
4. DNA ligase inserts the correct sequence

Question 12

Describe mismatch repair

Answer 12

When the wrong base in produced in the unmethylated daughter strand, it can be compared with the parent strand. It can then be repaired by mismatch repair enzymes

Question 13

How do mismatch repair enzymes recognize the daughter strand?

Answer 13

The daughter strand is not methylated (no CH3 added)

Question 14

What are the most chemically reactive elements in DNA?

Answer 14

N or C in nitrogenous base rings are very reactive

Question 15

How do chemical reactions affect DNA?

Answer 15

Lesions are created:
Lesions can prevent replication
Lesions can cause mutagenesis - the lesion is replicated and becomes fixed in DNA forever
Lesions may block transcription or translation

Question 16

Describe spontaneous DNA damage

Answer 16

Spontaneous damage causes deamination of bases e.g. when cytosine is deaminated, uracil is formed. Uracil binds to adenine rather than guanine

Question 17

How is spontaneous damage repaired?

Answer 17

BAR - Base excision repair

Question 18

Describe BER

Answer 18

1. DNA glycolase breaks glycosidic bond between deaminated base and sugar
2. Exonuclease cleaves DNA at the site, creating a larger gap
3. DNA polymerase links the correct deoxynucleotides together and DNA ligase inserts the missing bases into the chain

Question 19

Describe depurination

Answer 19

Depurination:

Glycosidic bond between purine bases (A or G) and sugar is hydrolysed, casuing the purine base to be lost

Question 20

How many purines are lost per cell per hour?

Answer 20

10,000 purines lost per cell per hour by depurination

Question 21

How is depurination repaired?

Answer 21

NER

Question 22

Describe pyrimidination

Answer 22

Glycosidic bond between pyrimidine (C or T) and sugar is lost
occurs less frequently than depurination

Question 23

What are bulky lesions/adducts caused by?

Answer 23

UV light causes pyrimidine dimer formation - adjacent C or T bases on same strand link together.
Where the dimers/lesions occur, localised denaturation of DNA also occurs as the dimers cannot pair with opposite bases.
Bulky lesions are repaired by NER

Question 24

Where is reactive O2 produced?

Answer 24

Aerobic metabolism causes reactive O2 to be produced via the fenton reaction
Reactive O2 attacks DNA