Mutations
Historically,
one reason for the study of genetics was to discover the chemical basis of
heredity. Naturally, the existence of mutations was necessary to the execution
of the classical experiments in genetics, and an understanding of mutations
will facilitate our study of these experi-ments. We have already covered the
chemical basis of heredity and the basics of gene expression. Perhaps here we
should explicitly state that a gene refers to a set of nucleotides that
specifies the sequence of an RNA or protein. We will define mutation and in the
next section mention the three basic types of mutations. In the following
section we will review the classical genetic experiments before turning to
recombina-tion.
A
mutation is merely an inheritable alteration from the normal. It is an
alteration in the nucleotide sequence of the DNA or, in the case of RNA
viruses, an alteration in the nucleotide sequence of its genomic RNA. We
already know that changes in coding portions of DNA may alter the amino acid
sequences of proteins and that changes in noncod-ing regions of DNA have the
potential for changing the expression of genes, for example by altering the
strength of a promoter. Of course, any cellular process that makes use of a
sequence of DNA can be affected by a mutation. The existence of mutations
implies that the sequence of DNA in living things, including viruses, is
sufficiently stable that most individuals possess the same sequence but
sufficiently unstable that alterations do exist and can be found.
The terms
wild-type, mutant, mutation, and allele are closely related but must be
distinguished. Wild-type is a reference, usually found naturally. It can mean
an organism, a set of genes, a gene, a gene product like a protein, or a
nucleotide sequence. A mutation is an inheritable change from that reference. A
mutant is the organism that carries the mutation. Two mutations are said to be
allelic if they lie in the same gene. However, now that genes can be analyzed
at the nucleotide level, in some situations alleles refers to nucleotides
rather than to genes.
Until it
became possible to sequence DNA easily, mutations could readily be identified
only by their gross effects on the appearance of the cell or the shape, color,
or behavior of an organism. Some of the most easily studied biological effects
of mutations in bacteria and viruses were changes in the colony or plaque
morphology. Other easily studied effects of mutations were the inability of
cells to grow at low or high temperatures or the inability to grow without the
addition of specific chemicals to the growth medium. Such readily observed
properties of cells constitute their phenotype. The status of the genome giving
rise to the phenotype is called the genotype. For example, the Lac-
phenotype is the inability to grow on lactose. It can result from mutations in lactose
transport, β-galactosidase
enzyme, lac gene regulation, or the
cells’ overall regulation of classes of genes that are not well induced if
cells are grown in the presence of glucose. Such cells would have a mutation in
any of the following genes: lacY, lacZ, lacI, crp, or cya.
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