INTRONS ???

John Ladasky ladasky at leland.Stanford.EDU
Mon Jun 10 19:34:44 EST 1996


In article <4pi24g$mto at dfw-ixnews2.ix.netcom.com>,
Michael L Roginsky  <d_micro at ix.netcom.com> wrote:
>In <4pcbhn$3mg at newsbf02.news.aol.com> santh55857 at aol.com (SAnth55857)
>writes: 
>>
>>I read recently the book THE EPIC HISTORY OF BIOLOGY, published by
>Plenum
>>Press, and it said that introns are strings of meaningless genetic
>>information. But is this really so? I thought that they were
>meaningful,
>>but just pointless today, though they were once useful in evolutionary
>>history.
>>
>>Can someone clarify? Is the author right?
>
>I have to be given  lot more information before considering evolutionay
>history. Too many variables are at work dead-ending the evolutionary
>claims. However, natural selection is certainly observable, and totaly
>right except for humans  tinkering with medical experimentation
>allowing those that would be prevented from multiplying to go on and
>spread "bashed genes" to offspring. To be precise we have great concern
>for having "created" super germs by using broad spectrum drugs as a
>matter of practice. I submit this as an engineer's view of what is
>occurring worldwide.

	An interesting point, but totally peripheral to the issue of in-
trons.  BTW, so is the newsgroup bionet.biology.tropical.  I will address
the issue of introns briefly, and then suggest that this discussion move
to bionet.molbio.evolution.  Followups have been set there.  There are
likely to be people in that newsgroup who know a lot more about this
subject than I do!

	When researchers first started reading protein sequences, they used
chemical methods directed at identifying the amino acids themselves.  Then
they cracked the nucleic acid code, and learned to read messenger RNA.  Each
amino acid was coded for by a string of three nucleic acids, and each string
followed directly after the other.  There were no "gaps."  When researchers
then attempted to read sequences directly from the DNA, they found that
there were gaps.  Typically, one would see a few hundred nucleotides in the
DNA sequence that corresponded to the messenger RNA -- but then there would
be a few hundred more nucleotides of sequence not seen in the messenger RNA,
before the original sequence resumed.

	Introns aren't exactly junk.  At least, they aren't now.  There are
conserved intron sequences that cause the primary RNA transcript to fold
into a defined structure that includes the ends of the intron.  This "ribo-
zyme" construct then splices itself out of the primary RNA.  Once all the 
introns have spliced out, you have a mature messenger RNA.  Introns also
carry "enhancer" sequences at the DNA level.  These sequences are believed
to target transcription-enhancing proteins to the vicinity of certain
genes.

	There are a lot of theories about the formation of introns and
their utility.  Introns are not found in bacterial DNA, but the debate
rages on about whether they existed at one point and then were eliminated.
So the evolutionary picture is unclear.  Sequences related to introns are
found in "jumping genes" -- mobile genetic elements that can occasionally 
splice themselves out of the genome and reinsert themselves elsewhere.
Jumping genes were first described by Barbara McClintock -- no one be-
lieved her for decades.  Some people think that jumping genes are vestigial
viral sequences that have been co-opted by their hosts for novel purposes.
Exactly what the reason for mobile genetic elements might be is unclear.
The same goes for introns.  I am not aware of any grand consensus about
the origin, original function, or present function of these sequences...

	O.K., tag!  You're it!


-- 
Unique ID : Ladasky, John Joseph Jr.
Title     : BA Biochemistry, U.C. Berkeley, 1989  (Ph.D. perhaps 1998???)
Location  : Stanford University, Dept. of Structural Biology, Fairchild D-105
Keywords  : immunology, music, running, Green



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