Why is the eukaryotic genome monocistronic

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Transcription control in prokaryotes and eukaryotes: an introduction

If one compares the transcription control in prokaryotes and eukaryotes, an important difference is first noticeable. While the prokaryotes only have one RNA polymerase, the eukaryotes have three polymerases that perform different tasks and can also be distinguished in terms of their sensitivity to α-amanitin:

Tab. 1
The RNA polymerases of the eukaryotes
PolymeraseTranscribed geneslocalizationα-amanitin sensitivity
I.ribosomal RNA (45S precursors of 28S, 18S and 5.8S rRNA)Nucleolusnot sensitive
IIsmall nuclear RNAs (U1, U2 etc.) and all genes that code for proteinsCell nucleusvery sensitive (inhibition: 1 µg / ml)
IIItRNAs, 5.8S rRNA, small nuclear RNA U6, repeated DNA sequences (Alu, B1, B2 etc., 7SK, 7SL RNA)Cell nucleusless sensitive (inhibition: 10 µg / ml) (species-dependent)
Note
The polymerase II of the eukaryotes thus corresponds formally to the RNA polymerase of the prokayotes.

But also with regard to the genome organization, prokaryotes and eukaryotes differ considerably.

Genome organization in prokaryotes and eukaryotes

  • In prokaryotes, the DNA is operonally structured, i.e. genes that functionally belong together are often located one after the other in the form of an operon on the DNA and are read together. The mRNA, which consequently contains the information for several proteins, is called polycistronic mRNA; Different proteins are only created on the ribosome. An example of this is the polycistronic mRNA of the bacterial lac operon, which is translated into the three proteins β-galactosidase (cleavage of lactose), permease (facilitated lactose uptake) and transacetylase (conversion of lactose into allolactose). This type of mRNA synthesis allows a simple, coordinated regulation: in the presence of the inducer lactose, all three proteins for lactose metabolism are formed, and in the presence of glucose gene expression is repressed. There is no polycistronic mRNA in eukaryotes. This means that genes whose products form a unit also have to be regulated individually. Often these genes are even located on different chromosomes! A typical example is globin synthesis: the gene for the α-subunit of hemoglobin is located on chromosome 16, the gene for β-hemoglobin on chromosome 11. However, this individual regulation has the advantage of greater flexibility! During embryonic development, human hemoglobin does not consist of α2β2-Subunits, but has the composition α2γ2. The expression of the different hemoglobin genes is regulated by certain signals.
  • In contrast to prokaryotes, the coding areas of eukaryotic DNA (exons) are repeatedly interrupted by introns. These non-coding DNA regions are initially also transcribed, i.e. translated into mRNA, but must later be removed from the mRNA by splicing before the mRNA is transported as mature mRNA from the nucleus into the cytoplasm of the cell. The number of introns varies considerably; while some genes have more than 50 introns, other genes completely lack them. There are a few examples of introns in prokaryotic genes, but these introns are removed differently (by self-splicing).
  • Eukaryotic DNA often occurs in multiple copies (repetitive DNA). In humans, around 20-30% of the DNA is available as highly repetitive (satellite) DNA, i.e. the corresponding DNA regions can be repeated up to 300,000 times. In many cases the function of the highly repetitive DNA is not yet known. Repetitive sequences are rare in prokaryotes; mostly the copy number of rRNA genes is increased (and mostly only very moderately).

The prokaryotic mRNA is often polycistronic, i.e. it codes for more than one protein. The individual genes can sometimes even overlap. In contrast, the eukaryotic mRNA is monocistronic. The mature mRNA of the eukaryotes is processed at the 5 'and 3' ends and, in contrast to the precursor mRNA, no longer contains any introns.