Classification, the three domains and the kingdoms

Last time

Before we start: last time, in Types of selection, selective breeding and antibiotic resistance, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Sorting life into groups

There are millions of species, and biologists need a way to talk about them all. They sort them into groups within groups, from three huge domains down to the single species. The groups follow evolutionary relationships, and the evidence is increasingly the sequence of D N A. This lesson gives you the system and the features that tell the groups apart.

The hierarchy

The levels are domain, kingdom, phylum, class, order, family, genus and species. Each level groups organisms that share more features than the one above. A common memory aid is: Dear King Philip Came Over For Good Soup. Each word starts with the same letter as a rank. Every species has a binomial name: the genus with a capital letter, then the species in lower case, written in italics, such as Panthera tigris for the tiger.

The three domains

The three domains are Bacteria, Archaea and Eukarya. The first two contain only prokaryotes, with no nucleus. They are separated by differences in ribosomal R N A sequences, in the cell wall and membrane chemistry, and in R N A polymerase. Bacteria have peptidoglycan cell walls and membranes with ester linked lipids. Archaea have no peptidoglycan, have ether linked lipids, and have histone like proteins, and often live in extreme habitats. Eukarya have a nucleus and membrane bound organelles.