Genetic Splicing

[ennui]

I've read from various sources that generally speaking, eukaryotes have monocistronic DNA. It codes for one protein. Bacteria/prokaryotes on the other hand, have polycistronic DNA. They can produce an mRNA strand which codes for multiple proteins.

 

Yet I've read a few articles in New Scientist saying that the average amount of protein products from a single eukaryotic gene is somewhere between 5 and 6. I know that splicing exists in eukaryotic genomes; but wouldn't this mean that the DNA is polycistronic?

 

Can anyone help me understand this?

[CharonY]

No, because it either refers to several ORFs that are being co-transcribed, or (more frequently) to the amount of polypeptides that the final processed mRNA encodes. In effect, the processing step is ignored for this type of classification.

[MedGen]

I believe the only cases of polycistronic RNA's in eukaryotes are the retrotransposons, LINE's (e.g.L1).

[CharonY]

Actually if one would like to be super-precise it ain't that straightforward. For instance, I recall that trypanosomes also transcribe polycistronic mRNA, however they get processed to monocistronic ones before translation. I am pretty sure that there are more exceptions.

[ennui]

No, because it either refers to several ORFs that are being co-transcribed, or (more frequently) to the amount of polypeptides that the final processed mRNA encodes. In effect, the processing step is ignored for this type of classification.

 

Thanks for the reply. But could you explain this in more detail? Is this to say that in eukaryotes, you generally have one ORF that can be spliced differently (e.g. antibody recombination); but in prokaryotes you have several ORFs that are tandemly linked by an intron spacer?

[CharonY]

Prokaryotes do not have introns as such. Essentially you have several ORFs one after the other (in some cases even coupled). All under the control of the same promoter. And the resulting mRNA does not get further processed (there are exceptions, but I will ignore them for now).

[MedGen]

Actually if one would like to be super-precise it ain't that straightforward. For instance, I recall that trypanosomes also transcribe polycistronic mRNA, however they get processed to monocistronic ones before translation. I am pretty sure that there are more exceptions.

 

Ahh, the wonderful mess that is biology, everytime something gets nailed down along comes another exception.