The complete package of genetic material for a living thing, a genome is one copy of the entire DNA in a cell of an organism (animal, plant or microbe) and includes both the chromosomes within the nucleus and the DNA in mitochondria. A copy of the genome is found in most cells. Our genome is 3,400,000,000 base pairs, packaged into 23 pairs of chromosomes: bacteria may have only 1,500,000 base pairs in one chromosome.