A genome is the complete set of genetic material (DNA) possessed by an organism. Most of the DNA is usually found in the chromosomes of the cell nucleus, but DNA is also present in organelles, such as the mitochondria of animal cells, the chloroplasts of plants, and the plasmids of bacteria). In humans, every cell other than the red blood cell contains a complete genome. (A red blood cell doesn't have a nucleus.) Genome is also used to refer to the RNA of RNA viruses and other parasitic quasi-life forms.

Actually, not all of the DNA consists of active genes (euchromatin). Only 2% of the human genome, for example, is euchromatic.

The smallest genome of free-living organisms is possessed by bacteria (from about half a million base pairs, which constitute about 500 genes). Below bacteria, DNA viruses have genomes as small as a few thousand base pairs. HIV has about 9,000. Humans are close to the top the list of genome sizes, with about 3 billion base pairs, which are organized into 24 chromosomes and a number of genes that is still a matter of guessing, but is likely to be on the order of 30,000. Above us are the fugu (more genes, but less 'junk' DNA), rice (!), the amphibians (up to 30 times as much DNA as humans), and the whisk fern (one of the simplest plants). The simpler organisms that have genomes bigger than ours have large amounts of repetitive DNA that has no genetic importance that we know of.

The Human Genome Project