All creatures can be divided into two groups: prokaryotes or eukaryotes, depending on their cells’ structure. Prokaryotes are unicellular living organisms; they do not transform into a multicellular form, capable of autonomous existence. They can be bacteria, including cyanobacteria or blue-green algae and archaea. These organisms are the oldest and the most primitive ones on the planet. Prokaryotes can be found everywhere: in the air, in water, in soil, inside living organisms. In the atmosphere, bacterial spores are present at altitudes up to almost twenty km, and they penetrate the ground to a depth of approximately four km. Bacterial cells are very diverse in shape, such as rods, rounded, hexagonal, star-shaped, stem-like ones. Some of them unite in pairs, others create chains; moreover, they may build clusters like bunches of grapes.
Unlike prokaryotes, eukaryotes are nuclear living organisms, and their cells contain a nucleus. They can be both unicellular and multicellular, but their cell structure is the same. The group of eukaryotes includes plants, animals, including humans, and fungi. The main thing that distinguishes prokaryotes from eukaryotes is the absence of a cell nucleus. This means that prokaryotic DNA is not organized into chromosomes and is not surrounded by a nuclear envelope. Eukaryotic cells are much more complicated. Their DNA is packed into chromosomes, which are located right in the nucleus.