Mutations (Edexcel GCSE Biology: Combined Science)

• Mutations are rare, random changes that occur in the sequence of DNA bases in a gene or a chromosome
• Mutations occur continuously
• As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the protein that the gene codes for
• Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
• There are different ways that a mutation in the DNA base sequence can occur

Insertions

• A new base is randomly inserted into the DNA sequence
• An insertion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs
  • 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 groups of three bases further on in the DNA sequence

An example of an insertion mutation

Deletions

• A 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 by the group of three bases in which the mutation occurs
• 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

Substitutions

• 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 group of three bases in which the mutation occurs; it will not have a knock-on effect

An example of a substitution mutation

• Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
• However, a small number of mutations code for a significantly altered protein with a different shape
• This may affect 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

• On rare occasions mutations lead to the development of new alleles and so new phenotypes
• Occasionally, the new allele (and its resulting phenotype) gives the individual a survival advantage over other members of the species
• For example:
  • A bird develops a mutation leading to a change in feather colours
  • This makes it more attractive to birds of the opposite sex
  • Which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation

• Mutations can also lead to harmful changes that can have dramatic effects on the body - for example, sickle cell anaemia in humans

Sickle cell anaemia

• Sickle cell anaemia was the first genetic disease to be described in terms of a gene mutation
• A gene mutation is a change in the base sequence of DNA
• The mutation changes the molecule haemoglobin, causing the red blood cells (RBC’s) to become stiff and sometimes sickle-shaped when they release oxygen to the body tissues
• The sickled cells tend to get stuck in narrow blood vessels, blocking the flow of blood
• As a result, those with sickle cell disease suffer painful “crises” in their joints and bones
• They may suffer strokes, blindness, or damage to the lungs, kidneys, or heart. They must often be hospitalized for blood transfusions and are at risk for a life-threatening complication called acute chest syndrome
• Although many sufferers of sickle cell disease die before the age of 20, modern medical treatments can sometimes prolong these individuals’ lives into their 40s and 50s

Sickle cell anaemia is caused by abnormal haemoglobin which changes the shape of red blood cells

• Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following:
  • Gamma rays, x - rays and ultraviolet rays - all types of ionising radiation which can damage bonds and cause changes in base sequences
  • Certain types of chemicals - for example chemicals such as tar in tobacco

• Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer