Mutation (Cambridge O Level Biology)

• The term mutation can be defined as follows:

A random change in the DNA base sequence

• Most mutations have no effect on the phenotype of an organism, as the protein for which a gene codes may work just as well as the protein from the non-mutated gene
• Mutations can lead to harmful changes that can have dramatic effects on the body, for example sickle cell anaemia in humans

Mutation example: sickle cell anaemia

• The mutation that causes sickle cell anaemia occurs in the gene that codes for haemoglobin, and so affects the structure of the haemoglobin protein
• Individuals with this mutation have red blood cells that are stiff and sickle-shaped
• 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 when blood flow to these regions is insufficient
  • They may suffer strokes, blindness, or damage to the lungs, kidneys, or heart.
  • They must often be hospitalised for blood transfusions

Sickle cell anaemia diagram

Sickled red blood cells are unable to bend, so can get stuck in the blood vessels,  blocking blood flow to important organs

Chromosome mutations

• Chromosome mutations occur when there is a change in the number or structure of chromosomes
  • E.g. Down's syndrome is a genetic condition in which individuals have 47 chromosomes rather than 46; this occurs when the chromosomes do not fully separate during meiosis
    • Down's syndrome is sometimes referred to as Down syndrome, or as Trisomy 21

Individuals with Down's syndrome have an extra copy of chromosome 21

Mutation as a source of genetic variation

• Mutation contributes to genetic variation in populations, as it can give rise to new alleles which may be passed on to offspring
• Other sources of genetic variation are:
  • Meiosis
    • The cell division that gives rise to gametes produces daughter cells which contain new combinations of alleles
  • Random mating
    • The selection of a mate within a population may occur at random, and will result in offspring with a new combination of alleles
  • Random fertilisation
    • Only one male cell will fuse with a female cell during fertilisation, determining the combination of alleles in the fertilised egg cell

Factors that affect mutation rate

• Mutations happen spontaneously, meaning that they do not need a specific trigger event, but their frequency can be increased by exposure to:
  • Ionising radiation, e.g. gamma rays, x-rays and ultraviolet rays
  • Some chemicals, e.g. chemicals such as tar in tobacco
• Increased rates of mutation can increase the probability that cells may become cancerous, which is why the factors above are linked to increased incidence of cancer