Bacterial protein synthesis — identifying active translation sites In prokaryotic cell biology, which cytological structure in a bacterium is a clear indicator that active protein synthesis (translation) is occurring?
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AA chromosome (the nucleoid region) indicating DNA storage
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BThe cytoplasmic (cell) membrane alone without ribosomal aggregates
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CA flagellum used for motility
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DA polysome (polyribosome) composed of multiple ribosomes on one mRNA
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EAn inclusion body storing reserve materials
Answer
Correct Answer: A polysome (polyribosome) composed of multiple ribosomes on one mRNA
Explanation
Introduction:In microbiology, recognizing where and how proteins are made inside bacterial cells is essential. Translation is the process by which ribosomes read messenger RNA (mRNA) and synthesize polypeptides. A hallmark of vigorous translation is the presence of polysomes (also called polyribosomes), visible by electron microscopy or detected biochemically as ribosome–mRNA complexes.
Given Data / Assumptions:
- Bacteria translate proteins using 70S ribosomes (50S + 30S subunits).
- During active translation, many ribosomes can bind a single mRNA simultaneously.
- Other cellular structures (chromosome, flagellum, membranes) may be present regardless of translation activity.
Concept / Approach:
A polysome forms when multiple ribosomes initiate on an mRNA and elongate concurrently. This arrangement boosts protein output and is a direct indicator of translation in progress. In bacteria, transcription and translation are often coupled, further enhancing polysome formation near the nucleoid.
Step-by-Step Solution:
Identify structures uniquely tied to translation: ribosomes and their assemblies on mRNA.Differentiate structural presence (e.g., chromosome, membrane) from functional evidence of translation.Recognize that multiple ribosomes on a single mRNA (a polysome) signal active protein synthesis.Conclude that among the choices, a polysome most directly indicates ongoing translation.Verification / Alternative check:
Polysome profiling (sucrose gradients) separates heavy polysomes from monosomes; increased polysome fractions correlate with higher translation rates. Electron micrographs classically show “beads-on-a-string” ribosome arrays on an mRNA.
Why Other Options Are Wrong:
Chromosome (nucleoid) stores genetic information but is not proof of translation at a given moment. The membrane and flagellum are not markers of translation. Inclusion bodies are storage or aggregated proteins, not necessarily active synthesis sites.
Common Pitfalls:
Confusing ribosome abundance with active translation; free ribosomes may be idle. Only polysome engagement with mRNA confirms active protein synthesis.
Final Answer:
A polysome (polyribosome) composed of multiple ribosomes on one mRNA