In the yeast signal transduction pathway, after both types of mating cells have released the mating factors and the factors have bound to specific receptors on the correct cells,

In the yeast signal transduction pathway, after both types of mating cells have released the mating factors and the factors have bound to specific receptors on the correct cells:

a. binding induces changes in the cells that lead to cell fusion.

Explanation:

There have been several studies about yeast cells and how they respond to the mating factors, a-factor and α-factor:

option b: the cells then produce the a factor and the  factor, is not correct as they are mating factors.

These studies have greatly contributed to help understanding the way  signals are transduced from a cell-surface receptor into a cell.

When yeast a cells are exposed to α-factor, they exhibit three responses, which are initiated in the moment the mating factor (a signaling molecule that acts between organisms, hence called a “pheromone”) binds to the receptor:

(1) they arrest in the G1 phase of the cell cycle, not option c: one cell nucleus binds the mating factors and produces a new nucleus in the opposite cell.

(2) they synthesize a variety of proteins involved in cell fusion, not option e: a growth factor is secreted that stimulates mitosis in both cells.

(3) they grow towards their mating partner, not option d: the cell membranes fall apart, releasing the mating factors that lead to new yeast cells.

The activated receptor triggers a so-called “MAP kinase cascade" protein kinase, followed by a transcriptional activator protein, Ste12. Ste12 inducing synthesis of assorted proteins that play an important role in arresting the cell cycle (the Far1 protein) and for cell fusion (the Fus1 protein).

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