Mutations - When DNA Changes

Mutations are rare because DNA is a very stable molecule for several reasons:

1. Because DNA is normally made of 2 complementary strands (double helix) repairs can be made when one side or the other is damaged, but not if both sides are damaged.
2. All the potentially reactive side groups of the sugar molecules making up the backbone of each DNA strand are already covalently bonded
3. The nucleotide bases lie protected inside the sugar-phosphate backbone, their potentially reactive side groups out of harms way secured by hydrogen bonds.
4. Finally DNA's helix and nucleosome coils provide a geometric tightness which deters most chemical reactions from taking place at normal temperatures.

Although DNA is very resistant to change -- change it must if organisms are to adapt to fluctuations in their environment. Natural Selection must have something to select. Thus mutation is a necessary and important part of evolution.

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The Cause of Mutations

Mutations are rare since the replication process is very accurate and reliable. Most organisms have evolved repair mechanisms -- enzymes which scour the DNA in search of errors to correct. But errors do occur because of powerful forces and chemicals which alter one or more of the nucleotide pairs in DNA.

Any agent that causes a mutation is called a mutagen. Mutagens can be physical or chemical in nature. Typical mutagens are:

• Physical:
  • gamma and X-rays
  • ultraviolet radiation
  • subatomic particles (high energy electrons)
• Chemical:
  • cigarette smoke
  • asbestos
  • free radicals and oxidizing agents
  • nucleotide look-a-like molecules

No matter what the mutagen the end result is a chemical change in DNA -- a primary lesion.

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Levels of mutation

There are 3 levels of mutation.

1. point mutations which affect individual nucleotides
2. chromosomal mutations affecting entire chromosomes
3. transpositions -- insertion of copies of DNA into new positions in the genome.

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