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, tow 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.
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. Yet another protection against mutations 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. 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.
Ultraviolet radiation in sunlight is a familiar mutagen. It can cause an extra chemical bond to form between thymine that are adjacent on a 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 cell cannot be repaired or does not die, it often turns cancerous. This is why many years of sunburns can cause certain types of skin cancer.
A type of disorder called an "inborn error of metabolism" results from inheriting a mutation that alters an enzyme. Such an enzyme block in a biochemical pathway has two general effects: the substance that the enzyme normally acts on builds up, and the substance resulting from the enzyme's normal action becomes scarce. It is similar to blocking a garden hose: water pressure builds up behind the block, but no water comes out after it.
The biochemical excesses and deficiencies that an inborn error of metabolism triggers can drastically affect health. The specific symptoms depend upon which pathways and substances are affected.