the prokaryote-eukaryote divide

All cellular life sorts into two great architectural styles, and the difference comes down to one question: is the DNA shut away inside a nucleus or not? Cells without a nucleus are prokaryotes, a word that literally means 'before the kernel'; cells with a true nucleus are eukaryotes, meaning 'true kernel'. This single difference in floor plan cascades into countless differences in how the two kinds of cell live and read their genes.

A prokaryote, such as a bacterium, keeps its usually circular genome loose in the cell with no surrounding membrane and few internal compartments, and it is typically small, around a thousandth of a millimetre across. Because there is no nucleus, transcription and translation happen in the same space at the same time: ribosomes can start reading a messenger RNA while it is still being made. A eukaryote, such as a yeast, plant, or animal cell, packages its much larger genome into linear chromosomes inside a membrane-bound nucleus, and it has many other compartments such as mitochondria. Here transcription happens in the nucleus and translation out in the cytoplasm, which separates the two steps in space and time and creates room for elaborate RNA processing in between.

This divide matters for almost everything that follows, because the molecular machinery and the regulation differ between the two groups. Bacterial genes are often bundled into operons read as one unit; eukaryotic genes are usually read one at a time, are interrupted by introns, and are wrapped around proteins into chromatin. Knowing which kind of cell you are dealing with tells you, in advance, roughly how its information will be stored, copied, and read.