How are gap junctions formed?

• A. A single connexon formed from connexin proteins
• B. Two connexons made from connexin proteins align to form a gap junction
• C. They are formed by clathrin-coated pits
• D. They are formed by desmosomal plaques

Answer: (B)

Gap junctions form when two hemichannels, called connexons, from neighboring cells dock head-to-head to create a continuous pore between the two cell cytoplasms. Each connexon is built from six connexin proteins, so when two of these hexamers align, they establish a gap junction channel that lets ions and small signaling molecules pass directly between cells. A single connexon by itself does not bridge the two cells; it must pair with a matching connexon from the adjacent cell. Structures involved in endocytosis (clathrin-coated pits) or cell–cell adhesion structures like desmosomes perform different roles, not the formation of the gap junction channel.