Mutations And Variation Flashcards
What is a mutation?
- A change in the base sequence of DNA.
What is a point mutation?
- Only one nucleotide affected.
- Changes codon in which it occurs.
- Caused by substitution.
What are the possible repercussions of a point substitution?
Damaging:
- The new codon could code for a new amino acid. If the new AA is different enough to the original one then the protein as a whole could be affected. Could be non-functional or inactive.
No effect:
- Could have no effect: degenerate code, or new AA is similar enough to old one (variable group) that protein is not impacted enough to affect function.
- Could cause premature stop codon.
- The new AA produced may be on a part of the protein that doesn’t have an impact on its function (e.g not on an enzyme’s active site).
Beneficial:
- Rare, may give a new and useful characteristic in the phenotype.
What is a frameshift mutation?
- Insertion or deletion of a nucleotide into the DNA.
- Can change whole protein, as triplet code means codons are always read as non-overlapping groups of three.
- Shifts the reading frame, will change every successive codon from the point of mutation.
What are the possible repercussions of a frameshift substitution?
No effect:
- Unlikely, unless the number of nucleotides inserted/deleted is a multiple of 3. Even so could still have a negative effect.
Damaging:
- Likely, could change entire primary structure of the protein, and therefore 2 and 3. Therefore change its ability to perform its function. Inactive/non-functioning.
Beneficial:
- V. Unlikely.
What is an example of a beneficial mutation in humans?
- Some people have a protein present in the cell surface membranes that prevents HIV viral agents binding to and entering their cells. This results in immunity to HIV.
- Lactose digestion. Post-suckling most mammals in the world become lactose intolerant. This ability prevents osteoporosis and provides an alternative food source during food scarcity.
What are the causes of mutation?
- Mutagens: physical, chemical, biological.
What are some examples of physical mutagens? How do they mutate DNA?
- ionising radiation such as X-rays.
- break one or both DNA strands - some breaks can be repaired but mutations can occur in the process.
What are some examples of chemical mutagens? How do they mutate DNA?
- Deaminating agents.
- Chemically alter bases in DNA such as converting C to U in DNA, changing the base sequence.
What are some examples of biological mutagens? How do they mutate DNA?
- Alkylating agents: methyl or ethyl groups attached to bases resulting in the incorrect pairing of bases during replication.
- Base analogs: incorporated into DNA in place of the usual base during replication, changing the base sequence.
- Viruses: viral DNA may insert itself into a genome, changing the base sequence.
Why are the vast majority of mutations ‘silent’/neutral?
- Have no effect on organism’s phenotype.
- Can occur in intron (non-coding DNA).
- Could result in same AA being produced due to degenerate code.
- Primary structure changes may occur, but not affect the overall structure or function of the proteins synthesised.
What is a nonsense mutation?
- Nonsense result in a codon becoming a stop codon instead of an AA-coding codon.
- The result is a shortened protein being synthesised which is normally non-functional.
- These mutations normally have negative or harmful effects on phenotypes.
What is a missense mutation?
- Missense mutations result in the incorporation of an incorrect AA(s) into the primary structure when the protein is synthesised.
- The result depends on the role the AA plays in the structure, and therefore function, of the protein synthesised. The mutation could be silent, beneficial, or harmful.
- A conservative mutation occurs when the base change leads to an AA being coded for which has similar properties to the original. Less severe effect.
- Non-conservative mutation: opposite, different properties. More likely to have an effect and cause problems.
What is a deletion chromosome mutation?
- A section of chromosome breaks off and is lost within the cell.
What is a duplication chromosome mutation?
- Sections get duplicated on a chromosome.
