The structure of an immunoglobulin G (antibody) molecule is shown schematically below. The black solid lines depict individual polypeptides and so there are four polypeptides in the quaternary structure of this molecule. Each of the spheres represents a stretch of about 100 amino acids folded independently of the rest of the polypeptide and performs a specific function in the molecule. Therefore, each sphere was given its own individual name (VH, CH1, CH2, etc.). Without knowing any additional details, you can predict that there must be TWELVE ___________________ in this molecule.

• A. α-helices.
• B. Domains.
• C. Subunits.
• D. Motifs.

Answer: (B)

Verified step by step guidance

1. Begin by examining the schematic representation of the immunoglobulin G (IgG) molecule. Notice that it consists of four polypeptide chains, depicted by the black solid lines.
2. Identify the spheres labeled VH, CH1, CH2, CH3, VL, and CL. Each sphere represents a domain, which is a distinct structural and functional unit within the polypeptide chain.
3. Count the number of spheres in the diagram. There are twelve spheres in total, indicating twelve domains within the IgG molecule.
4. Understand that each domain is approximately 100 amino acids long and folds independently, contributing to the specific function of the antibody.
5. Conclude that the correct term to fill in the blank is 'Domains,' as each sphere represents a domain in the quaternary structure of the immunoglobulin G molecule.