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. Hence, proteins mediated a wide range of other functions, such as generation and transmission of nerve impulses, control of growth and differentiation. Proteins indeed play crucial roles in virtually all biological processes.