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|WikiProject Molecular and Cellular Biology||(Rated Start-class, High-importance)|
Encoding, not encoded by?
Hello. Not a domain expert here, but should the first sentence of this article say not "sequence encoded by a gene" but "sequence encoding a gene"?
I think that the ORF is the sequence that is between a start and a stop codon...
Exons that do not correspond to protein
are not can be exons
The current Wikipedia entrance on Exons claims that exons are INCORRECTLY referred to stretches of DNA that do correspond to coding DNA. The reasoning behind this is a reference to a paper that discusses this types of exons. One paper does not merit to change a definition of something that is found in all textbooks and all glossary as: Coding sequence of DNA present in mature messenger RNA. EXpressing regiON. This whole entrance is INCORRECT and should be changed.
Regarding the remark above this one: if you refer to a mRNA you are correct, if you refer to hnRNA or the gene you are wrong since these might contain INTeRvening regiONs
- Glossary definition of exon, from the Lodish textbook: Segment of a eukaryotic gene (or of its primary transcript) that reaches the cytoplasm as part of a mature mRNA, rRNA, or tRNA molecule.
- As far as I know, everybody in genomics recognizes that exons need not contain protein-coding sequence; 3' and 5' UTR exons are found in most eukaryotes, and there are even entirely non-protein-coding transcripts that undergo splicing. Exons that do contain protein-coding sequence are referred to as CDS exons when the distinction is important. --Mike Lin 21:20, 11 July 2006 (UTC)
The Wikipedia entry is correct not all exons code for proteins. Although a search of online definitions  finds many entries that imply exons must code for proteins. Most will, although a significant amount of exons code for UTRs see the entry on 3'UTRs.
I've been seeing a lot of "exons do not have to code for protein" in the literature: but no-where have I seen it explained clearly. Is it simply that portions of the mature mRNA (poly-A sites and UTR's, regulatory sequences(?) that sort of thing) which are not represented as protein but are, again, present in the mature mRNA, are exons? Geno-Supremo (talk) 16:23, 2 February 2009 (UTC)
Conflicts with Complementary DNA
Got this from the Complementary DNA entry:
The central dogma of molecular biology outlines that in synthesizing proteins, DNA is transcribed into mRNA, which is translated into protein. One difference between eukaryotic and prokaryotic mRNA is that eukaryotic mRNA can contain introns (intervening sequences), which are not coding sequences, per se, and must be spliced out of the mRNA before it is translated into protein. Prokaryotic mRNA has no introns, so it is not subject to splicing.
And this from the Exon Entry:
An exon is any region of DNA within a gene that is transcribed to the final messenger RNA (mRNA) molecule, rather than being spliced out from the transcribed RNA molecule. Exons of many eukaryotic genes interleave with segments of non-coding DNA (introns). The term exon was coined by American biochemist Walter Gilbert in 1978:
So we have "eukaryotic mRNA can contain introns" and "An exon is any region of DNA within a gene that is transcribed to the final messenger RNA (mRNA)". Those can't both be true.
I'm guessing that the statement under Exon is supposed to be "eukaryotic RNA can contain introns". Not mRNA. But I don't know from biology. :) —The preceding unsigned comment was added by 18.104.22.168 (talk) 20:13, 6 March 2007 (UTC).
The current opening is confusing:
"An exon is any region of DNA within a gene that is transcribed to the final messenger RNA (mRNA) molecule, rather than being spliced out from the transcribed RNA molecule. Exons of many eukaryotic genes interleave with segments of non-coding DNA (introns)"
but not incorrect
I suggest this rewrite-
An exon is the region of a transcribed gene present in the final functional RNA molecule, whereas intragenic introns are spliced out.
Note: a) part of every mature mRNA is non-coding UTR but exonic. As in the diagram and text. b) other RNAs, tRNAs etc have introns c) However, introns, by definition do not code for proteins. (At least in the mRNAs they are removed from.) d) There is some lack of clarity as to whether the exon is part of the gene (DNA) or mRNA, or both.
Many web definitions are not correct: http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-E/exon.html http://www.answers.com/topic/exon?cat=technology as they imply that exons always code for proteins, whereas they do not, as correctly pointed out in the article.
This prominent Molecular Biology online text book has a definition similar to that proposed, but it is more complex:
Exon - one of the officers rank in the Yeomen of the Guard.
The first mention of Exon is in the ceremony of All Nights, which is fully described in the chapter relating to Charles II. They were added to the staff of officers in 1668 just about the time when Marsham’s account of All Night was written.
The derivation and meaning of the word Exon has been and is a puzzle to many, but it is undoubtedly the French pronunciation of the word exempt. An exempt was an officer in the old French Garde Du Corps. “Exempts des Guedes du Corps” are described in a military dictionary as “Exons belonging to the Body Guards,” There was in France an officer of police called “Un Exempt (exon) de Police.”
When Charles II formed his Horse Guards he created a commissioned officer who was styled indiscriminately the exempt or the Exon, and in each of the two troops this officer ranked with the Captain. There is further confusion connected with the title of Exon, for in his commission he is styled corporal. But it appears that in Elizabeth’s reign “corporal” was a commissioned officer, and the term was synonymous with Captain.
Down to the time of the Coronation of George III, which took place on 22 September 1761, corporal was only another word for Exon, as may be seen on referring to the official programme of the Coronation, wherein mention is made of “the Corporals or Exons of the Yeomen of the Guard.” The exempt in the French Garde du corps always had charge of the Night Watch, and the Exon is the English Body Guard was especially appointed for that service.
Curiously enough the word Exempt is also used in the orders of the Yeomen of the Guard with its English meaning.
Origin of term
While the quote does say the term derives from expressed region if you read it carefully, I specified it directly since I think it may not be obvious if you don't read carefully (I didn't actually notice it at first). If there's any disagreement that exon derives from expressed region,  supports the claim Nil Einne (talk) 12:52, 30 September 2010 (UTC)
I'm surprised that there isn't a link to operon here? I don't know about the two to know if it should definitely be mentioned in this article or not (my very-lay-understanding (*) is that an operon is a general term for the collection of several introns, exons, promotor sequence(s), etc involved in a single gene regulation subsystem. Rather like the equivalent of a single file from a 10-million-file-30-billion-line computer program? (to take a perhaps intelligent design POV - not that I know any more about the "modern" (non-religious, ? sic) ID movement)
(*) I went to high school starting in 1994, when only the best schools mentioned even alternative splicing or introns. (I remember feeling quite proud of myself about making a set of lecture notes for my bio teacher that went into "detail" (sic).. How naive - I should make her a new set of notes that really goes into detail - introns, t-, m-, r- and other kinds of rna, psedugenes and retrotransposons, self-translating (???) rRna, etc)