Questions: Prokaryotic Cells

Question 1

A new antibiotic is designed to block assembly of 80S ribosomes. Which organisms would this drug harm?

• A. Bacteria only, because they use 80S ribosomes for protein synthesis
• B. Both bacteria and human cells equally, since all cells use the same ribosomes
• C. Human cells, because bacteria use 70S ribosomes — not 80S
• D. Archaea only, since they are the prokaryotes with 80S ribosomes

Answer: (C)

Prokaryotes (bacteria and archaea) use 70S ribosomes; eukaryotes (including human cells) use 80S ribosomes. A drug targeting 80S ribosomes would harm human cells while leaving bacteria unaffected — the opposite of what an antibiotic should do. This ribosome size difference is medically exploited in reverse: antibiotics like erythromycin and tetracycline target 70S ribosomes to kill bacteria without harming human cells.

Question 2

What is the nucleoid region in a prokaryotic cell?

• A. A membrane-bound organelle that houses the main chromosome, analogous to a eukaryotic nucleus
• B. A dense region of cytoplasm where the chromosome is concentrated, with no surrounding membrane
• C. A specialized region where ribosomes are manufactured before being released into the cytoplasm
• D. The outer membrane that separates Gram-positive from Gram-negative bacteria

Answer: (B)

The nucleoid is not a compartment — it has no bounding membrane. It is simply the region of cytoplasm where the single circular chromosome is most densely packed. This is the defining difference from a eukaryotic nucleus: the absence of a nuclear membrane means prokaryotic DNA is in direct contact with ribosomes, enabling simultaneous transcription and translation.

Question 3

In prokaryotic cells, a ribosome can begin translating an mRNA molecule while it is still being transcribed from DNA — something impossible in eukaryotic cells.

• T. True
• F. False

Answer: True

This is a direct consequence of the prokaryotic cell's defining feature: no nuclear membrane. In eukaryotes, transcription occurs inside the nucleus and the mRNA must be processed and exported before ribosomes (in the cytoplasm) can access it — separating the two processes in time and space. Prokaryotes have no such separation, so translation can begin on the emerging mRNA immediately.

Question 4

Prokaryotic cells have no internal structures — they are uniform bags of cytoplasm bounded by a membrane.

• T. True
• F. False

Answer: False

This is a common overgeneralization. Prokaryotes lack membrane-bound organelles, but they do have internal organization: ribosomes (70S) throughout the cytoplasm, the nucleoid region where DNA is concentrated, plasmids, and often a cell wall outside the membrane. Some bacteria also have internal membrane systems for photosynthesis. Additionally, prokaryotes have primitive cytoskeletal proteins (homologs of actin and tubulin), contrary to older textbook claims.

Question 5

Why is the absence of a membrane-bound nucleus — rather than small size or structural simplicity — considered the defining characteristic of prokaryotic cells?

Model answer: The nuclear membrane is functionally, not just structurally, significant: it determines whether transcription and translation are separated in space and time. All other 'simple' features of prokaryotes (small size, no organelles) are consequences or correlates, but a cell without a nucleus still operates as a prokaryote even if it were large or metabolically complex. The nuclear membrane defines whether gene expression follows the prokaryotic or eukaryotic pathway.

Size is not definitive — some prokaryotes are larger than some eukaryotes. Metabolic simplicity is not definitive — prokaryotes are often more metabolically diverse than eukaryotes. But the nuclear membrane determines the entire architecture of gene expression: whether mRNA is processed before translation, whether transcription and translation can occur simultaneously, and how the genome is organized. It is the structural feature with the most downstream functional consequences.