Mutations

A mutation is any change in genetic material that is passed on to the next generation. The process of acquiring change in genetic material forms the fundamental underpinning of evolution . Mutation is a source of genetic variation in all life forms. Depending on the organism or the source of the mutation, the genetic alteration may be an alteration in the organized collection of genetic material, or a change in the composition of an individual gene .

Mutations may have little impact, or they may produce a significant positive or negative impact, on the health, competitiveness, or function of an individual, family, or population.

Mutations arise in different ways. An alteration in the sequence, but not in the number of nucleotides in a gene is a nucleotide substitution. Two types of nucleotide substitution mutations are missense and nonsense mutations. Missense mutations are single base changes that result in the substitution of one amino acid for another in the protein product of the gene. Nonsense mutations are also single base changes, but create a termination codon that stops the transcription of the gene. The result is a shortened, dysfunctional protein product.

Another mutation involves the alteration in the number of bases in a gene. This is an insertion or deletion mutation. The impact of an insertion or deletion is a frameshift, in which the normal sequence with which the genetic material is interpreted is altered. The alteration causes the gene to code for a different sequence of amino acids in the protein product than would normally be produced. The result is a protein that functions differently—or not all—as compared to the normally encoded version.

Genomes naturally contain areas in which a nucleotide repeats in a triplet. Trinucleotide repeat mutations, an increased number of triplets, are now known to be the cause of at least eight genetic disorders affecting the nervous or neuromuscular systems.

Mutations arise from a number of processes collectively termed mutagenesis. Frameshift mutations, specifically insertions, result from mutagenic events where DNA is inserted into the normally functioning gene. The genetic technique of insertional mutagenesis relies upon this behavior to locate target genes, to study gene expression, and to study protein structure-function relationships.

DNA mutagenesis also occurs because of breakage or base modification due to the application of radiation, chemicals, ultraviolet light, and random replication errors. Such mutagenic events occur frequently, and the cell has evolved repair mechanisms to deal with them. High exposure to DNA damaging agents, however, can overwhelm the repair machinery.

Genetic research relies upon the ability to induce mutations in the lab. Using purified DNA of a known restriction map, site-specific mutagenesis can be performed in a number of ways. Some restriction enzymes produce staggered nicks at the site of action in the target DNA. Short pieces of DNA (linkers) can subsequently be introduced at the staggered cut site, to alter the sequence of the DNA following its repair. Cassette mutagenesis can be used to introduce selectable genes at the specific site in the DNA. Typically, these are drug-resistance genes. The activity of the insert can then be monitored by the development of resistance in the transformed cell. In deletion formation, DNA can be cut at more than one restriction site and the cut regions can be induced to join, eliminating the region of intervening DNA. Thus, deletions of defined length and sequence can be created, generating tailor-made deletions. With site-directed mutagenesis, DNA of known sequence that differs from the target sequence of the original DNA, can be chemically synthesized and introduced at the target site. The insertion causes the production of a mutation of pre-determined sequence. Site-directed mutagenesis is an especially useful research tool in inducing changes in the shape of proteins, permitting precise structure-function relationships to be probed. Localized mutagenesis, also known as heavy mutagenesis, induces mutations in a small portion of DNA. In many cases, mutations are identified by the classical technique of phenotypic identification—looking for an alteration in appearance or behavior of the mutant.

Mutagenesis is exploited in biotechnology to create new enzymes with new specificity. Simple mutations will likely not have as drastic an effect as the simultaneous alteration of multiple amino acids. The combination of mutations that produce the desired three-dimensional change, and so change in enzyme specificity, is difficult to predict. The best progress is often made by creating all the different mutational combinations of DNA using different plasmids , and then using these plasmids as a mixture to transform Escherichia coli bacteria . The expression of the different proteins can be monitored and the desired protein resolved and used for further manipulations.

See also Cell cycle (eukaryotic), genetic regulation of; Cell cycle (prokaryotic), genetic regulation of; Chemical mutagenesis; Chromosomes, eukaryotic; Chromosomes, prokaryotic; DNA (Deoxyribonucleic acid); Laboratory techniques in immunology; Mitochondrial DNA; Mitochondrial inheritance; Molecular biology and molecular genetics