Amino acids are the fundamental structural units of proteins. There are
20 amino acids found frequently in mammalian proteins, and these
molecules may be linked to one another to form a chain called a
polypeptide chain. The structure of a protein then depends on: (1) the
quantity of each amino acid present; (2) the sequence of amino acids in
the polypeptide chain; (3) the length of the polypeptide chain, that is,
the molecular weight; and (4) the folding and the side (nonlinear)
arrangement of the polypeptide chain molecules, that is, the secondary
and tertiary structures.
How can we investigate protein synthesis by using radioactive isotopes?
Since proteins are made up of amino acids, the logical conclusion, after
what we have learned about DNA synthesis and RNA synthesis, is that the
best way would be to mark an amino acid and follow its incorporation
into a molecule of protein. We could label a mixture of several amino
acids, but, for the sake of clarity, we will describe the incorporation
of a single labeled amino acid.