Another main difference between the two is that transcription and translation occurs simultaneously in prokaryotes and in eukaryotes the RNA is first transcribed in the nucleus and then translated in the cytoplasm. RNAs from eukaryotes undergo post-transcriptional modifications including: capping, polyadenylation, and splicing. These events do not occur in prokaryotes. Eukaryotes contain mRNAs that are monocystronic.
Termination in prokaryotes is done by either rho -dependent or rho -independent mechanisms. Eukaryotic Pol II is made up of 10 subunits.
Rpb1 and Rpb2 make up the bulk of the complex 6. Prokaryotes utilize a transcription factor sigma which is the smallest subunit of the structure that can easilty associate and dissociate from the core subunits. Promoter sequences vary as well between prokaryotes and eukaryotes. Termination is quite diiferent among prokaryotes and eukaryotes also. In prokaryotes there are two paths that can be taken for termination.
One is the rho-independent pathway and the other is the rho-dependent pathway. The rho-independent pathway depends on a palindromic termination sequence that encodes a stem-loop structure and once the polymerase encodes this palindromic sequence it pauses and the weak interaction of the RNA-DNA hybrid allows it to dissociate from the polymerase 1.
In the rho-dependent pathway a protein known as rho binds to the newly transcribed mRNA and slides along it towards the polymerase. That's the five prime cap. And the five prime cap is simply a guanine nucleotide. So I'm gonna draw a G inside, Guanine, and it's going to have a methyl group somewhere on the molecule. So I'm gonna draw a methyl group. And the bond between this guanine and the nucleotide right near it is a bond that's different than the bond that you'd typically find between two nucleotides.
And so that's really all the five prime cap is. And the five prime cap is actually the ribosomal binding site in eukaryotes. So that means that in eukaryotes, the ribosome's going to recognize this particular part and bind to it. So after the five prime cap, we have this other noncoding region which the ribosome's not going to translate.
And then the ribosome is going to hit the start codon again. AUG tells it to start, and it's gonna start translating, so it's going to translate this entire section until it hits the stop codon. And then we hit something that looks different than what we've seen in the prokaryotic mRNA, so this section with blue nucleotides, and that's called the poly-A tail.
And the poly-A tail is a bunch of nucleotides that are all A's, or adenines, so I'm gonna draw A's inside all of these nucleotides. And the poly-A tail is actually pretty long, so it's typically anywhere between and nucleotides long.
So that's pretty long. So I didn't exactly draw it to scale. And the purpose of both the five prime cap, and the poly-A tail is to prevent this mRNA from being degraded by enzymes. So it acts as kind of a signal that does not allow enzymes to break it down or degrade it. And so you might be wondering, well, what about prokaryotic mRNA? How come they don't have anything similar to prevent them from being degraded. And the brief answer to that question is that in prokaryotic cells, transcription, that's an R, and translation, both happen in the same place.
So prokaryotic cells don't exactly have a nucleus.
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