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The chemical agents which may cause mutation are called mutagens.
Most of them are also carcinogens. The following figure shows the chemical
structures of a few potent mutagens. Figure 7-F-7
explains the mechanism of mutation by 5-bromouracil.
Figure 7-F-6. Structures of potent
mutagens.
- Acridines (e.g., proflavin) are positively charged molecules. They may
be inserted between two DNA strands, thereby altering DNA's structure and
rigidity. As a result, DNA replication will not be faithful.
- Alkylating agents are chemicals that add an alkyl group (CnH2n+1)
to another molecule. Alkylation of a base may change the normal base
pairing. For example, the alkylating agent EMS converts guanine to
7-ethylguanine which pairs with thymine. The mispairing will lead to
mutation. Some alkylating agents may also cross-link DNA, resulting in
chromosome breaks.
- Nitrous acid is a deaminating agent that converts cytosine to uracil, adenine
to hypoxanthine, and guanine to xanthine. The hydrogen-bonding potential
of the modified base is altered, resulting in mispairing.
- Hydroxylamine and free radicals may modify base structures,
resulting in mispairing.
Figure 7-F-7. Mechanism of mutation induced by
5-bromouracil. This molecule has two tautomeric isoforms. Its keto
form (BUk) pairs with adenine whereas its enol form (BUe)
pairs
with guanine. Suppose in the first replication the keto form was
incorporated into a new DNA strand. During the second replication, if the
keto form undergoes a tautomeric shift to the enol form, it will cause A:T to G:C
mutation.
Figure 7-F-8. (a) Base structures induced
by free radicals. (b) The base change induced by NH2OH.
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