10: Translation

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The RNA polymerase has done its job (or in the case of prokaryotes, may still be in the process of doing its job), so now what happens to the RNA? For RNA that is destined to provide instructions for making a protein, then it needs to be translated, which is a job for Superman^TM! Oops, actually it’s a job for ribosomes.

10.1: Introduction to Ribosomes
Ribosomes are a complex of RNA and protein that bind to and processively move down (from 5’ to 3’ end) a strand of mRNA, picking up aminoacyl-tRNAs, checking to see if they are complementary to the RNA tri-nucleotide being “read” at the moment, and adding them to the new polypeptide chain if they are. The RNA part of the ribosomes are generated by the organism’s general purpose RNA polymerase in prokaryotes, and generated by the RNA polymerases I and III in eukaryotes.
10.2: Prokaryotic Ribosomes
The prokaryotic ribosomes contain 3 RNA strands and 52 protein subunits which can be divided into 1 RNA and 21 proteins in the small ribosomal subunit (aka the 30S subunit) and 2 RNA and 31 proteins in the large ribosomal subunit (50S subunit). The small subunit locates the start site and moves along the RNA. The large ribosomal subunit contains the aminoacyl transferase enzyme activity that connects amino acids to make a protein. Neither subunit is sufficient to carry out translation by itself.
10.3: Eukaryotic Ribosomes
Like the RNA molecules in prokaryotic ribosomes, the eukaryotic rRNA molecules are also post-transcriptionally cleaved from larger transcripts. This processing, and the subsequent assembly of the large and small ribosomal subunits are carried out in the nucleolus, a region of the nucleus specialized for ribosome production, and containing not only high concentrations of rRNA and ribsomal proteins, but also RNA polymerase I and RNA polymerase III.
10.4: The Genetic Code
We have blithely described the purpose of the DNA chromosomes as carrying the information for building the proteins of the cell, and the RNA as the intermediary for doing so. Exactly how is it, though, that a molecule made up of just four different nucleotides joined together (albeit thousands and even thousands of thousands of them), can tell the cell which of twenty-odd amino acids to string together to form a functional protein?
10.5: tRNA are Rather Odd Ducks
In prokaryotes, tRNA can be found either as single genes or as parts of operons that can also contain combinations of mRNAs or rRNAs. In any case, whether from a single gene, or after the initial cleavage to separate the tRNA transcript from the rest of the transcript, the resulting pre-tRNA has an N-terminal leader (41 nt in E. coli) that is excised by RNase P. That cleavage is universal for any prokaryotic tRNA.
10.6: Prokaryotic Translation
As soon as the RNA has emerged from the RNAP and there is sufficient space to accommodate a ribosome, translation can begin in prokaryotes. In fact, for highly expressed genes, it would not be unusual to see multiple RNA polymerases transcribing the DNA and multiple ribosomes on each of the transcripts translating the mRNA to protein!
10.7: Eukaryotic Translation
Eukaryotic translation, as with transcription, is satisfyingly similar (from a student studying point of view, or from an evolutionary conservation one) to the prokaryotic case. The initiation process is slightly more complicated, but the elongation and termination processes are the same, but with eukaryotic homologues of the appropriate elongation and release factors.
10.8: Regulation of Translation
Gene expression is primarily regulated at the pre-transcriptional level, but there are a number of mechanisms for regulation of translation as well. One well-studied animal system is the iron-sensitive RNA-binding protein, which regulates the expression of genes involved in regulating intracellular levels of iron ions.

Thumbanil: Diagram of RNA translation. (CC BY 3.0 - unported; Kelvinsong).