| The American Heritage® Book of English Usage. |
A Practical and Authoritative Guide to Contemporary English. 1996.
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4. Science Terms: Distinctions, Restrictions, and Confusions
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| § 25. exon / intron / axon |
| In 1844, when the American inventor Samuel Finley Breese Morse transmitted “What hath God wrought” as a series of dots, dashes, and pauses along a telegraph wire stretched between Washington, D.C., and Baltimore, Maryland, he inadvertently mimicked a cipher system nature has used for millenia—DNA and RNA. Each of the genes contained in DNA and RNA is composed of a particular arrangement of discrete “dots” or “dashes” of information, punctuated by “pauses” of noninformation. When adeptly translated by specialized cellular components, this coded information yields millions of protein “messages” that allow the body to function. The molecular dots or dashes are called exons; the punctuations of silence are known as introns. The terms exon and intron were first introduced into the scientific literature in the late 1970s when research began to show that a gene was not simply read from one end to another when a protein was formed. Instead, genes were found to be composed of information units that could be read, or expressed, in a variety of combinations, thus making any one gene the template to any number of proteins. Exon, then, is built from ex pressed and the suffix -on, “fundamental unit,” a suffix that also is used in such words as photon and electron. Research also showed that exons did not exist in an unbroken string within a gene but were separated by introns, units of nonexpression that, at least according to current understanding, serve no function other than to provide silent patches between the information-laden exons. Intron comes from the combination of intragenic, which means “within the gene,” and the suffix -on. | 1 |
| The similar-sounding term axon is neurological rather than genetic and refers to the filamentous process of a nerve cell. Its function is to conduct nerve impulses away from the body of the nerve cell and toward other nerve cells or other cells or tissues. Axon comes from Greek axon, “axis,” and first appeared as a neurological term in scientific literature in the early 1900s. | 2 |
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| The American Heritage® Book of English Usage. Copyright © 1996 by Houghton Mifflin Company. All rights reserved. |
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