In The Genetic Code, 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. That explanation is correct, but it is also simplified, and overlooks a crucial component of the translation process. 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 next section.
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.
The cloverleaf structure shown above is actually a two dimensional simplification of the actual tRNA structure. The cloverleaf is therefore called a secondary structure. In reality, 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 structure of the anticodon of tRNA helps to explain the degeneracy of the genetic code. Previously, in the SparkNote on the Genetic Code, 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.
The flexibility of some anticodons is the result of the fact that the 3' end of the anticodon is more spatially confined than the 5' end. As a result, the 5' end of the anticodon is free to hydrogen bond with several base groups located at the 3' position of the codon. This idea is called the wobble hypothesis and has been confirmed by X-ray studies that show that while the 3' and middle positions are held tightly in a specific orientation by stacking interactions, the 5' position is not. The 5' position is called the wobble position because it can move around to allow for its pairing with different bases.
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."
Before translation can occur, a molecule of tRNA must be bound with the appropriate amino acid. This two-step process is called "charging". In the first step, called adenylylation, an ATP molecule is hydrolyzed, releasing two phosphates, and, transfigured into AMP, forms a high-energy bond with an amino acid.
The specificity of the charging reaction is maintained through two mechanisms. First, amino acids recognize the correct tRNA through distinguishing features on the tRNA, such as the acceptor stem, D stem, and anti-codon stem. Second, the entire process of charging is goverened by a proofreading system that ensures the appropriate amino acid has been loaded onto each tRNA molecule. The proof- reading mechanism checks the reaction at both steps to ensure proper pairing.