(3) What causes mutations?

Completely random mutations are relatively uncommon, in DNA code copying, it is estimated that there is a 1 in 10⁹ (1 in a billion) chance of a mutation, though other factors can come into play to increase this e.g. exposure ionising radiation and ingesting carcinogenic molecules..

Mutations are happening all the time and can occur quite spontaneously - a random event.

There are various different ways that the rate of mutations can increase and change the base sequence in DNA e.g.

A mutation can happen if an error occurs in chromosome replication (DNA replication) i.e. it might not be as perfect as that shown in the diagram below (see DNA and RNA structure and Protein Synthesis).

The chance of a mutation is increased if an organism is exposed to certain chemicals, particularly those known as carcinogenic substances e.g. some constituents of tobacco tar.

A carcinogen is defined as any substance (e.g. carcinogenic chemical), radionuclide, or radiation that promotes carcinogenesis, the formation of cancer.

This may be due to the ability of the substance or radiation to damage the genome or to the disruption of cellular metabolic processes.

Some molecules cause mutations by interfering with the unzipping of DNA and producing errors in the replication.

Radiation from radioactive materials is particularly effective in causing mutations, hence the dangers associated with exposure to alpha, beta and gamma ionising radiations.

The energy of the particles is great enough to break chemical bonds, inducing changes in the molecular structure of DNA.

See Alpha, beta & gamma radiation -  ,dangers of radioactive emissions - health and safety issues and ionising radiation gcse physics revision notes

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