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In the previous section, we explained how each codon in messenger RNA (mRNA) codes for a specific amino acid, and that in the process of translation the mRNA brings the amino acids together to form proteins. However, there needs to be a component that deciphers this code. That component is transfer RNA (tRNA), which acts as a kind of link between the information encoded in the mRNA and the amino acids. If the mRNA is a code, then the tRNA is the key that interprets that code into physical proteins.
This section will describe the structure of tRNA and describe how tRNA can "carry" amino acids; knowledge of these aspects of tRNA will be vital for understanding the actual process of proteins synthesis covered in the next section.
The Structure of tRNA
Transfer RNA molecules vary in length between 60 and 95 nucleotides, with the majority measuring about 75 nucleotides (much smaller than the normal mRNA strand). Regions of self-complementarity within tRNA creates a cloverleaf-shaped structure.
Figure 6.10: tRNA Cloverleaf Structure
The cloverleaf, being a cloverleaf, is comprised of three characteristic loops. The loop on the bottom of the cloverleaf contains the anticodon, which binds complementarily to the mRNA codon. Because anticodons bind with codons in antiparallel fashion, they are written from the 5' end to 3' end, the inverse of codons. For example, the anticodon in the figure above should be written 3'-CGU-5'. At the 3' end of the tRNA molecule, opposite the anticodon, extends a three-nucleotide acceptor site that includes a free -OH group. A specific tRNA binds to a specific amino acid through its acceptor stem.
The cloverleaf structure shown above is actually a two-dimensional simplification of the actual tRNA structure. The cloverleaf is therefore called a secondary structure. The cloverleaf folds further into a tertiary structure, a sort of vague L-shape. At one end of the L lies the anticodon; at the other is the acceptor stem. The L-shaped structure simply amplifies the two active ends of tRNA: the anticodon and the acceptor stem.
The Wobble Hypothesis
The structure of the anticodon of tRNA helps to explain the degeneracy of the genetic code. Previously, we saw that more than one codon could specify a particular amino acid. However, now we know that tRNA acts as a go between for the codons of mRNA and amino acids. Each tRNA binds to a specific amino acid, but the anticodons of some tRNA molecules can bind to two or three different codons. This idea is called the wobble hypothesis. The wobble hypothesis means that if the first and second positions are the same, certain different bases in the third position will code for the same amino acid. Codons that specify for the same amino acid are called "synonyms."
