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Written by Tim Sheppard MBBS BSc. Last updated 9/11/10

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What is translation?

With the whole thing delicately planned, we find ourselves with everything set - the original code (DNA), the messenger carrying a copy (mRNA), the translator (a ribosome containing rRNA) and the transporter to bring in the amino acids (tRNA). All that remains for us to see is how they all fit together.

The mRNA molecule, containing the copy of the code, travels from where the DNA is stored (in the nucleus) to where the translating machinery is kept (in the cytoplasm). Here there are many tRNA molecules which have picked up their various different amino acids. Most importantly for starting off the translation, one specific tRNA molecule picks up the methionine amino acid. This is the tRNA molecule with UAC (uracil, adenine and cytosine) in that order in its anticodon loop.

In the section on types of RNA, I described how ribosomes are made up of various different things - proteins and ribosomal RNA, and that there are basically two subunits out of which everything else is made. However, when the mRNA and tRNA are all set and ready to go, they don't go looking for the ribosomes, because the ribosomes aren't ready yet. The two subunits of a ribosome are formed around the mRNA and tRNA, in the following way:

The 40s subunit binds to something called IF3. Then, along with a few other molecules, IF3 helps the first tRNA molecule (the one attached to methionine) to bind to this 40s subunit. The IF3 then comes apart from the subunit, and the mRNA molecule binds to the appropriate place - so that the UAC of the tRNA molecule is binding with hydrogen bonds to the AUG of the mRNA molecule. The reason the anticodon loop is called an anticodon is because it binds to the codon on an mRNA molecule, so UAC is the anticodon, while AUG is the codon. In fact, because this is the first codon in the mRNA molecule, it is called the initiation codon. It is said that this codon 'codes' for methionine, because the tRNA molecule which it codes for is attached to methionine.

Finally the 60s subunit comes in, forming the final ribosome, with the mRNA molecule running through it, and the first tRNA molecule and amino acid all happy and ready. With the ribosome all set up, it's probably a good idea to explain something about how parts of the ribosome are identified. When the code is 'read', the mRNA runs from 5' to 3', so you could imagine reading left to right, with the 5' end on one side, and the 3' end on the other.

When the ribosome and mRNA are all set up, there are effectively two sites on the ribosome - the P site and the A site, each with a codon from the mRNA in it. The P site or peptidyl site is where the methionine tRNA molecule goes to first. Every other tRNA molecule bound to an amino acid will go to the A site or amino-acyl site first, where the amino acid bound to the tRNA will form a peptide bond with the amino acid bound to the tRNA molecule in the P site.

This is quite difficult to grasp, but is the essence of translation. The methionine tRNA is bound to the P-site, and whichever amino acid is coded for next will come along with its tRNA molecule. When the two amino acids are next to each other, a peptide bond will form between them, releasing water, and the whole collection moves on. The first tRNA molecule is released, and its amino acid is bound to the amino acid on the second tRNA molecule. Because everything has moved on, this means that the second tRNA molecule is now in the P-site, and a third tRNA molecule will come into the A site, again with its amino acid forming a peptide bond with the amino acid on the P site. In this way, a long chain of amino acids is made up.

And so the chain continues. The codons in the mRNA molecule match up with the anticodon loops in the tRNA molecules, and since the anticodon loop determines which amino acid is attached, then the order of the amino acids is determined by the order of the codons in the mRNA molecule. It is quite hard to follow, but put simply, the order of the nucleotides in the original DNA sequence determines the order of the amino acids in the final protein. The code from the DNA is sent as a message (via mRNA) to the ribosomes, where tRNA molecules (starting with one attached to methionine) bring in amino acids. They will keep doing this, and the mRNA will keep moving on, step by step, until the stop codon is reached. This is a sequence of bases on the mRNA molecule (UAA, UAG, or UGA) which tells the ribosome to stop reading. And so the protein is finished. Well, almost.

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