|Édouard Chatton, Roger Stanier, C. B. van Niel cell biology|
Prokaryote and eukaryote
Prokaryote are unicellular organisms without nuclei enclosed in a membrane. Eukaryote are organisms with cells whose nuclei are enclosed in a membrane. Eukaryotes can be multicellular. Prokaryotes have circular chromosomes; eukaryotes have linear chromosomes. Prokaryotes are divided into archaea and bacteria—based on the structure of their 16S ribosomal RNA and so forth. It hasn’t really been as simple as that.
Is it a vegetable or an animal? Which came first, archaea or bacteria? Is cyanobacteria (blue green algae) an algae (a plant) or bacteria (an animal)? Did archaea once have nuclei enclosed in a membrane, and then lost it? Did bacteria spawn neomura, which in turn spawned archaea and eukaryota? Or did a common ancestor spawn both bacteria and neomura? Should prokaryotes be divided first on whether their membranes are single or double? Are eukaryotes bacteria that ate themselves? Many questions are given different answers in different theories that emphasize different things.
We may think of bacteria as if they weren’t alive, no blood, no fur, no big brown eyes. But how are they different from angels since plenty of them would have room to dance on the head of a pin.