Any alteration in the nucleotide sequence that spells out a genetic message in DNA is known as a mutation. Certain chemicals and various forms of ionizing radiation, such as x-rays, cosmic rays, and atomic radiation, can break the chemical bonds in DNA. This can result in the loss of segments of DNA or the incorporation of the wrong base when the broken bonds are reformed. Environmental factors that increase the rate of mutation are known as mutagens. Even in the absence of environmental mutagens, the mutation rate is never zero. In spite of proofreading, some errors are made during the replication of DNA, and some of the normal compounds present in cells, particularly reactive oxygen species, can damage DNA, leading to mutations.
Types of Mutations The simplest type of mutation, known as a point mutation, occurs when a single base is replaced by a different one. For example, the base sequence C-G-T is the DNA code word for the amino acid alanine. If guanine (G) is replaced by adenine (A), the sequence becomes C-A-T, which is the code for valine. If, however, cytosine (C) replaces thymine (T), the sequence becomes C-G-C, which is another code for alanine, and the amino acid sequence transcribed from the mutated gene would not be altered. On the other hand, if an amino acid code is mutated to one of the three termination code words, the translation of the mRNA message will cease when this code word is reached, resulting in the synthesis of a shortened, typically nonfunctional protein.
Assume that a mutation has altered a single code word in a gene, for example, alanine C-G-T changed to valine C-A-T, so that it now codes for a protein with one different amino acid. What effect does this mutation have upon the cell? The answer depends upon where in the gene the mutation has occurred. Although proteins are composed of many amino acids, the properties of a protein often depend upon a very small region of the total molecule, such as the binding site of an enzyme. If the mutation does not alter the conformation of the binding site, there may be little or no change in the protein's properties. On the other hand, if the mutation alters the binding site, a marked change in the protein's properties may occur. Thus, if the protein is an enzyme, a mutation may change its affinity for a substrate or render the enzyme totally inactive. To take another situation, if the mutation occurs within an intron segment of a gene, it will have no effect upon the amino acid sequence coded by the exon segments (unless it alters the ability of the intron segment to undergo normal splicing from the primary RNA transcript).
In a second general category of mutation, single bases or whole sections of DNA are deleted or added. Such mutations may result in the loss of an entire gene or group of genes or may cause the misreading of a sequence of bases. Figure 5-14 shows the effect of removing a single base on the reading of the genetic code. Since the code is read in three-base sequences, the removal of one base not only alters the code word containing that base, but also causes a misreading of all subsequent bases by shifting the reading sequence. Addition of an extra base causes a similar misreading of all subsequent code words, which often results in a protein having an amino acid sequence that does not correspond to any functional protein.
What effects do these various types of mutation have upon the functioning of a cell? If a mutated, nonfunctional enzyme is in a pathway supplying most of a cell's chemical energy, the loss of the enzyme's function could lead to the death of the cell. The story is more complex, however, since the cell contains a second gene for this enzyme on its homologous chromosome, one which has not been mutated and is able to form an active enzyme. Thus, little or no change in cell function would result from this mutation. If both genes had mutations that rendered their products inactive, then no functional enzyme would be formed, and the cell
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