why don't eukaryotes have operons?

[pezzang]

Does anyone know? because its extensive size of DNA as compared to prokaryotes? any ideas?

Also, what's between the coding regions in a prokaryotic operon? (intergenic area between multiple genes in operon)

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[flaahless]

Euks don't have polycistronic DNA (except for like, slime molds). Why they don't have it? Beats meeh, evolution... or maybe our natural inclination to be inefficient.

Prokes lack introns so there isn't anything between their genes. One gene ends where another begins I believe. Also, prokes lack the splicing machinery to excise the DNA that would/could be between the genes.

 

[killinsound]

drosophila actually have a couple polycistronic dna... but... thats besides the mcat

 

[Kfire326]

actually i JUST studied the following for my biochem final on wed:

Eukaryotes have something called Locus Control Regions (LCRs). An LCR = an enhancer that binds activators that increase transcription of more than one gene in the same locus. However, these genes are not all transcribed onto the same mRNA. example: B-globin. One enhancer will activate transcription of all the subunits of adult Hb, another can activate all the subunits of fetal Hb.

--> bind activators to 1 enhancer = activate multiple genes, just like in prokaryotic operons, if you activate 1 promotor, you activate multiple genes.

I guess this isn't a REASON, but it's an alternative...basically the same concept.

edit: and yea, fyi: this is not anywhere near MCAT material...

 

[arsenewenger]

Does anyone know? because its extensive size of DNA as compared to prokaryotes? any ideas?

Also, what's between the coding regions in a prokaryotic operon? (intergenic area between multiple genes in operon)

It has to do with the simplicity of their system and so ,relative to Euks.,extensive regulation is not needed.


Another good explanation is evolution.

 

[Swiperfox]

I believe I read this somewhere. We lack operons because gene regulation is so complex that you could not possibly fit genes that code for regulation points close enough to the genes they regulate. Operons depend upon having the structural gene closely downstream, while Eukaryotic genes do not have this luxury due to these complex control mechanisms.

 

[QofQuimica]

Does anyone know? because its extensive size of DNA as compared to prokaryotes? any ideas?

Also, what's between the coding regions in a prokaryotic operon? (intergenic area between multiple genes in operon)

Interesting question. I wonder if it has anything to do with the fact that we have to export our mRNAs out of the cell nuclei, which prokaryotes do not have to do. But I'm just speculating here, and the previous posters are correct that the answer to this question is irrelevant for MCAT purposes.

 

[Retro Virus]

The Eukaryotic evolutionary strategy in just about everything is specificity through regulation. Operons make variable expression within the gene set hard to regulate because if you want some extra protein X only and you have an XYZ operon, you end up producing lots of products Y and Z, so you're out of luck. In theory I guess you could use alternative splicing to disable some of the genes on the processed mRNA, but it would be more energetically costly and I don't think that happens often.

Eukaryotes can still produce an operon-like effect (simultaneous gene activation) by using a regulatory small molecule that acts on many transcription factors, a single transcription factor that acts on many regulatory sequences, or a single regulatory sequence that acts on many genes. But using the one mRNA for one gene approach lets them tweak their biochemistry much more finely when needed.

 

[pezzang]

is it possible that ribosome binding site is present at intergenic region of operons? Does the definition of intergenic region include coding sequences as well?

 

[Retro Virus]

is it possible that ribosome binding site is present at intergenic region of operons? Does the definition of intergenic region include coding sequences as well?

Mostly yes to the first question - ribosome bindings sites (SD sequences) are usually in the 5' UTR, a little bit upstream of coding regions, except in some very wierd cases where genes overlap or a particular stretch of nucleotides is BOTH a coding region for one gene and a non-coding regulatory sequence for another gene. Also because ribosomes release at the end of genes you would want a new SD before each gene unless you were encoding a polyprotein or had some other mechanism to stop and restart translation. I don't know if you want to call that an intergenic region, though, because if we are talking about ribosome binding sites then we are talking about mRNA, which has non-coding sequences but generally doesn't have them in long stretches.

As for the second question, I think no. My understanding is that intergenic region refers to remotely regulatory and structural regions (enhancers, insulators, repeats, etc.) and other parts of the genome that don't have known gene-enoding function. But, we almost never use the term in any of my classes.

Also congrats to you in having an interest in cell bio that goes beyond the MCAT, because when you get into it it's really cool stuff.