Advanced Placement (AP) Biology Practice Exam

In the steroid signal transduction pathway, what becomes a transcription factor?

1. The hormone-receptor complex

2. The ligand molecule

3. The plasma membrane

4. The target cell

The correct answer is: The hormone-receptor complex

In the steroid signal transduction pathway, the hormone-receptor complex becomes a transcription factor. When steroids, which are lipophilic molecules, enter a target cell, they bind to specific intracellular receptors in the cytoplasm or nucleus. This binding activates the receptor, leading to a conformational change that allows the complex to function as a transcription factor. Once formed, the hormone-receptor complex can bind to specific DNA sequences in the promoter region of target genes, modulating the transcription of those genes. This process ultimately influences the production of specific proteins, which carry out the biological responses associated with the steroid hormone. The ability of the hormone-receptor complex to act directly on DNA highlights its role as a transcription factor in the regulation of gene expression. In contrast, the ligand molecule refers to the steroid itself before it binds to its receptor and doesn't directly function as a transcription factor. The plasma membrane serves as a barrier that the steroid must cross but does not participate in the transcriptional activity. The target cell describes the cell that responds to the steroid hormone and contains the necessary receptors, but it is the hormone-receptor complex that exerts the transcriptional effect.