Which one of the following structure is NOT found in prokaryotes?

Correct — A, Nuclear envelope. Prokaryote means 'before the nucleus', and the absence of a nuclear envelope is what the word records. A bacterial cell keeps its single circular DNA molecule in a region called the nucleoid, which has no membrane around it and no nucleolus inside it, so transcription and translation happen in the same compartment. Everything else on the list is present. A plasma membrane bounds every cell, prokaryote or not. Ribosomes are present, though of the smaller 70S type rather than the 80S ribosomes of the eukaryotic cytoplasm. A cell wall is present in almost all bacteria, made of peptidoglycan rather than the cellulose of plants.

• (b)Cell membrane — Every living cell has a plasma membrane; without one it could not maintain a separate internal chemistry. In bacteria the membrane also carries out work that mitochondria do in eukaryotes, including respiration.
• (c)Ribosome — Ribosomes are universal, because all cells make proteins. Bacterial ribosomes are 70S rather than 80S, and that difference is what several antibiotics exploit to attack bacteria without harming human cells.
• (d)Cell wall — A cell wall is present in almost all bacteria and gives them their shape and osmotic protection. Its peptidoglycan is the target of penicillin, and the way it takes the Gram stain divides bacteria into the two great groups.

Prokaryotic cells lack a nuclear envelope and lack every membrane-bound organelle — no mitochondria, no chloroplasts, no endoplasmic reticulum, no Golgi apparatus, no lysosomes. What they do have is a plasma membrane, ribosomes, a nucleoid carrying circular DNA, often plasmids, and usually a cell wall. Eukaryotic cells have a true nucleus bounded by a double membrane with pores, linear chromosomes wound on histones, and the full set of organelles. Bacteria and archaea are prokaryotes; all other life is eukaryotic.

The item is a definition dressed as a list. Once you translate the word prokaryote, the answer is forced, and the other three options are things every cell has. The genuinely useful detail is the ribosome difference: the 70S bacterial ribosome is chemically distinct enough from the 80S human one that antibiotics such as streptomycin and tetracycline bind one and not the other, which is what makes them selectively toxic. Mitochondria and chloroplasts also carry 70S ribosomes, which is one of the pillars of the endosymbiotic theory.

• Prokaryotes have no nuclear envelope; their DNA lies in a nucleoid with no surrounding membrane.
• They also lack mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus and lysosomes.
• Prokaryotic ribosomes are 70S; eukaryotic cytoplasmic ribosomes are 80S.
• Bacterial cell walls are made of peptidoglycan, the target of penicillin.
• Mitochondria and chloroplasts carry 70S ribosomes and circular DNA, which supports the endosymbiotic theory.

• Assuming prokaryotes have no ribosomes because they have no other organelles.
• Confusing the nucleoid, which is a region, with a nucleus, which is a bounded organelle.
• Thinking bacterial cell walls are made of cellulose like plant walls.

A definition item disguised as a structure list. Translating the word prokaryote answers it outright.

• practice — not a real PYQ

  The genetic material of a bacterial cell is located in a region called the

  • (a)nucleus
  • (b)nucleoid
  • (c)nucleolus
  • (d)nuclear envelope

  Answer(b) nucleoid — an unbounded region of the cytoplasm, not a membrane-bound organelle.

• practice — not a real PYQ

  Ribosomes found in prokaryotic cells are of which type?

  • (a)80S
  • (b)70S
  • (c)60S
  • (d)40S

  Answer(b) 70S — the difference from the 80S eukaryotic ribosome is what several antibiotics exploit.