Prokaryotes vs. eukaryotes

Two cell plans: one with a nucleus, one without.

The idea

Prokaryotes (bacteria and archaea) are typically 1–10 µm across and keep a single circular chromosome in a nucleoid region with no membrane around it; they have no membrane-bound organelles, small 70S ribosomes built from 50S and 30S subunits, often extra DNA rings called plasmids, and a cell wall (peptidoglycan in bacteria). They reproduce asexually by binary fission: the chromosome is copied, the cell elongates, and a septum pinches it into two identical cells. Eukaryotes (animals, plants, fungi, and protists) are usually 10–100 µm, store linear chromosomes wound around histones inside a nuclear envelope, contain mitochondria, endoplasmic reticulum, and Golgi, use larger 80S ribosomes (60S + 40S), and divide by mitosis. Tellingly, mitochondria and chloroplasts carry their own circular DNA and 70S ribosomes — evidence that they descend from engulfed bacteria (endosymbiosis).

Work through an example

E. coli in a rich medium at 37 °C divides every 20 minutes. Starting from one cell, 2 hours means 6 doublings and 2⁶ = 64 cells; 8 hours means 24 doublings and 2²⁴ = 16,777,216 cells. The ribosome difference is medically useful: streptomycin and tetracycline bind the bacterial 30S subunit, so they stop bacterial protein synthesis but leave your own 80S cytoplasmic ribosomes alone.

What to watch for

The S in 70S and 80S stands for Svedberg units, a measure of how fast a particle sediments in a centrifuge, not its mass — which is why the subunits do not add up (50S + 30S = 70S, 60S + 40S = 80S). And "no organelles" does not mean "nothing inside": prokaryotes are packed with ribosomes and a simple cytoskeleton, and cyanobacteria even have internal photosynthetic membranes; what they lack is membrane-bound organelles.