What is a gene?

Marty Sachs msachs at uiuc.edu
Mon Feb 14 18:55:06 EST 1994

In article <2jjffj$dp5 at news.mic.ucla.edu> Ron Kagan <rkagan at ewald.mbi.ucla.edu>
>Re: What is a gene?
>In article <1994Feb12.181821.927 at galileo.cc.rochester.edu> RICHARD P.
>>In <CL2F6J.3DF at pnfi.forestry.ca> lmarshal at pnfi.forestry.ca writes:
>>> In <2jeiua$913 at news.mic.ucla.edu> Ron Kagan
><rkagan at ewald.mbi.ucla.edu> writes:
>>> >How about: 
>>> >	  A gene is a segment of DNA/RNA that, when
>>> >transcribed/reverse-transcribed, 
>>> >		leads to a biologically functional product.
>>> What about a genic location holding an allelic sequence that
>>> produces a protein that is non-functional.  Not a gene anymore?
>>I think that any stretch of DNA/RNA which leads to a RNA/protein product
>>be referred to as a gene, functional or not (except perhaps for the
>>pseudogene which is transcribed).  I also think that the definition of
>>physical boundaries of a gene in the statement above should be expanded
>>include all proximal and distal regulatory regions (promotors,
>enhancers, etc)
>>involved in the transcription of that gene.  This would not include
>>regulatory molecules - just the appropriate regions of DNA.
>>Rick Willis 
>>University of Rochester Cancer Center
>I think that the correct term for any stretch of DNA/RNA which leads to a
>RNA/DNA product is *transcription unit*, not gene.  The reason that I
>include biological function in my definition of "gene" is that a
>definition should include the most essential characteristic(s) of the
>entity being defined; that is, the one(s) that best explains all (or
>most) of the other characteristics of the entity.
>In the case of a gene, even without knowing about transcription or
>reverse transcription, we know that genes are units of heredity, are
>factors that determine characteristics (phenotype) of an organism,  are
>carriers of biological information, and are subject to the forces of
>natural selection.   A non-functional transcription product does none of
>these.  It is soon lost (in terms of evolutionary time) from the "gene
>pool" because there are no selective pressures to maintain it, and it
>carries no information.  In this regard, it is like a string of random
>letters: "xfrk", which does not constitute a word, even though words are
>composed of letters, because it conveys no meaning (information).
>Aside from "transcription unit", there is a term for a segment of DNA
>that is recognizable as having once encoded a functional product, but no
>longer does: "pseudogene".
>*It is only because a gene encodes a biological function that make it all
>of the things that we know a gene to be: a unit of information, heredity,
>and natural selection.*
>Therefore, I stick to my original definition:
>A gene is a segment of DNA/RNA that, when
>leads to a biologically functional product.
>Ron Kagan
>rkagan at ewald.mbi.ucla.edu

Genes have alleles (alternative states; polymorphism).  Some of these alleles
are null (some null alleles do not encode a transcript).  A null allele of a
gene is not a pseudogene (which are nonfunctional duplicates of functional
genes, i.e., unique loci).

Some consider the term "gene" to be synonymous with "unit of transcription". 
However, this falls apart with the concept of the "operon" and is falling apart
in eukaryotes with the daily identification of "biologically functional
products" that result from "alternative splicing" (either resulting in a single
product from more than one transcription unit or several products resulting
from a single transcription unit).

So, what is a gene?

Marty Sachs
USDA/ARS & Agronomy/UIUC    msachs at ux1.cso.uiuc.edu [internet]
S108 Turner Hall            msachs at UIUCVMD [bitnet] 
1102 S. Goodwin Ave.        (217) 244-0864 [phone]
Urbana, IL  61801           (217) 333-6064 [FAX]

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