Effects of Mutations

The nature of the genetic code provides some protection against mutation. Sixty-one codons specify the twenty types of amino acids, and therefore some amino acids correspond to more than one codon type. Usually, two or three codons specifying the same amino acid differ only in the third base of the codon. A mutation that changes the third codon base can encode the same amino acid. For example, the DNA triplets GGA and GGG each specify the amino acid proline. If a mutation changes the third position of GGA to a G, the amino acid for that position in the encoded protein does not change—it is still proline.

Figure 4.25

(a) The DNA code for the amino acid glutamic acid is CTT. (b) If something happens to change the first thymine in this section of the molecule to adenine, the DNA code changes to CAT, which specifies the amino acid valine. The resulting mutation, when it occurs in the DNA that encodes the protein hemoglobin, causes sickle cell disease.

Figure 4.25

(a) The DNA code for the amino acid glutamic acid is CTT. (b) If something happens to change the first thymine in this section of the molecule to adenine, the DNA code changes to CAT, which specifies the amino acid valine. The resulting mutation, when it occurs in the DNA that encodes the protein hemoglobin, causes sickle cell disease.

If a mutation alters a base in the second position, the substituted amino acid is very often similar in overall shape to the normal one, and the protein is not changed significantly enough to affect its function. This mutation, too, would go unnoticed. (An important exception is the mutation shown in fig. 4.25.) Yet another protection against mutation is that a person has two copies of each chromosome, and therefore of each gene. If one copy is mutated, the other may provide enough of the gene's normal function to maintain health. (This is more complicated for the sex chromosomes, X and Y, discussed in chapter 24, p. 988.) Finally, it also makes a difference whether a mutation occurs in a body cell of an adult or in a cell that is part of a developing embryo. In an adult, a mutant cell might not be noticed because many normally functioning cells surround it. In the embryo, however, the abnormal cell might give rise to many cells forming the developing body. All the cells of a person's body could be defective if the mutation were present in the fertilized egg.

Mutations may occur spontaneously if a chemical quirk causes a base in an original DNA strand to be in an unstable form just as replication occurs there. Certain chemical substances, called mutagens, cause mutations. Researchers often use mutagens to intentionally alter gene function in order to learn how a gene normally acts. Table 4.4 lists some mutagens.

Ultraviolet radiation in sunlight is a familiar mutagen. It can cause an extra chemical bond to form between thymines that are adjacent on a DNA strand. This bond forms a kink, which can cause an incorrect base to be inserted during DNA replication. If sun damage is not extensive, repair enzymes remove the extra bonds, and replication proceeds. If damage is great, the cell dies. We experience this as a peeling sunburn. If a sun-damaged

Commonly encountered mutagens

Mutagen

Source

Aflatoxin B

Fungi growing on peanuts and other foods

2-amino 5-nitrophenol

Essentials of Human Physiology

Essentials of Human Physiology

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