If the mutated gene is used for protein synthesis, the mRNA transcribed from
the gene carries the incorrect base for that position. The mRNA may insert an
incorrect amino acid in the protein. If this happens, the mutation is called a
missence mutation.
Mutations that change or destroy the genetic code are called nonsense muta-
tions. If nucleotides are added or deleted from mRNA, the mutation is called a
frame shift mutation.
A mutation occurring in the laboratory is called an induced mutation;muta-
tions occurring outside the laboratory are called spontaneous mutation. A spon-
taneous mutation occurs when a mutation-causing agent is present.
Base substitution and frame shift mutations occur spontaneously. Agents in
the environment or those introduced by industrial processing can directly or
indirectly cause mutations. These agents are called mutagens. Any chemical
or physical agent that reacts with DNA can potentially cause mutations.(^126) CHAPTER 7 Microbial Genetics
Type of Mutation Description
Point mutation Also known as base substitution, this is the most common
type of mutation and involves a single base pair in the
DNA molecule. In point mutation, a different base is sub-
stituted for the original base, causing the genetic code to
be altered. The substituted base pair is used when DNA is
replicated.
Missence mutation A mutation when a new amino acid is substituted in the
final protein by the messenger RNA during transcription.
Nonsense mutations A mutation when a terminator codon in the messenger
RNA appears in the middle of a genetic message instead
of at the end of the message, which causes premature
termination of transcription.
Frame shift mutation Pairs of nucleotides are either added or removed from a
DNA molecule.
Loss-of-function mutation This mutation causes a gene to malfunction.
Spontaneous mutation Naturally occurring mutation that happens without the
presence of a mutation-causing agent.
Induced mutation Induced in a laboratory.
Table 7-2. Types of Mutations