Advanced Placement (AP) Biology Practice Exam

What type of pathway requires the formation of a dimer to proceed?

1. G-protein linked receptor pathway

2. Tyrosine kinase pathway

3. Ion channel receptor pathway

4. Glycoprotein receptor pathway

The correct answer is: Tyrosine kinase pathway

The tyrosine kinase pathway is characterized by the requirement for receptor dimers to become activated. When a signaling molecule, such as a growth factor, binds to a tyrosine kinase receptor, it typically causes two of these receptors to come together, forming a dimer. This dimerization triggers autophosphorylation, where each receptor phosphorylates tyrosine residues on the other. This phosphorylation event activates the receptor's kinase activity, allowing it to relay signals inside the cell and initiate a cascade of downstream signaling events that ultimately lead to cellular responses like growth, differentiation, or metabolism. The other pathways do not involve dimer formation in the same way. For instance, G-protein linked receptor pathways often operate through the activation of guanine nucleotide-binding proteins but do not require receptor dimers for their signaling function. Ion channel receptor pathways allow ions to flow through channels directly in response to ligand binding, while glycoprotein receptors play roles in various recognition processes but do not inherently rely on dimerization for their activity. Thus, the unique feature of the tyrosine kinase pathway lies in its reliance on dimerization for activation and subsequent signaling.