Question

unit 4 test

1. read each question carefully. write your response in the space provided for each part of each question. answers must be written out in paragraph form. outlines, bulleted lists, or diagrams alone are not acceptable and will not be scored.

the compound eyes of arthropods contain photoreceptor cells surrounded by other cells such as cone cells and pigment cells. scientists studied how interactions between neighboring cells in the developing eyes of larval fruit flies affect the differentiation of r7 precursor cells that normally become the photoreceptors of adult wild - type flies. one of their experiments is illustrated in figure 1.
r7 and r8 cells are both present in a cluster of cells in the developing eye. r8 cells have cell - surface proteins called boss. r7 precursor cells have cell - surface receptor tyrosine kinases (rtks) called sev and contain the g protein ras that is involved in intracellular signaling. rtks are proteins with an extracellular portion that binds to a ligand and an intracellular portion that transfers phosphate groups from atp to other proteins. scientists examined the fate of r7 precursor cells in fly larvae with wild - type (normal) forms of the boss, sev, and ras proteins or in larvae with single mutations in the sev (sev⁻) or double mutations in both sev (sev⁻) and ras (rasᴰ). rasᴰ is a form of ras that is always active.
figure 1. the fate of r7 precursor cells in the presence of r8 cells when the r7 precursor cells are wild - type (wild type) or have mutant sev proteins (single mutant) or have mutant sev and ras proteins (double mutant)
(a) describe the first interaction that must occur between the r7 precursor cell and the r8 cell for the r7 precursor to differentiate to an r7 photoreceptor cell.
(b) describe how the defect in the single - mutant (sev⁻) r7 precursor cell causes the cell to become a cone cell instead of a photoreceptor cell.

Explanation:

Part (a)

Step 1: Identify Key Components

The R8 cell has the Boss protein, and the R7 precursor has the Sev RTK. These are cell - surface proteins involved in signaling.

Step 2: Determine the Interaction

For the R7 precursor to differentiate into an R7 photoreceptor, the first interaction must be the binding of the Boss protein (on the R8 cell) to the Sev RTK (on the R7 precursor cell). This binding is the initial step in the signaling cascade that leads to the differentiation.

Step 1: Understand the Wild - Type Pathway

In the wild - type, the binding of Boss to Sev (a receptor tyrosine kinase, RTK) activates the Sev RTK. This activated RTK then activates the Ras protein through a series of intracellular signaling events. The activated Ras is part of the signaling cascade that leads to the R7 precursor differentiating into an R7 photoreceptor.

Step 2: Analyze the Single - Mutant (Sev⁻) Case

In the Single - Mutant (Sev⁻) R7 precursor cell, the Sev protein is non - functional (mutant). Since Sev is the receptor for the Boss ligand, the binding of Boss to Sev cannot occur (or the Sev RTK cannot be activated even if bound, depending on the nature of the mutation). Without the activation of Sev RTK, the downstream signaling cascade that activates Ras and leads to the differentiation into a photoreceptor is not initiated. As a result, the cell follows the default pathway of differentiation, which in this case is to become a cone cell.

Answer:

The first interaction is the binding of the Boss protein (present on the R8 cell) to the Sev RTK (present on the R7 precursor cell).

Part (b)