Introduction
DNA contains all the information for encoding the amino acids that make up proteins. DNA is made up of nucleotides, and proteins are made up of amino acids. The information in DNA is first transcribed into messenger RNA (mRNA), which is also made up of nucleotides. The process of converting the genetic message found in mRNA to an amino acid sequence (protein synthesis) is called translation. How does the cell convert the nucleotide sequence of mRNA into the amino acid sequence of proteins? This process involves a large enzyme called the ribosome and an adapter molecule between the two languages of mRNA sequence and peptide sequence called transfer RNA (tRNA).
The Ribosome
The synthesis of proteins is catalyzed by the ribosome. The ribosome is made up of a large and small subunit, and is a large enzyme comprised mostly of ribosomal RNA (rRNA), with proteins interspersed like islands in a sea of RNA. Besides the rRNA, the ribosome contains binding sites for tRNA and mRNA. The rRNA forms most of the ribosomal structure and performs the catalytic steps of peptide synthesis, the mRNA delivers the genetic message, and tRNA translates the genetic code into peptide sequence.
Transfer RNA
tRNA is involved in the translation of the nucleic acid message into the amino acids of proteins. tRNA itself is an RNA molecule with a conserved inverted L structure. One end of the tRNA contains an anticodon loop which pairs with a mRNA specifying a certain amino acid. The other end of the tRNA has the amino acid attached to the 3' OH group via an ester linkage.
tRNA with an attached amino acid is said to be "charged". The enzyme that attaches the amino acid to the 3'-OH is called an aminoacyl tRNA synthetase (aaRS). There is a specific tRNA for each amino acid, 20 in all. Similarly, there is a specific aaRS for each tRNA.
Only the first 2 nucleotides in the tRNA anticodon loop are strictly required for the decoding of the mRNA codon into an amino acid. The third nucleotide in the anticodon is less stringent in its base-pairing to the codon, and is referred to as the "wobble" base. Since the genetic code is degenerate, meaning that more than one codon can specify a single amino acid, the anticodon of tRNA can pair with more than one mRNA codon and still be specific for a single amino acid.
Protein synthesis begins when a charged tRNA, mRNA, and the small and large ribosomal subunits (30S and 50S for E. coli) come together to form the initiation c complex (70 S in E. coli). The initiation complex contains a peptidyl binding site, (P site) and an aminoacyl binding site (A site). The first tRNA to bind is always the initiator tRNA, tRNAfMET. tRNAfMET binds to the start codon of mRNA, AUG. The first amino acid of the protein is thus methionine.
