DNA Mutations Flashcards
(53 cards)
What is a mutation?
Heritable change in nucleotide sequence from a lesion (i.e. error) in DNA that is not repaired prior to replication round. May cause beneficial, detrimental or neutral changes.
What is the difference between spontaneous mutations and induced mutations?
Spontaneous: occur during normal growth due to replication errors (replication errors 10^-6/1000bp)
Induced: caused by external factors called mutagens that chemically modify DNA (10^-3/1000bp)
What is the difference between a wildtype and a mutant?
Wildtype: Strain of an organism isolated from nature with a certain observable phenotype
Mutant: Cell whose genome carries change in nucleotide sequence from that of the wildtype genotype, resulting into an observable mutant phenotype
What is a parental strain?
The strain from which a mutant is derived which may be either the wild-type strain or a previously mutated strain.
What is the difference between wildtype and mutant E.coli grown on MacConkey agar with a maltose carbon source?
Wildtype: pH indicator will turn red as maltose is fermented
Mutant strains (Mal-): pH indicator will remain yellow as maltose will not be fermented defectiveness in maltose metabolism
What are two ways mutants may be isolated?
Selection: favouring growth of mutants that have an advantage under specific conditions to differentiate from wild type. Conditions are severe enough that mutants are easily detectable.
Screening: examining all colonies for a specific trait (without providing selective advantage) to differentiate mutants from wildtype. Difficult and time consuming.
What are selectable mutations?
Mutation that provides growth advantage to mutant strain under specific environmental conditions, allowing its progeny to outcompete and replace the parent strain.
What are non-selectable mutaitons?
Mutations that neither provides an advantage or disadvantage over parent cells when grown in laboratory conditions. May be detected by screening morphologies for differences.
What is the difference between a nutritional auxotroph and prototroph?
auxotroph: A mutant strain that has an additional nutritional requirement compared to the parental strain from which it was derived. Written as (nutrient^-)
prototroph: A wild-type strain that can synthesize all required nutrients without needing additional growth factors beyond what is normally supplied in minimal medium. Written as (nutrient^+)
What are point mutations?
Mutations that change only one base pair, includes transitions (change within base category A-G, C-T) and transversions (change between base category, A/G-C/T)
What are silent mutations?
Point mutation in coding region of DNA results in codon that codes for the same amino acid as the wildtype. Results in normal protein.
What are missense mutations?
Point mutation in coding region of DNA result in codon that codes for a different amino acid than the wildtype. Impact on folding depends on location of amino acid and chemistry.
What is a nonsense mutation?
Point mutation in coding region of DNA that result in codon that codes for a stop rather instead of the wildtype amino acid. Unless occuring at the end of the gene, the product is incompletely made and truncated proteins are inactive or lack normal activity.
What is a frameshift mutation?
Mutation caused by an insertion or deletion that alters the reading frame by shifting to the right (+) or the left (-)
What is reversion?
The spontaneous/induced back mutation to reverse a point mutation and restore the original wild-type phenotype, producing a revertant.
What are two types of revertants?
- Same site revertant: mutation that restores activity is at the same site as the original mutation. aka true revertant if it restores the original sequence and phenotype.
- Second site revertant: mutation is at a different site in DNA that may compensate for the effect of the original mutation as a suppressor.
What are three classes of suppressor mutations?
- Mutation somewhere else in the same gene that restores enzyme function.
- A mutation in another gene that restores the function of the product of the original mutated gene.
- mutation in another gene that results in the production of an enzyme that can replace the nonfunctional one
What is a tRNA suppressor mutation and how does it rescue nonsense and missense mutations?
Type of second site suppressor mutation that happens in a tRNA anticodon.
Nonsense mutations: binds to the mutated stop codon and brings in its designated amino acid instead
Missense mutations: binds to mutated amino acid codon and brings in a functionally similar one.
Why are tRNA mutations possibly lethal? Why isn’t it?
If a tRNA mutated into a suppressor, it would no longer bind to its original codon, and the amino acid wouldn’t be incorporated where its needed. It isn’t lethal because most microorganisms have multiple tRNA genes for the same amino acid.
What are macrolesions and examples?
Macrolesions are changes in large segments of DNA that can span 1 or more genes. Includes:
- Large insertions or deletions
- Duplications: segment of DNA is repeated at different site
- Translocations: segment of DNA is moved to a different location on the same strand
- Inversions: segment is removed and inserted in the opposite direction (opposite strand)
What are types of spontaneous mutations?
- Apurinic/Apyrimidinic sites
- Deamination
- Tautomeric shift
- Insertions and deletions
What are Apurinic/Apyrimidinic sites? How do they occur?
Loss of nitrogenous bases as a result of the spontaneous hydrolysis of glycosyl bond between base and deoxyribose. This is typically corrected in DNA repair, but if not, A MUTATION IS FORMED by the addition of ANY base via DNA polymerase.
What is deamination?
The loss of amino group from cytosine, adenine and guanine (NOT thymine/uracil)
What are the results of deamination?
- Adenine becomes hypoxanthine, and pairs with cytosine instead of thymine
- Guanine becomes xanthine, pairs with cytosine as normal
- Cytosine becomes uracil, pairs with Adenine instead of guanine.
