Genetic Mutations (A Level only) (AQA A Level Biology): Flashcards
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Define the term gene mutation
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Define the term gene mutation
A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide.
When and how do gene mutations occur?
Mutations occur continuously and spontaneously.
Errors in the DNA often occur during DNA replication.
Name the six types of change to a DNA base sequence that can cause a gene mutation
Insertion
Deletion
Substitution
Inversion
Duplication
Translocation
What is a substitution mutation?
A substitution mutation occurs when a base in the DNA sequence is randomly swapped for a different base.
A substitution mutation only changes the amino acid for the in which the mutation occurs; it does not have a effect.
A substitution mutation only changes the amino acid for the triplet in which the mutation occurs; it does not have a knock-on effect.
What are the three forms a substitution mutation can take?
Silent mutation – does not alter the amino acid sequence of the polypeptide (because the genetic code is degenerate)
Missense mutation – alters a single amino acid in the polypeptide chain
Nonsense mutation – creates a premature stop codon, producing an incomplete polypeptide
Why does an insertion mutation have such a large effect on the polypeptide produced?
It creates a new, different triplet of bases.
It also has a knock-on effect, changing all the triplets further on in the DNA sequence (a frameshift).
This may dramatically change the amino acid sequence and the ability of the polypeptide to function.
True or False?
A deletion mutation causes a frame-shift by changing the groups of three bases further on in the DNA sequence
True.
Like an insertion, a deletion mutation causes a frame-shift, changing all subsequent triplets and possibly the ability of the polypeptide to function.
What happens during an inversion mutation?
The DNA of a single gene is cut in two places, and the cut portion is inverted 180° before being rejoined to the same place within the gene.
A large section of the gene is 'backwards', so multiple amino acids are affected, frequently resulting in a non-functional protein.
Why is a duplication mutation often not harmful, and why is it important?
The original version of the gene remains intact, so the mutation is not harmful.
Over time the second copy can undergo mutations enabling it to develop new functions, making duplication an important source of evolutionary change.
In a translocation mutation, a section of a gene is cut off and attaches to a gene.
In a translocation mutation, a section of a gene is cut off and attaches to a separate gene.
What is the effect of most mutations on the polypeptide produced?
Most mutations do not alter the polypeptide, or only alter it slightly, so that its structure or function is not changed.
Why do some mutations not cause a change in the amino acid sequence?
Because the genetic code is degenerate, some mutations do not change the amino acid coded for.
Because they change only one triplet, mutations usually have a smaller effect on the resultant polypeptide than insertions or deletions.
Because they change only one triplet, substitution mutations usually have a smaller effect on the resultant polypeptide than insertions or deletions.
Why do insertion and deletion mutations often produce a non-functional polypeptide?
They change all base triplets downstream of (after) the mutation.
This frameshift alters the amino acid sequence so much that a non-functional polypeptide is produced.
How can a mutation that produces a significantly altered polypeptide affect a protein's function?
The polypeptide may have a different shape.
If the shape of an enzyme's active site changes, the substrate may no longer be able to bind.
A structural protein (like collagen) may lose its strength if its shape changes.
Using the TYR gene, explain how a mutation can affect an organism's phenotype
A mutation in the TYR gene affects the structure of an enzyme needed to produce the pigment melanin.
The lack of melanin affects the phenotype, causing albinism (very pale skin and often white hair).
What is a mutagenic agent?
A mutagenic agent is a factor that increases the frequency (rate) of mutations in DNA.
Name and describe the three types of mutagenic agent
Physical mutagens – cause mutations by physical means, typically ionising radiation (e.g. X-rays, gamma rays)
Chemical mutagens – interfere with the DNA molecule chemically
Biological mutagens – organisms or biological molecules that interfere with DNA, such as viruses (e.g. HPV)
True or False?
A mutagenic agent and a carcinogen are exactly the same thing
False.
A carcinogen causes cancer, whereas a mutagen causes DNA mutations. Some carcinogens do cause mutations, but a mutagen can cause mutations without leading to cancer.
The rate at which mutations occur can be estimated as around one mutation per genes per generation.
The rate at which mutations occur can be estimated as around one mutation per 100,000 genes per generation.
During which process, and how predictably, do mutations occur?
Mutations occur spontaneously and randomly during DNA replication.
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