what is a gene mutation?
a gene mutation is a change in the sequence of bases in a DNA molecule
what can mutations result in?
mutations may result in an altered polypeptide; as the DNA base sequence of a gene determines the sequence of amino acids that make up a polypeptide, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for.
what is a polypeptide?
a polymer consisting of a large chain of amino acids bonded together by peptide bonds.
when do mutations occur?
mutations occur spontaneously during DNA-replication
what are the ways in which a mutation in the DNA base sequence could occur?
- substitution
- insertion
- deletion
what are substitution, insertion, and deletion mutations an example of?
- they are all examples of point mutation, which are mutations that involve a change in the DNA base sequence at a single location
how can other types of mutations affect genes or entire chromosomes?
- genes can be replicated or lost
- chromosomes can be divided unequally during meiosis, resulting in cells with extra or missing chromosomes
substitution of nucleotides
a mutation that occurs when a base in the DNA sequence is randomly swapped for a different base
what will a substitution mutation change?
a substitution mutation will only change the amino acid for the triplet in which the mutation occurs, and will have no impact on triplets located elsewhere in the gene
what does substitution mutation include?
- silent mutations
. the mutation does not alter the amino acid sequence the polypeptide; this is due to the degenerate nature of the genetic code - missense mutations
. the mutation alters a single amino acid in the polypeptide chain, e.g. sickle cell anaemia is caused by a single substitution mutation changing a single amino acid in the haemoglobin protein - nonsense mutations
. the mutation creates a premature stop codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function, e.g. cystic fibrosis can be cased by a nonsense mutation
> a stop codon provides a signal for the cell
translation of the mRNA molecule into an
amino acid sequence
insertion of nucleotides
a mutation that occurs whom nucleotide is randomly inserted into the DNA sequence is known as an insertion mutation
what would insertion mutation change?
it changes the amino acid that would have been coded for by the original base triplet, as it creates, a new different triplet of bases
> every group of three bases in a DNA sequence codes for an amino acid
what effect does an insertion mutation have?
a knock-on effect on other base triplets by changing the triplets further on in the DNA sequence
> this means that insertion mutations cause what is known as frameshift mutation; they don’t only change the triplet where the insertion has occurred, but every triplet downstream of the insertion
- this may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function
deletion of nucleotides
a mutation occurs when a nucleotide is randomly deleted from the DNA sequence
what does deletion mutation change?
deletion mutation changes the triplet in which the deletion has occurred, and also changes every group of three bases further on in the DNA sequence
> this is known as frameshift mutation
> This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function
do mutations alter the polypeptide?
most mutations do not alter he polypeptide or only alter it slightly so that its appearance or function is not changed
how is it possible for mutations to not alter the polypeptide?
it is possible because the genetic code is degenerate; the base sequence can be changed without necessarily altering the amino acid
how is the small number of mutations code for a significantly altered polypeptide?
- a mutation changes the DNA base sequence
- one or many amino acids in the primary structure of a protein is altered
- different bonds form in the secondary and tertiary structures of the protein
- the final 3D structure of the protein is altered
what advantage can a significantly altered polypeptide results in?
very rarely this can give rise to protein that provides an organism with an advantage, e.g. resistance to an antibiotic, or the ability to digest a new type of food.
how can mutations that provide an advantage drive the process of evolution by causing natural selection to occur?
- individuals with an advantage are more likely to survive and reproduce
- the advantageous mutation is more likely to be passed on
- the mutation becomes more common in the population
how are mutations that affect polypeptide structure likely to be harmful?
they affect the ability of proteins to perform their function, e.g.
- in cystic fibrosis, a membrane channel protein no longer functions
> a fault in the CFTR gene leads to production of non-functional chloride channels, reducing the movement water by osmosis into cell secretions
> this results in the production of thick, sticky mucus in the air passages, the digestive tract and the respiratory system
what causes sickle-cell disease?
the haemoglobin protein no longer functions
> sickle -cell disease is caused by a single substitution mutation that causes haemoglobin proteins to clump together
> this affects the shape of red blood cells, preventing easy blood flow and causing severe pain and problems with blood supply to important organs
what would happen if mutations in the genes involved with cell division occurs?
- it can lead to uncontrolled cell division and the development of tumours that can become cancerous
what would happen if mutations occur in the gametes or sex cells?
mutations could then be passed on to future generations, meaning that every cell in the body of an organism’s offspring will contain the mutation
