▪ I. exon1
(ˈɛksɒn)
[app. intended to express the pronunciation (ɛgzɑ̃) of Fr. exempt.
Cf. exaun, occurring as a spelling of exempt in 1678; also exant (quot. 1655 below), used in the sense of exempt n. 4 a. The ‘exempts’ or ‘exons’ of the Yeomen of the Guard, according to Thoms Bk. of the Court, were first appointed in 1668.]
The ordinary title of the four officers of the Yeomen of the Royal Guard, ‘styled corporals in their commissions’ (Thoms) and ranking below the ‘Ensign’; = exempt n. 4 b.
| [1655 in Nicholas Papers (1892) II. 354 The Court gaue Moreland (Cromwells express) two Exants of y⊇ guards to conduct him in safety.] 1767 Royal Calendar 83 Yeomen of the Guards..Exons [4 names follow]. 1843 Macaulay Ess., Mad. d'Arblay, It never occurred..to the Exons, and Keepers of the Robes, that, etc. 1873 Daily News 19 June 6/5 Colonel Bourke (the Exon in Waiting). 1891 New Army List 132 Yeomen of the Guard..Exons [4 names]. |
▪ II. exon2 Genetics.
(ˈɛksɒn)
[f. expressed ppl. a. + -on1: see quot. 1978.]
A section of a DNA or RNA molecule that codes for a protein, in cases where such sections are separated by non-coding ones. Cf. intron.
| 1978 W. Gilbert in Nature 9 Feb. 501/1 The notion of the cistron..must be replaced by that of a transcription unit containing regions which will be lost from the mature messenger—which I suggest we call introns (for intragenic regions)—alternating with regions which will be expressed— exons. 1982 P. N. Gray in T. M. Devlin Textbk. Biochem. xviii. 909 The introns are removed by endonucleolytic enzymes, and the exons are joined or spliced together forming a continuous informational sequence. 1983 Nature 21 July 215/3 The residues forming the active site are encoded in three different exons, similar to other serine proteases. 1985 Sci. Amer. Oct. 57/1 (caption) In eukaryotes the protein-coding sequences of DNA (exons) are often separated by intervening sequences (introns) that must be excised from a primary transcript to make messenger RNA (mRNA). |
Hence eˈxonic a.
| 1978 Nature 9 Feb. 501/3 Middle repetitious sequences within introns may create hot spots for recombination to rearrange the exonic sequences. 1979 Ibid. 6 Dec. 571 (caption) The length of the exonic sequence 4{pp} is 119 {pm} 29 base pairs. 1981 Sci. Amer. May 55/1 Protein-coding exonic sequences evolve slowly, mainly by point mutations in which one base replaces another, whereas such noncoding sequences as introns evolve much faster by insertion and deletion events of variable extent. |