Observe the diagram and answer the following question.
What is indicated 'R' in the diagram?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Antigen binding site
R marks the notched tips of the two arms of the Y. These variable ends of the heavy and light chains fit a particular antigen, so they are the antigen binding sites.
Option (1) Light chain — Incorrect. A light chain is the whole shorter chain on the outside of each arm (marked S), not just its tip.
Option (2) Antigen binding site — Correct. Each arm of the Y-shaped antibody ends in a region formed by a light and a heavy chain, which binds the antigen. R, at the tip of an arm, is this antigen binding site.
Option (3) Heavy chain — Incorrect. The heavy chains are the two long chains (P and Q) that run down the lower part of the Y.
Option (4) Disulphide bond — Incorrect. The disulphide bonds are the small S–S links between the chains.