Deoxyribonucleic acid, or DNA, is a molecule in the cells of all life-forms that contains genetic codes for inheritance. DNA, discussed elsewhere in this book, is as complex in structure as it is critically important in shaping the characteristics of the organism to which it belongs, and therefore it is not surprising that a subtle alteration in DNA can produce significant results. Alterations to DNA are called mutations, and they can result in the formation of new characteristics that are heritable, or capable of being inherited.
Every cell in the body of every living organism contains DNA in threadlike structures called chromosomes. Stretches of DNA that hold coded instructions for the manufacture of specific proteins are known as genes, of which the human race has approximately 40,000 varieties. If the DNA of a particular gene is altered, that gene may become defective, and the protein for which it codes also may be missing or defective. Just one missing or abnormal protein can have an enormous effect on the entire body: albinism, for instance, is the result of one missing protein.
Mutations also can be errors in all or part of a chromosome. Humans normally have 23 pairs of chromosomes, and an extra chromosome can have a tremendous negative impact. For example, there should be two of chromosome 21, as with all other chromosomes, but if there are three, the result is Down syndrome. People with Down syndrome have a unique physical appearance and are developmentally disabled. Nor is an extra chromosome the only chromosomal abnormality that causes problems: if chromosomes 9 and 22 exchange materials, a phenomenon known as translocation, the result can be a certain type of leukemia. Down syndrome also results from translocation.
Germinal mutations are those that occur in the egg or sperm cells and therefore can be passed on to the organism's offspring. Somatic mutations are those that happen in cells other than the sex cells, and they cannot be transmitted to the next generation. This is an important distinction to keep in mind in terms of both the causes and the effects of mutation. If only the somatic cells of the organism are affected, the mutation will not appear in the next generation; on the other hand, if a germinal mutation is involved, what was once an abnormality may become so common in certain populations that it emerges as the norm.
Most of the forms of mutation we discuss in this essay appear suddenly (i.e., in a single generation) and affect just a few generations. Yet even such seemingly "normal" characteristics as our ten fingers and ten toes or our two eyes or our relatively hairless skin (compared with that of apes) are ultimately the product of mutations that took shape over the many hundreds of millions of years during which animal life has been evolving. Evolution, in fact, is driven by mutation, along with natural selection (see Evolution).
Over the eons, advantageous mutations, examples of which we look at later, have allowed life to develop and diversify from primitive cells into the multitude of species—including Homo sapiens —that exist on Earth today. If DNA replicated perfectly every time, without errors, the only life-forms existing now would be those that existed about three billion years ago: single-cell organisms. Mutations, therefore, are critical to the development of diverse life-forms, a phenomenon known as speciation (see Speciation). Mutations that allow an organism to survive and reproduce better than other members of its species are always beneficial, though a mutation that may be beneficial in some circumstances can be harmful in others. Mutations become especially important when an organism's environment is changing—something that has happened often over the course of evolutionary history. And though we cannot watch evolution taking place, we can see how mutations are used among domesticated plants and animals, as discussed later.
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