What precursor is produced when RNA polymerase I transcribes a set of eukaryotic rRNA genes? Select the correct rRNA precursor molecule.

Introduction / Context:
Ribosome biogenesis in eukaryotes begins with transcription of a large polycistronic rRNA precursor by RNA polymerase I. This precursor is processed to yield multiple mature rRNAs incorporated into ribosomal subunits.

Given Data / Assumptions:

• RNA polymerase I operates primarily in the nucleolus.
• The initial Pol I transcript (45S pre-rRNA in higher eukaryotes) is processed to 28S, 18S, and 5.8S rRNAs.
• 5S rRNA is transcribed separately by Pol III.

Concept / Approach:
Match polymerase to product: Pol I → 45S rRNA precursor; Pol III → 5S rRNA; Pol II → mRNA. Therefore, the correct precursor from Pol I is the 45S pre-rRNA.

Step-by-Step Solution:
Identify polymerase: RNA polymerase I.Recall product: 45S pre-rRNA.Exclude single mature rRNAs as initial products; they arise via processing.

Verification / Alternative check:
Pulsed-labeling and processing maps show the 45S transcript is cleaved and modified to produce the mature rRNAs.

Why Other Options Are Wrong:
Options listing 5S, 18S, or 28S as single direct products misrepresent that these are processed derivatives, not the primary Pol I transcript. 5S is a Pol III product.

Common Pitfalls:
Equating bacterial 16S/23S nomenclature with eukaryotic processing; eukaryotic large precursor organization differs.

Final Answer:
A single 45S rRNA precursor molecule.