Final answer:
The Ramachandran plot visualizes the dihedral angles of amino acid residues in protein structures, with points representing the phi and psi angles of residues. The plot is divided into most favored, allowed, and disallowed regions, with amino acid residues predominantly found in the most favored and allowed regions, and rarely in disallowed regions due to steric clashes.
Step-by-step explanation:
The Ramachandran plot is a graphical representation used to visualize dihedral angles φ (phi) and ψ (psi) of the amino acid residues in protein structure. Each point on the plot represents the φ and ψ angles of a residue within a polypeptide chain. There are typically three regions on the plot: the most favored regions (where the combinations of φ and ψ are commonly found in proteins), the allowed regions (less common but still permissible conformations), and the disallowed regions (where steric clashes prevent the amino acids from adopting those φ and ψ angle combinations). In terms of the number of residues in each region, the most favored regions contain majority of amino acid residues as these conformations are energetically favorable. Allowed regions may contain a smaller number of residues that are found in stable, but less energetically favorable conformations. Disallowed regions have minimal to no residues because the steric hindrance typically makes these conformations energetically unfavorable and, therefore, rarely observed. Proteins designed in labs, under certain conditions, can sometimes have amino acids located in those disallowed regions, although it's not common in naturally occurring proteins.