3.1 Errors In DNA Replication

Question 1

What causes DNA replication stress? (3)

Answer 1

1. Replication machinery has defect
2. Factors hinder replication fork progression
3. Have defects in response pathways

I.e. Machine defect, no fork progression, response pathway defect

Question 2

What is slippage (2)

Answer 2

1. Parent strand slips and forms a loop so a nucleotide cannot be seen to DNA polymerase. So daughter strand has complimentary nucleotide missing.
2. Daughter strand slips so DNA polymerase adds another nucleotide

Note this mutation is carried through all new cell lineages

Question 3

What is nucleotide misincorporation and how count it be caused?

Answer 3

1. DNA polymerase adds an incorrect complementary base relative to parent strand
2. Exonuclease removes and polymerase usually repairs BUT
3. Defects in DNA exonuclease would mean proof reading wouldn’t occur as efficiently

Question 4

What is the function of the exons lease domain in DNA polymerase? (2)

Answer 4

1. Detects base pair mismatches
2. removes incorrect nucleotide so polymerase can repair

I.e proof reading

Question 5

What factors hinder replication fork progression?

Answer 5

1. Base excision repair defect
   - faulty BER causes DNA single strand breaks
2. Faulty BRCA
   - during replication, the SSBs turn into double strand breaks

Question 6

What is the function of functioning BER and BRCA pathways? (2)

Answer 6

1. BER - repairs nucleotide problems on DNA by cutting and replacing problems
2. BRCA - repairs problems with replication fork

Question 7

Why are DSBs very dangerous?

Answer 7

Double strand breaks

1. Lead to genomic mutations - cancer
2. Lead to cell growth inhibition - ageing
3. Cell death

Question 8

What gene is associated with huntingtons?

Answer 8

HTT

Question 9

What happens within the HTT gene causing Huntingtons?

Answer 9

More CAG - polyglutamine repeats

A healthy individual would have 6-39
An unhealthy gene would have 35-121

Note - more repeats, younger onset

Question 10

Is Huntingtons autosomal dominant or recessive? What is the percentage chance an affected father/mother will pass it on to his/her children?

Answer 10

Autosomal dominant therefore 50% chance

Question 11

What does the mutant protein in Huntingtons cause?

Answer 11

Neurone degeneration so causes uncontrolled movement, emotional problems and loss of thinking (cognition)

Question 12

What are the similarities and difference between exo and endonucleases?

Answer 12

Both are involved with proofreading and catalyse hydrolysis of single nucleotides in DNA chain.

Exo work at ends (3’ and 5’ ends) of polynucleotide chain
Endo works in middle of chain by separating nucleotides

Question 13

What can mutation generation and accumulation cause? Give an example.

Answer 13

Mutations can occur over and over causing cells to go from

Normal - premalignant - malignant

E.g. Colorectal cancer
Normal - hyperproliferation-late adenoma (benign tumor) - carcinoma - metastasis (spread of cancer to other areas of body)

Question 14

What causes ageing?

Answer 14

1. Genomic instability, cell growth inhibition and cell death usually caused by DSBs

Question 15

Give another name for the cell death pathway. What will happen if this is mutated?

Answer 15

1. Apoptosis

2. Cells will not die when they could be becoming cancerous so will lead to cancer!

Question 16

What is the name of diseases where ageing occurs prematurely? Give an example.

Answer 16

Progeroid

E.g.

1. Werner syndrome
2. WRN mutation
3. DNA rep defects and DNA damage
4. Autosomal recessive
5. Patients prematurely display features associated with normal ageing
6. Can cause other things like cataracts, skin ulcers, type 2 diabetes, atherosclerosis etc

Question 17

What is the major groove and minor groove on DNA. Where are they useful?

Answer 17

Major groove is where backbones are far apart.
Minor grooves are where they are close together.
Useful as DNA binding proteins bind to major groove as backbone doesn’t get in the way

Question 18

How does 2m of DNA fit into a 6micrometre nucleus?

Answer 18

The packing technique used (DNA around histones, compressed into solenoid, further folding into chromosome) is very efficient!

Question 19

When transcription occurs, how do transcription factors differentiate between active chromatin (euchromatin) and inactive (heterochromatin)?

Answer 19

Acetyl groups covalently binds to euchromatin so that histones loosen grip on DNA so TF’s can access it

Methyl groups covalently binds to heterochromatin silencing gene expression by inactivating alleles