MGD S9 - Mutagenesis

Question 1

How does a change in the base sequence of and nucleotide lead to a change in the primary sequence of a protein and what is the effect of this?

Answer 1

Change in genetic code (base sequence) can result in different amino acids being coded for.

This causes a change to the primary sequence of the protein which can lead to a change in shape and function of the protein.

Question 2

What is a point mutation?

Answer 2

One base is substituted for another.

Question 3

What is the difference between a transition and transversion point mutation?

Answer 3

Transition:

Purine swapped for another purine
Pyrimidine swapped for another pyrimidine

Transversion:

Purine swapped for a pyrimidine or Vice versa

Question 4

What types of mutation can result from a point mutation in the coding region of a protein?

Answer 4

Silent mutations
Missense mutations
Nonsense mutations

Question 5

What are the possible effect(s) of a point mutation in a non-coding region?

Answer 5

Altered binding sites
Altered promoter sequences
Altered splice sites

Question 6

What are insertion or deletion mutations?

Answer 6

A sequence is added or removed for the nucleic acid.

Question 7

How can insertion/deletion mutations vary in severity?

Give examples of multi-nucleotide deletion/insertion mutations of different severity

Answer 7

Single nucleotide base mutations

Few nucleotide mutations
E.g. Triplet repeats

Millions of nucleotide mutation
E.g. Tandem duplications

Question 8

What is a frameshift mutation?

Answer 8

An insertion or deletion of a number of nucleotides not divisible by 3

This causes the shift of all subsequent triplet codes up one nucleotide

Question 9

What is a silent mutation?

Answer 9

A mutation that doesn’t result in a different amino acid being specified

Question 10

What is a Missense mutation?

Answer 10

A mutation that replaces one amino acid with another

Question 11

What is a Nonsense mutation?

Answer 11

A mutation that results in a premature stop codon

Question 12

What is a spontaneous mutation?

Answer 12

A mutation that is:

Not caused by exposure to a known mutagen

Can be due to errors in DNA replication

Can be due to DNA bases having a slight chemical instability

Question 13

Why does the rate of spontaneous mutation differ between genes?

Answer 13

Depends on size of gene

Dependent on DNA sequence

Question 14

Each new individual has multiple new mutations in their genetic code, why are most individuals unaffected?

Answer 14

Most are in non-coding regions due to chance

Question 15

What is the cause of induced mutations?

Answer 15

Chemicals and radiation can be mutagenic, they cause mutation

Question 16

What is a carcinogen?

Answer 16

A chemical that causes cancer

Question 17

Give examples of mutagens and how they mutate the genetic code

Answer 17

Alkylating agents:
Remove a base

Acridine agents:
Add or remove a base

X rays:
Break chromosomes/delete a few nucleotides

UV radiation:
Creates thymine dimers

Question 18

What is a Wild-type trait?

Answer 18

A trait that is the most common in a population

Question 19

What is a mutant phenotype and what causes it?

Answer 19

Caused by a mutation

Mutant phenotype is a phenotype that differs from the most common/wild type trait in a population

Question 20

What is a mutant allele and what causes it?

Answer 20

Caused by mutations to a gene

Mutant allele is an allele that differs from the common/wild-type allele in a population

Question 21

What are the 3 modes of DNA repair?

Answer 21

Mismatch repair

Excision repair:

Nucleotide excision
Base excision repair

Question 22

What is mismatch repair?

Answer 22

Occurs when enzymes detect nucleotides that don’t base pair in newly synthesised DNA

The incorrect base is excised and repaired

This process is also called proofreading

Question 23

What is nucleotide excision repair and what type of damage does it repair?

Answer 23

Involved in the repair of larger/more bulky lesions to DNA that distort the Helix

Can replace 10 - 30 bases

Question 24

What is base excision repair, how is it performed and what type of damage does it repair?

Answer 24

Takes two forms:

Short patch BER:
- One nucleotide replaced

Long Patch BER:
- 2 to 10 nucleotides replaced

The base is excised and replaced by DNA polymerase and DNA ligase

Question 25

What happens to a cell if damage is too severe to repair?

Answer 25

Proteins p53 (Guardian of the genome) monitors repair and if damage is too severe it promotes apoptosis

Question 26

What is the effect of mutations to DNA repair protein coding genes?

Answer 26

There is an overall increase in mutation / DNA is not repaired efficiently

Question 27

How does Cancer arise?

Answer 27

If DNA is damaged to the point where apoptosis doesn’t occur as it should in a damaged cell then the damage leads to uncontrolled growth and cancerous cells are produced.

Question 28

Describe briefly how a tumour arises

What is the major factor influencing tumour growth?

Answer 28

Tumours are derived from individual abnormal cells

They arise from a lack of normal growth control

Generated in a multistep process

Tumours are more likely to arise from rapidly dividing cells

Question 29

Describe very briefly the nature of a tumour and how their behaviour is determined

Answer 29

All cells of a tumour are the same type

The behaviour of the tumour depends on the cell type

Question 30

What are Oncogenes?

Give a brief description

Answer 30

Genes involved in the control of cell division

• They’re present in normal cells
• Many different classes
• May stimulate/inhibit growth

Question 31

What are tumour suppressor genes?

Answer 31

Genes involved in protecting the cell against one step on the path to cancer

Question 32

What is an Oncogene called when present in a normal cell and how do they come to promote tumour growth?

Answer 32

When genes present in a normal cell they’re called ‘Proto-oncogenes’

It is after increased expression or mutation occurs to the gene that they become oncogenes

Question 33

How might a virus stimulate tumour growth?

Answer 33

Specific retroviruses have been shown to contain genes able to transform cells to a cancerous phenotype

Question 34

How might you identify sickle cell disease with PCR and gel electrophoresis/southern blotting?

Answer 34

Put the DNA through PCR

Apply restriction endonucleases

In sickle cell disease the restriction site for MstII is destroyed

Therefore with gel electrophoresis and southern blotting there will be one less DNA fragments as a restriction site is ‘missing’

Question 35

Why can not all genetic disease be identified with PCR/gel electrophoresis/southern blotting in the same manner as sickle cell disease?

Answer 35

Not all mutations result in the loss of a restriction site

Question 36

How is southern blotting used in the detection of genetic disease?

Give examples of the types of mutations that can be detected

Answer 36

Can be used to identify mutations that have a large effect on molecular weight of DNA/DNA fragments produced by endonucleases

Technique of choice to analyse larger segments of DNA within and around a gene to identify mutation (Eg. Sensitive to mutations that affect restriction sites)

Can be used to analyse triplet repeat disorders such as Huntington’s disease and Fragile X syndrome

Question 37

Explain the process of array comparative genomic hybridisation (Array CGH)

Answer 37

1. An array of DNA probes covering the entire genome is applied to the surface of a solid matrix
2. Patient and normal DNA each labelled with a differently coloured fluorescent probe
3. Equal amounts of labelled DNA then hybridised to the probe array and the hybridisation signals (the different colours relative strength at different probes) is analysed
4. For probes where normal DNA exceeds that of the patients DNA the patient is shown to have a deletion in the chromosomal region from which that probe was derived

Question 38

What ethical issues might be involved in genetic testing?

Answer 38

Genetic testing prenatally that identifies a disease raises the question of ethicality of abortion of a foetus with an identified defect

If Patient X gets tested for a genetic disease and tests positive for a dominant X-linked genetic disease then is it ethical to inform the mother even if the mother is asymptomatic?