A polynucleotide segment of RNA which forms "messenger" RNA and is thus present after splicing. Exons are the portion of RNA that contain what we currently believe to be the useful parts of the genetic code, used for protein synthesis. Compare intron.

"An exon is a segment of an interrupted gene that is represented in the mature RNA product."1

Exons are the segments of DNA that are copied onto RNA during translation, and thus are expressed ('expressed region'); or to put it more directly, exons are the segments of DNA that are made into genes. It would, in fact, not be too misleading to say that exons are genes.

On the DNA molecule genes are often 'interrupted' by bits of 'Junk DNA' that do not contribute to the final product of that gene. These interruptions are called 'introns'. Introns must be removed from the RNA transcript (which is a direct copy of the DNA) before it can be used to produce proteins. This process is called RNA splicing, and the final product is mRNA (sometimes, to be very precise, we say 'mature mRNA' or simply 'mature RNA'), which consists entirely of exons.

RNA splicing is a complex subject, but all exons have specific beginning markers, called start codons, and end markers, called stop codons. Introns also have specific markers at their ends2; once the RNA polymerase identifies the start of a gene, it simply removes every segment defined by these markers3 until it reaches the end of the gene.

A gene with no introns is called colinear; these genes are common in yeast and bacteria, but much less common in eukaryotes. We do not understand the reason that introns multiply in some genera and species so much more than others. However, it is worth noting that introns can collect mutations and/or multiply themselves at a much faster rate than exons, because there is no evolutionary pressure on them. This makes them useful for extrapolating genetic relationships among species.

1. Quote from Essential Genes, by Benjamin Lewin, Pearson Prentice Hall, 2006.

2. The most common markers for introns are guanine + thymine on the start or 'left side' and adenine + guanine on the end or 'right' side, so this is sometimes referred to as the GT-AG rule -- but the RNA uses uracil where DNA uses thymine, so sometimes the term 'GU-AG rule' is used. These combinations mark over 98% of splices, but there are other combinations.

3. i.e., After it recognizes a start codon indicating the start of the gene, it starts transcribing information; if it finds a GT segment before reaching the stop codon, that must be an intron, so that is removed. Once it reaches AG it starts collecting again... And so on, until the stop codon is reached.

This was written to help me understand some of the information about exons better. If you have questions or facts that you think I should add, please let me know.

In some cases exons form distinct subregions of the gene, called domains, and at some points in the editing of the primary transcript there may be a choice of exons. Which one to use is then dependent on environmental conditions in the cell. That is, one gene can make several different proteins.

For example a B cell, a kind of lymphocyte, produces antibodies. Early on in its differentiation it needs to retain them at the cell membrane, but later it needs them to circulate, and it achieves this by switching exons, so that the "same" gene now codes for slightly different proteins.

Apparently the latest Human Genome Project figures show that as much as 60% of our genome may have splice variants like this!

In such cases the exons might have evolved as distinct genes and become permanently associated (having a single promoter site and a single terminator).

However not all exons represent bounded domains. In these cases they are probably single genes whose integrity has been invaded by junk DNA introns. Since introns usually begin and end with characteristic two-base sequences, presumably natural selection has spared intrusive junk that just happens to fit the code for domain boundaries.

About the other senses of the word, given by Webster, an exon is one of four officers commanding the Yeomen of the Royal Guard, styled corporal in their commissions, and ranking below an ensign. The word, dating from 1767, derives from the French pronunciation of 'exempt'.
And Exon. is actually the abbreviation of the Latin name of the city of Exeter, viz Exonia, and of its corresponding adjective Exoniensis.
exercise, left as an = E = Exploder

Exon /eks'on/ excl.

A generic obscenity that quickly entered wide use on the Internet and Usenet after Black Thursday. From the last name of Senator James Exon (Democrat-Nebraska), primary author of the CDA.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

Ex"on (?), n. [NL., from E. Exe (Celt. uisge water) the name of a river.]

A native or inhabitant of Exeter, in England.


© Webster 1913.

Ex"on, n. [F. expect an under officer.]

An officer of the Yeomen of the Guard; an Exempt.



© Webster 1913.

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