Gene Expression (Part 2): Translation in Eukaryotes

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Do you remember that we decided that DNA and RNA are two separate languages? Transcription factors read “TATAAA” as “park here.” There are hundreds of other examples of how enzymes recognize only certain sequences, but we’re gonna keep it simple for now.

Well during translation, Dr. Ribosome and his tRNA graduate students “translate” the mRNA into yet ANOTHER language: polypeptide. Ok, after some thought, that’s too cheesy even for me. But since it gets the point across, I’ll leave it at that.

Once the mRNA strand has left the nucleus and entered the cytoplasm (or “cytosol” to some people), it is fair game for ribosome binding. That 5′ cap is EXTREMELY important here because it marks the location for the ribosome to bind.

WAIT! I want to add a little sidenote here that would have REALLY aided me in getting the concept of why all the little proteins are necessary. Remember how we needed transcription factors to start transcription? We’re gonna need translation initiation factors in order to start translation as well. But what’s really the point? You’ll have to read that article to find out, but for now, read on.

SO, the ribosome uses initiation factors to bind on the RNA, more factors find the AUG start codon, then the ribosome uses factors to bring the tRNA molecule with Methionine to the AUG Start codon… WHEN DOES THE RIBOSOME ACTUALLY DO SOME WORK BY ITSELF!?!?

Ummmmmmm, pretty much after that first methionine. You see, once everything is set up, the rest is automatic. If you look at the picture above, you’ll see that there is an “A Site,” a “P Site,” and a tRNA molecule floating away. The “A Site” is short of aminoacyl site, or “amino ACID” site. This is where tRNA molecules land if their anticodon matches the codon that is showing.The “P Site” is short for the peptidyl, or “PEPTIDEyl” site. This is where the amino acids bound to the tRNA molecules are added to the growing protein using a peptide bond. Finally, there is the exit site, where the empty tRNA molecule is allowed to float off and become recharged with another amino acid (of the same type).

In the case of the methionine start codon, the ribosome was moved so that the AUG codon was the only one that was showing in the Aminoacyl site. Once the methionine tRNA was bound, the RIBOSOME MOVED (not the tRNA molecule) and this pushed the tRNA molecule over to the Peptidyl site. This left an empty A-Site for another tRNA to enter and bind, causing the ribosome to move again. This time, the methionine tRNA is pushed into the exit site and ejected from the ribosome.

This move-eject-bind procedure will go on until the ribosome encounters a stop codon. If you look at the chart above, you’ll see three different combinations that will result in a stop. This happens because there’s a special molecule that possess and anticodon but NOT an amino acid. When this molecule moves into the P site, the ribosome will attempt to move the growing amino acid chain to bind with the new amino acid. The only problem is that with this special molecule, there is no amino acid. So the protein just floats off, and the ribosome falls apart, thereby ending translation.

Short Version

1. Factors bind ribosome to mRNA
2. Methionine tRNA initiates reading
3. Move – peptide bond/eject – Move – peptide bond/eject
4. Stop codon ends it all

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Best of Luck,

Grey