The process of protein synthesis, translation, is centrally important in biology, and for the MCAT, so let's review and expand on the basics a bit. Protein synthesis takes place on ribosomes, complex assemblages of rRNA and protein. Consisting of a large and small subunit, ribosomes are complex assemblies of over fifty separate proteins and three RNA molecules. Ribosomes are somewhat larger and more complex in eukaryotes. rRNA, through internal base pairing folding into defined structures in ribosomes, comprises the majority of ribosome mass.
Activation of an amino acid to be used in translation begins when the carboxyl group of the amino acid is joined to the 3' -OH of tRNA by a specific aminoacyl-tRNA synthetase to form aminoacyl-tRNA, the activated precursor in protein synthesis. Protein synthesis begins with initiation (occupation of the P (peptidyl) site by initiator tRNA), proceeds through subsequent elongation steps (binding of the next aminoacyl-tRNA in the A (aminoacyl) site, shifting to next codon, etc.), to end with termination.
Rather than the entire mRNA being translated, mRNA molecules contain signals that define the beginning and end of the polypeptide chain. While prokaryotic mRNA can have multiple start sites and serve as the template for the synthesis of several proteins, eukaryotic mRNA has only one start site and serves as the template for only a single protein.
Protein synthesis in prokaryotes begins with formylmethionine, brought by initiator tRNA to the ribosome. The initiation complex begins on the small subunit when a portion of the start signal on mRNA pairs with a section of rRNA and the initiator codon on mRNA pairs with the fMet initiator tRNA. Upon pairing with the large subunit, initiation is complete, and elongation can begin. fMet-tRNAf occupies the P site and the A site is empty. Subsequent elongation occurs with alternation of amino-acyl-tRNA binding, peptide bond formation, and translocation. Protein synthesis ends upon reaching a stop codon where a release factor binds rather than another aminoacyl-tRNA. In overview, the small ribosomal subunit is tasked with ordering the amino acids according to mRNA sequence; the large subunit is tasked with linking the amino acids.