6.1.1 Gene Mutations Flashcards
What is a 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 do mutations occur?
Mutations occur continuously.
Can mutations occur spontaneously?
Mutations can occur spontaneously (i.e. for no reason) during DNA replication
When can the probability of a mutation occurring increase?
The probability of a mutation occurring can increase with the presence of certain factors known as mutagens,
What is a mutagen?
An agent, such as radiation or a chemical substance, which causes genetic mutation.
What are some examples of mutagens?
Ionising radiation such as X-rays can break the DNA strands which can then be altered during the repair process
Deaminating chemicals can alter the chemical structure of bases, converting one base into another
Methyl or ethyl groups can be added to bases, leading to incorrect base pairing
Viruses can insert sections of viral DNA into the DNA of cells
Why do these mutations usually have no effect on us?
As most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed (as the genetic code is degenerate i.e. several different triplets often code for the same amino acid)
Many mutations occur in non-coding sections of DNA and so have no effect on the amino acid sequence at all
When can a mutation in a gene sometimes lead to a change in the polypeptide?
Insertion of one or more nucleotides
Deletion of one or more nucleotides
Substitution of one or more nucleotides
Describe what happens when there is an insertion of nucleotides?
A mutation that occurs when a nucleotide (with a new base) is randomly inserted into the DNA sequence is known as an insertion mutation
An insertion mutation changes the amino acid that would have been coded for by the original base triplet, as it creates a new, different triplet of bases
Remember – every group of three bases in a DNA sequence codes for an amino acid
An insertion mutation also has a knock-on effect by changing the triplets (groups of three bases) further on in the DNA sequence
This is sometimes known as a frameshift mutation
This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function
Describe the deletion of nucleotides.
A mutation that occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence
Like an insertion mutation, a deletion mutation changes the amino acid that would have been coded for
Like an insertion mutation, a deletion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence
Like an insertion mutation, this is sometimes known as a frameshift mutation
This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function
Describe the substitution of nucleotides.
A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base
Unlike an insertion or deletion mutation, a substitution mutation will only change the amino acid for the triplet (a group of three bases) in which the mutation occurs; it will not have a knock-on effect
What three forms can substitution mutations take?
Silent mutations – the mutation does not alter the amino acid sequence of the polypeptide (this is because certain codons may code for the same amino acid as the genetic code is degenerate)
Missense mutations – the mutation alters a single amino acid in the polypeptide chain (sickle cell anaemia is an example of a disease caused by a single substitution mutation changing a single amino acid in the sequence)
Nonsense mutations – the mutation creates a premature stop codon (signal for the cell to stop translation of the mRNA molecule into an amino acid sequence), causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function (cystic fibrosis is an example of a disease caused by a nonsense mutation, although this is not always the only cause)
What are beneficial mutations?
A small number of mutations result in a significantly altered polypeptide with a different shape
This may alter the ability of the protein to perform its function. For example:
If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site
A structural protein (like collagen) may lose its strength if its shape changes
In some cases, this alteration to a polypeptide may actually result in an altered characteristic in an organism that causes beneficial effects for the organism
In these cases, the original mutation is referred to as a beneficial mutation
What are harmful mutations?
By altering a polypeptide, some mutations can lead to an altered characteristic in an organism that causes harmful effects for the organism
In these cases, the original mutation is referred to as a harmful mutation
Many genetic diseases are caused by these harmful mutations (e.g. haemophilia and sickle cell anaemia)
What are neutral mutations?
Neutral mutations offer no selective advantage or disadvantage to the individual organism
This can occur either because:
A mutation does not alter the polypeptide
A mutation only alters the polypeptide slightly so that its structure or function is not changed
A mutation alters the structure or function of the polypeptide but the resulting difference in the characteristic of the organism provides no particular advantage or disadvantage to the organism
