by M E Skeel

Created on: January 29, 2011   Last Updated: January 31, 2011

Mutations are changes in the DNA.  There are three main causes of mutations:  duplication and transcription errors, damage to the DNA by chemicals, and damage by radiation.  Although mutations can affect any cell in the body, it is mutations to sex cells, sperm and eggs, that can be passed on to the next generation.  Damage to body cells can harm the individual but will not be passed on to offspring.

Whenever a cell divides, the DNA must be copied so that both cells have complete sets of DNA.  This process is called mitosis.  When sex cells are produced, which contain only half the DNA of the parent, a more complicated process called meiosis occurs.  In either process, mistakes can be made.  A piece of DNA may be inverted, deleted or changed.  In each case, the result is a cell with a different set of instructions than the original cell, in other words, a mutation.  In most cases, these changes will mean that instead of making a protein correctly, the instructions are now wrong and either nothing will be made or a different protein will be constructed.  Since each protein has specific uses in the cell and the body, these changes can be deleterious but occasionally a new product is formed that has some new use and thus these mutations, if they occur in the sex cells, can lead to evolutionary change.  Luckily most mutations have no effect because they occur in parts of the DNA that do not code for anything or they occur in a duplicate set of instructions so the correct protein can still be constructed elsewhere.

Transcription errors occur when the DNA instructions are passed on to the messenger RNA molecules.  If the code is incorrectly transmitted because of the substitution of an incorrect base in the newly constructed RNA molecule, the mistake can have similar effects:  either the protein is not made or a different protein is constructed and a mutation has occurred.  As with DNA duplication errors, most of these are harmless or harmful and very, very few lead to a positive change.

Although mutations can happen spontaneously because of mistakes in DNA duplication and transmission, not all mutations occur this way.  Both chemicals and radiation can cause mutations.  Again, for these to affect future generations, they must occur in the sex cells, but both chemicals and radiation can harm the individual by affecting other cells in the body. 

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