Translation

Samford University -- Department of Biological and Environmental Sciences
Genetics -- Biol 333

Protein Synthesis: Translation (Gene Expression part II)

The genetic information in the form of a sequence of bases in DNA is transcribed into individual "messages" in the form of a sequence of bases in RNA. This process of transcription is followed by the process of translation, in which the meaning of this informational RNA molecule is decoded. Translation is then the process by which an RNA base sequence is used to directed the synthesis of a specific polypeptide with a specific amino acid sequence. In eukaryotes, it follows RNA processing and occurs after the mRNA leaves the nucleus through a nuclear pore and travels to the cytoplasm. That is, the two processes, transcription and translation, are temporally and spatially separated. In prokaryotes, however, translation begins before transcription is finished (they occur simultaneously) and they occur in the same place. In order for translation to occur, several things must be present--the major ones being an mRNA molecule, a ribosome, and charged transfer RNAs (tRNAs).
Transfer RNAs: These single-stranded RNAs are 70-80 bases in length and all have common sequences. There are numerous tRNAs--at least one for every amino acid. Eukaryotic and prokaryotic tRNAs are very similar in many respects. tRNAs have many modified bases. An amino acid will become covalently bonded to its specific tRNA creating a charged tRNA. Each tRNA is designated with a superscript that indicates which amino acid it will bind. For example, tRNA^ala is the tRNA that will be charged with (bound to) alanine. When it is charged, it is indicated as ala-tRNA^ala (alanine attached to the alanine-specific tRNA). tRNA have various modified bases.
tRNA's 3-D Structure: tRNAs all have the same general shape, described as a cloverleaf with internal base pairing holding the cloverleaf in place. This cloverleaf actually folds in on itself producing a more complex 3-D structure. 3' CCA: All tRNAs have the trinucleotide CCA at the 3' terminus. It is here that the amino acid will attach. The 3' CCA is at one end of the folded tRNA and the anticodon (see below) is at the other end. Aminoacyl-tRNA Synthetases and Charging of a tRNA: The attachment of an amino acid to the tRNA is catalyzed by a group of enzymes called aminoacyl-tRNA synthetases. There is a specific enzyme for each amino acid/tRNA pair. Anticodon: Three bases in the middle of the tRNA sequence make up the anticodon (although they end up at one end of the tRNA after it forms the cloverleaf and folds up). These three bases are unique for each tRNA and (as we will see) hydrogen bond to the codon of the mRNA during translation. (See Degenerate below for the role of modified bases in the anticodon.)
Ribosomes: Ribosomes are the sites where the actual protein synthesis occurs and are similar in composition in prokaryotes and eukaryotes. Ribosomes have a large subunit and small subunit which come together during translation. mRNA: The mRNA molecule used in translation has several features. (Remember, in eukaryotes it has already undergone extensive processing including splicing.) Untranslated Regions, Cap, Poly-A Tail: Messenger RNA molecules include a region before the beginning of the coding sequence and after the coding sequence ends. The "front" end of the mRNA is referred to as the 5' untranslated region (5' UTR) and the sequence at the "back" end is called the 3' untranslated region (3' UTR). They also have had a cap added to the 5' end and a poly-A tail added to the 3' end. Polycistronic mRNAs: Prokaryotes often have long mRNAs that code for more than one polypeptide. These are referred to as polycistronic mRNAs. Also, transcription and translation occur simultaneously in prokaryotes.

Translation: The process of translation is divided into these three stages: Initiation: During initiation in both prokaryotes and eukaryotes, a special charged tRNA and the 5' end of the mRNA bind to the small ribosomal subunit. Initiation in both prokaryotes and eukaryotes involves various protein factors. In both eukaryotes and prokaryotes, the assembled ribosome has three potential tRNA binding sites: the A site (aminoacyl site), the P site (peptidyl site) and the E site (exit site). Elongation: The polypeptide grows from amino to carboxyl end as new amino acids arrive at the A site bound to their respective tRNAs. Several ribosomes (a polysome or polyribosome) may be translating a given mRNA at any time. Termination: When a codon for which there is no tRNA with a complementary anticodon comes into the A site, translation terminates. These terminator codons are UAA, UAG, and UGA (also called nonsense codons). Instead of a tRNA binding to the open A site, a release factor binds there and stops protein synthesis.
The Genetic Code: The genetic code is expressed as a table of codons showing which codons specify which amino acids. There are several features of the genetic code.
Triplet: 3 bases per 1 amino acid Universal: The same code is used in (almost) all organisms. Non-ambiguous: A given codon always encodes the same amino acid. Degenerate: More than one codon is possible for most amino acids (64 codons, 20 amino acids). This is accomplished primarily through "wobble" pairing at the third codon position.
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