The template-recognition site on tRNA is a sequence of three bases called the anticodon. Proteins are built from a basic set of 20 amino acids, and so that many amino acids are designated by more than one codon. In 64 codons, UAG, UAA, and UGA are the only three codons that do not specify an amino acid. The start signal for protein synthesis is more complex. Thus, the genetic code is nearly universal.
Proteins play key roles in biological systems.
For example, specific macro molecules called enzymes catalyze nearly all chemical reactions in biological systems. Specific proteins transport many small molecules and ions. For instance, hemoglobin transports oxygen in erythrocytes. And proteins are also the major component of muscle. Antibodies are highly specific proteins that recognize and combine with such foreign substances as viruses, bacteria, and cells from other organisms.