Introns and Exons

[Radical Edward]

I came across these little marvels doig a biology course last year, but they were just briefly described as Introns being the parts of a gene that are spliced in for protein synthesis, and exons are spliced out. I also came across another article regarding fruit flies I think it was, where they have shown that by skipping different Introns, one gene can code for several different proteins. anyone any idea of how this mechanism works, and if it is employed to any significant degree in higher animals.

[Guest Hippokrates]

to make something clear:

EXons are the ones that are EXpressed (=> that are translated into proteins) and introns get cut out of the sequence...

 

the act of splicing is performed by nuclear complexes consisting of proteins and RNA ("small nuclear riboproteoparticles" = snRNPs; called "snurps"). Since the RNA posesses the enzymatic activity it is also called "ribozyme". the snRNPs recognize particular sequences at the beginning and the end of an intron to find it.

 

concerning the possibility of alternative splicing:

whatever its present state, a cell is able to choose by using certain mechanisms the adequate gene (to transcribe & translate) out of a thousand available. e.g. regulator proteins that can attach to a promotor region and enable the transcription.

and i would suggest a simular mechanism for splicing, too... (just my speculation) picture this : the gene A can be alternatively spliced to produce protein A1 or A2 (the mRNA for A2 still has intron number II, which has been spliced out in the A1-mRNA.) now think of A1 of being capable of "protecting" intron II. So when there is a lot of A1, new transcripts will be spliced to A2-mRNAs -- but if there are overwhelming masses of A2 intron II will lose its protector, and the production will swich to A1-mRNA.

in the end you will have a (more or less) constant ratio between these two proteins.

since most of the sequences stay the same, the two products dont differ in a great extend. (one of the results below is about a immune globuline gene that can be either spliced to become a membrane located Ig or a secretory one...)

 

alternative splicing obiously has an emminent importance in higher animals. i found thousands of results when searching for alternative+splicing +human. i have to confess that i read all of them yet...

 

hippokrates

[fafalone]

snRNPs (U1snRNP, U2snRNP, etc) make up the spliceosome complex; and only U6 acts as a ribozyme. snRNPs also contain the Sm family of proteins. Some of these bind to conserved sites along the snRNA to form the core of the snRNP. They also contain RNA helicases (yes, there are double stranded RNAs). pre-mRNAs are spliced by teh same pair of reactions that occur as group II introns splice themselves, and these reamarkably resemble the snRNPs involved in splicing. This suggests that snRNAs are the catalytically active components of the snRNPs, NOT the proteins.

I do not believe snRNPs recognize sequences themselves; SR proteins (lots of serine and arginine) that span the intron/exon borders are believed to recruit the snRNPs.

[blike]

[EDIT] ignore me.

 

<<gene can code for several different proteins>>

 

That would be the evolutionary advantage, for those of you who read my previous question.

[fafalone]

Some introns are just left overs it wasn't a disadvantage to eliminate, especially since they're not activated by transcription factors. Other introns serve as promoter regions. In RNA, they serve as enzymes. I also hypothesize they stabilize structure; might become more disorganized if the entire strand was completely spliced.

 

"<><?><?><!@$#><%!#%@?>@?#$>< edit :P"

 

What may be an intron when one gene is being copied might be an exon for another gene.

[Guest Hippokrates]

proteins with different structure and function often consist of domains that are very similar (but in different sequence) and it s possible to trace them back to one particular exon in the DNA.

 

thus the genes for many proteins developed by the combination of a limited number of genetic sections (which represent exons) => a "modular assambly system"

 

according to this theory the introns facilitated the rearrangement of the exons in early evolutionary time....

[mister_me]

It's called the miracle of life. When one Intron (male) gets together with an Exon (female) they get jiggy with it. Pretty soon, protien synthesis occurs and something happens which hasnt yet been explained by scientists. It eventually leads to occurances in advanced animal lifeforms.

 

This is more commonly known as P2synth sexual intercourse.

[Ahmad]

I'm not sure if that is what you mean, but in prokaryotes, there are no introns or exons. But they contain specific sequences that mark a seperator between two different proteins. So for a single gene, transcribed in a single mRNA message, two proteins are produced.

[Ahmad]

Keywords: polycistronic RNA, multi-cistronic RNA, operons

[fafalone]

who cares about prokaryotes

[Ahmad]

The question was about a single mRNA message coding for different proteins, that is why I mentioned the thing in prokaryotes. In case you mean why it must be studied, then the answer is that it is important for recombinant DNA technology, utilizing bacteria to produce a specific protein we need.

 

There is also another point regarding introns and exons. Some mRNA messages are spliced differently in different kinds of tissues, yielding two distinct proteins based on the tissue.

[Ahmad]

You will find more information here:

 

http://www.indstate.edu/thcme/mwking/rna.html#clinical

 

Look for the calcitonin example.