Mutations - When DNA Changes
Mutations which occur in normal body cells (somatic
cells) do not affect offspring (at least not directly). Only
mutations in gametes (egg or sperm) are inherited.
Mutations are rare because DNA is a very stable
molecule for several reasons:
- 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
- All the potentially reactive side groups of
the sugar molecules making up the backbone of each DNA strand are
already covalently bonded
- The nucleotide bases lie protected inside the
sugar-phosphate backbone, their potentially reactive side groups
out of harms way secured by hydrogen bonds.
- 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.
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:
- gamma and X-rays
- ultraviolet radiation
- subatomic particles (high energy
- cigarette smoke
- free radicals and oxidizing
- nucleotide look-a-like
No matter what the mutagen the end result is a
chemical change in DNA -- a primary lesion.
Levels of mutation
There are 3 levels of mutation.
mutations which affect individual
mutations affecting entire
-- insertion of copies of DNA into new positions in the
To learn more about mutation, go to Mutations,
Mutagenesis, and Carcinogenesis.