The variety of proteins synthesized by a cell and the specific amino acid compositions of each protein are ultimately governed by the cellular DNA.
This DNA is enzymatically transcribed in the cell nucleus to produce a host of RNAs, including ribosomal RNA (rRNA), messenger RNA (mRNA), and transfer RNA (tRNA).
Different RNA polymerases are responsible for the synthesis of each family of RNAs. The base sequences of these RNAs are complementary to the base sequences of the DNA molecules transcribed.
rRNA is ultimately incorporated into the cytoplasmic ribosomes, which may be free in the cytosol or attached to the surface of the endoplasmic reticulum that faces the cytosol.
Each ribosome consist of two parts or subunits a small subunit and a large subunit. The small subunit binds mRNA entering the cytosol from the nucleus, and the functional complex is completed with the subsequent addition of the large subunit. Attached ribosomes are linked to the endoplasmic reticulum by their large subunits.
The nucleotides of mRNA are arranged as a linear sequence of codons (also known as triplets), each co- don consisting of three successive nitrogenous bases. The codon sequence of each mRNA molecule contains all the information necessary to (1) properly initiate polypeptide synthesis on the ribosome, (2) designate the specific sequence of amino acids to be incorporated (i.e., the primary structure of the polypeptide), and (3) terminate polypeptide synthesis and release the completed polypeptide. Table 22-1 shows the various co- dons of mRNA and their meanings in protein synthesis. This is called the “genetic code.” The code is said to be degenerate because in certain instances a single amino acid may be coded for by more than one codon.
Molecules of tRNA entering the cytosol from the nucleus combine with amino acids; this is a molecule- specific association in that each amino acid species is enzymatically combined with a particular type (or species) of tRNA. The products, called aminoacyl-tRNA, represent the form in which amino acids are incorporated into newly synthesized protein. Each species of tRNA contains, among other functional groups, an anticodon (sequence of three bases) that is recognized by a corresponding (complementary) codon of mRNA and ensures that the correct amino acid will be incorporated into its proper position in the primary structure of the polypeptide being synthesized.
Once the mRNA-ribosome complex has been formed, amino acids bound to their specific tRNA molecules are sequentially brought to the ribosome and incorporated into the growing polypeptide chain. This process, called translation, is believed to take place by an orderly and linear movement of the mRNA along the ribosome (or vice versa) so that each codon is translated in sequence.
The elongation of the polypeptide chain takes place by a series of enzyme- catalyzed reactions occurring at three adjacent sites of the ribosome; these are:
(1) The A site (i.e., amino acid or acceptor site),
(2) The P site (i.e., peptide site), and
(3) The E site (i.e., exit site).
To understand the process of elongation, consider an intermediate stage in the synthesis of a polypeptide. At this time, the growing polypeptide chain is attached to the P site of the ribosome by a molecule of tRNA and is termed peptidyl-tRNA. The mRNA codon located at the adjacent amino acid site specifies the particular aminoacyl-tRNA that can be bound there. With a new aminoacyl-tRNA in position in the A site, the bond linking the growing polypeptide to its tRNA is broken and replaced by a peptide bond with the amino acid of aminoacyl-tRNA. This leaves the peptidyl-tRNA (which is now one amino acid longer) temporarily in the A site.
The tRNA molecule freed in the process is moved to the E site, while at the same time the peptidyl-tRNA is moved from the A site to the P site; these movements constitute translocation. Binding of another aminoacyl-tRNA at the now vacant A site is accompanied by the release of the tRNA molecule from the E site. The released tRNA reenters the cytosol where it may combine with another amino acid to be used in protein synthesis.
Translocation is accompanied by the movement of the ribosome and/or mRNA so that the next codon is in position at the amino acid site. Thus, tRNA molecules employed in bringing amino acids to the ribosome are transiently bound first to the A site, then to the P site, and finally to the E site before returning to the cytosol.
Amino acids are sequentially added to the growing polypeptide until its primary structure is complete. Once the end of the message encoded in the strand of mRNA is reached, the completed protein is released from the ribosome. The ribosome separates from the mRNA and dissociates into its two subunits; these may be used again in another round of protein synthesis.
Many mRNA molecules are large enough to be simultaneously translated by a number of ribosomes. These ribosomes move in series along the mRNA (or the mRNA is moved along the ribosomes), the net result being that its coded message is translated into a number of identical proteins.
The release of one ribosome at the end of the message is accompanied by the attachment of a new ribosome at the beginning of the message. Such strings of ribosomes are called polyribosomes or polysomes, and most cellular protein synthesis takes place on these structures. Although each mRNA molecule may be attached to several ribosomes, each ribosome synthesizes but a single protein chain before dissociating into its subunits.