The biggest distinction between a eukaryotic cell and a prokaryotic cell is:

The story behind the answer

Correct: The presence or absence of a nucleus. Eukaryotic cells have a true, membrane-bound nucleus that houses DNA; prokaryotic cells (bacteria and archaea) lack a nucleus, with DNA instead in a nucleoid region. While size, organelles, and reproduction can differ, the defining hallmark is the nucleus.

Context: Eukaryotes (animals, plants, fungi, protists) also contain other membrane-bound organelles like mitochondria and the endoplasmic reticulum, likely arising via endosymbiosis. Prokaryotes are typically smaller and simpler but remarkably efficient and diverse.

Memory tip: “Eu-karyote = true kernel” (eu = true, karyon = kernel/nucleus), “Pro-karyote = no nucleus.” Think of the nucleus as the command center—eukaryotes have it enclosed; prokaryotes keep the plans on an open desk.