MGD S9 - Mutagenesis Flashcards
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?
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.
What is a point mutation?
One base is substituted for another.
What is the difference between a transition and transversion point mutation?
Transition:
Purine swapped for another purine
Pyrimidine swapped for another pyrimidine
Transversion:
Purine swapped for a pyrimidine or Vice versa
What types of mutation can result from a point mutation in the coding region of a protein?
Silent mutations
Missense mutations
Nonsense mutations
What are the possible effect(s) of a point mutation in a non-coding region?
Altered binding sites
Altered promoter sequences
Altered splice sites
What are insertion or deletion mutations?
A sequence is added or removed for the nucleic acid.
How can insertion/deletion mutations vary in severity?
Give examples of multi-nucleotide deletion/insertion mutations of different severity
Single nucleotide base mutations
Few nucleotide mutations
E.g. Triplet repeats
Millions of nucleotide mutation
E.g. Tandem duplications
What is a frameshift mutation?
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
What is a silent mutation?
A mutation that doesn’t result in a different amino acid being specified
What is a Missense mutation?
A mutation that replaces one amino acid with another
What is a Nonsense mutation?
A mutation that results in a premature stop codon
What is a spontaneous mutation?
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
Why does the rate of spontaneous mutation differ between genes?
Depends on size of gene
Dependent on DNA sequence
Each new individual has multiple new mutations in their genetic code, why are most individuals unaffected?
Most are in non-coding regions due to chance
What is the cause of induced mutations?
Chemicals and radiation can be mutagenic, they cause mutation
What is a carcinogen?
A chemical that causes cancer
Give examples of mutagens and how they mutate the genetic code
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
What is a Wild-type trait?
A trait that is the most common in a population
What is a mutant phenotype and what causes it?
Caused by a mutation
Mutant phenotype is a phenotype that differs from the most common/wild type trait in a population
What is a mutant allele and what causes it?
Caused by mutations to a gene
Mutant allele is an allele that differs from the common/wild-type allele in a population
What are the 3 modes of DNA repair?
Mismatch repair
Excision repair:
Nucleotide excision
Base excision repair
What is mismatch repair?
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
What is nucleotide excision repair and what type of damage does it repair?
Involved in the repair of larger/more bulky lesions to DNA that distort the Helix
Can replace 10 - 30 bases
What is base excision repair, how is it performed and what type of damage does it repair?
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
What happens to a cell if damage is too severe to repair?
Proteins p53 (Guardian of the genome) monitors repair and if damage is too severe it promotes apoptosis
What is the effect of mutations to DNA repair protein coding genes?
There is an overall increase in mutation / DNA is not repaired efficiently
How does Cancer arise?
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.
Describe briefly how a tumour arises
What is the major factor influencing tumour growth?
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
Describe very briefly the nature of a tumour and how their behaviour is determined
All cells of a tumour are the same type
The behaviour of the tumour depends on the cell type
What are Oncogenes?
Give a brief description
Genes involved in the control of cell division
- They’re present in normal cells
- Many different classes
- May stimulate/inhibit growth
What are tumour suppressor genes?
Genes involved in protecting the cell against one step on the path to cancer
What is an Oncogene called when present in a normal cell and how do they come to promote tumour growth?
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
How might a virus stimulate tumour growth?
Specific retroviruses have been shown to contain genes able to transform cells to a cancerous phenotype
How might you identify sickle cell disease with PCR and gel electrophoresis/southern blotting?
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’
Why can not all genetic disease be identified with PCR/gel electrophoresis/southern blotting in the same manner as sickle cell disease?
Not all mutations result in the loss of a restriction site
How is southern blotting used in the detection of genetic disease?
Give examples of the types of mutations that can be detected
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
Explain the process of array comparative genomic hybridisation (Array CGH)
- An array of DNA probes covering the entire genome is applied to the surface of a solid matrix
- Patient and normal DNA each labelled with a differently coloured fluorescent probe
- 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
- 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
What ethical issues might be involved in genetic testing?
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?
