A mutation is a permanent alteration in the sequence of nitrogenous bases of a DNA molecule. The result of a mutation is generally a change in the end-product specified by that gene. In some cases, a mutation can be beneficial if a new metabolic activity arises in a microorganism, or it can be detrimental if a metabolic activity is lost.
Types of mutations. The most common type of mutation involves a single base pair in the DNA molecule and is known as a point mutation. In this case, a different base is substituted for the normal base, thus altering the genetic code. Should a new amino acid be substituted in the final protein, the mutation is known as missense mutation. Certain mutations change the genetic code and destroy the information it contains. Such a mutation is referred to as a nonsense mutation.
In another type of cell mutation, a frameshift mutation, pairs of nucleotides are either added to or deleted from the DNA molecule, with the result that the “reading frame” is shifted. The amino acid sequence in the resulting protein changes as a result of this frameshift. If a mutation occurs without laboratory intervention, it is a spontaneous mutation; if it occurs as a result of laboratory intervention, it is an induced mutation.
Mutagens. Physical and chemical agents capable of bringing about mutations are called mutagens. Chemical mutagens include nitrous acid. This substance converts adenine to hypoxanthine, a molecule that will not pair with thymine, and thus interrupts the genetic code. A base analog is a chemical mutagen that resembles a nitrogenous base and is incorporated by error into a DNA molecule. Such a DNA molecule cannot function in protein synthesis. Certain dyes and fungal toxins (for example, aflatoxin) are known to be mutagens.
Physical mutagens include X rays, gamma rays, and ultraviolet light. X rays and gamma rays break the covalent bonds in DNA molecules, thereby producing fragments. Ultraviolet light binds together adjacent thymine bases, forming dimers. These dimers cannot function in protein synthesis, and the genetic code is thereby interrupted. Radiation damage can be repaired by certain bacterial enzymes, a process known as photoreactivation.
The probability of a mutation occurring during cellular division is known as the mutation rate. In bacteria, the spontaneous mutation rate is about one in a billion reproductions. This factor implies that in every population of a billion cells, there is at least one mutant. This mutant organism may never express its mutation. However, for example, if the mutation renders antibiotic resistance, then the mutants will survive when an antibiotic is applied to the population, and a new colony of antibiotic-resistant bacteria will emerge.
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