A genome is the complete collection of hereditary information for an individual organism. In cellular life forms, the hereditary information exists as DNA. There are two fundamentally distinct types of cells in the living world, prokaryotic and eukaryotic, and the organization of genomes differs in these two types of cells.

Prokaryotes comprise the bacteria and archaea. The latter were originally designated "extremophiles" because they favor such extreme environments as high acidity, salinity, or temperature. Prokaryotic cells tend to be very small, have few or no cytoplasmic organelles, and have the cellular DNA arranged in a "nucleoid region" that is not separated from the remainder of the cell by any membrane. Eukaryotes exist as unicellular or multicellular organisms. Among the unicellular eukaryotes are the protozoa, some types of algae, and a few forms of fungi, while the multicellular organisms include animals, plants, and most fungi.

Eukaryotic cells are larger than prokaryotic cells, have a complex array of cytoplasmic structures, and have a prominent nucleus that communicates with components in the cytoplasm through an elaborate nuclear envelope. The hereditary information occurs principally in the nucleus of eukaryotic cells; in addition, minuscule (but essential) amounts of hereditary information occur in some cytoplasmic organelles (specifically, in chloroplasts for plants and algae, and in mitochondria for all eukaryotic groups).

Eukaryotic cells pass through a "cycle," progressing from a newly formed cell to a cell that is dividing to produce the next generation of progeny cells. Prior to division, the cell is in an "interphase"; during division, the cell is in a "division phase." During interphase, the nuclear DNA is organized in a dispersed network of chromatin, which is a complex consisting of nucleic acid and basic proteins. Immediately prior to and during division, the chro-matin condenses to a series of discrete, compact structures called chromo

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