Questions: Ribosomes and Protein Synthesis Introduction

Question 1

A newly synthesized protein contains a signal sequence at its N-terminus that targets it to the ER membrane. Where was it most likely being translated?

• A. On a free ribosome in the cytoplasm, then transported to the ER after synthesis
• B. On a membrane-bound ribosome docked to the rough ER
• C. Inside the nucleus, before mRNA export
• D. On a mitochondrial ribosome in the matrix

Answer: (B)

Proteins destined for the ER, Golgi, secretion, or the plasma membrane are synthesized on ribosomes that dock to the rough ER *during* translation — the signal sequence is recognized co-translationally. The growing polypeptide is threaded directly into the ER lumen or membrane. Free ribosomes in the cytoplasm produce soluble cytoplasmic and nuclear proteins. Ribosomes do not exist inside the nucleus, and mitochondrial ribosomes synthesize only a handful of inner-membrane proteins.

Question 2

Prokaryotic and eukaryotic ribosomes perform the same molecular function and are therefore structurally identical.

• T. True
• F. False

Answer: False

Both translate mRNA into protein using the same genetic code and basic mechanism, but their physical structures differ significantly. Prokaryotic ribosomes are 70S (made of 30S and 50S subunits); eukaryotic ribosomes are 80S (40S and 60S subunits). The differences in ribosomal RNA and protein composition are clinically exploited by antibiotics: drugs like streptomycin and erythromycin bind specifically to bacterial ribosomal components that differ from their eukaryotic counterparts, killing bacteria without harming the patient's own cells.

Question 3

Trace the journey of a protein destined for secretion from the gene in the nucleus to its release outside the cell.

Model answer: The gene is transcribed into pre-mRNA in the nucleus → processed into mature mRNA → exported through nuclear pores → translated by a ribosome that docks to the rough ER → the protein enters the ER lumen → transported in vesicles to the Golgi → further processed and packaged → secretory vesicles fuse with the plasma membrane → protein is released extracellularly.

This pathway highlights that secreted proteins never exist free in the cytoplasm. From the moment the signal sequence emerges from the ribosome, the protein is threaded into the ER lumen and travels through membrane-enclosed compartments (ER → Golgi → vesicle) until secretion. Understanding the contrast with cytoplasmic proteins (made on free ribosomes, released directly into the cytosol) clarifies why the cell needs two ribosome populations.