The length of the DNA molecule that makes up the genome of E. coli is approximately 1,000 times longer than the length of an E. coli cell. Therefore the DNA must be tightly packaged within the cell. This requirement for some method to package DNA into a compact conformation also applies to eukaryote chromosomes. The chromosomes of eukaryotes are located in the nucleus of each cell, but all eukaryotes possess at least one other genome, in addition to the one in the nucleus, that is required for normal function. This is found in mitochondria .
Plants also possess a third genome in their chloroplasts .
Both of these additional, extra-nuclear genomes, in mitochondria and chloroplasts, share many similarities with prokaryote genomes. During the course of evolution the genomes in organelles have changed, but they are clearly related to those of bacteria. Many genes that are required for mitochondrial and chloroplast function are now found in the nuclear genome. Now I want to consider the sizes of different genomes. In the same way that computer memory is reported in bytes, the functional unit of computer memory, genomes are normally described in base pairs because this tells you something about the potential capacity of that genome and its ability to encode genes.