Which of the following is the primary hallmark distinguishing prokaryotes from eukaryotes?

• A. Presence of a nucleus
• B. Lack of a nucleus
• C. Membrane-bound organelles
• D. Cytoskeleton

Answer: (B)

The main distinction between prokaryotes and eukaryotes is where their genetic material is housed. Prokaryotes lack a true nucleus; their DNA sits in a nucleoid region within the cytoplasm, not enclosed by a nuclear membrane. Eukaryotes, in contrast, have a defined nucleus protected by a nuclear envelope. Because this feature of a nucleus (or its absence) is the fundamental organizational difference, the lack of a nucleus is the best answer. If a cell has a nucleus, it’s a hallmark of a eukaryote, not a prokaryote. Membrane-bound organelles and cytoskeletons are related differences, but they don’t define the simplest, most universal distinction as clearly as the presence or absence of a nucleus.