Which feature defines eukaryotes?

• A. Cells with a nucleus and membrane-bound organelles.
• B. Lack of a nucleus.
• C. Unicellularity only.
• D. Cell walls without cellulose.

Answer: (A)

The defining feature of eukaryotes is that their cells have a true nucleus enclosed by a membrane and contain membrane-bound organelles that compartmentalize cellular processes. This organization sets them apart from prokaryotes, which lack a nuclear envelope and most internal membranes, with their DNA located in a nucleoid region and metabolism carried out in a more open cytoplasm. Unicellularity occurs in both groups, so it doesn’t define eukaryotes, and cell walls are not universal or restricted in a way that would distinguish the group—plants and some algae have cellulose, fungi have chitin, while many organisms lack cell walls altogether.