Genome
The genetic material of an organism consists of deoxyribonucleic acid (DNA). A gene is a segment of DNA that encodes a protein (or a struc­tural ribonucleic acid [RNA] for example, ribosomal RNA), along with the regulatory elements that control expression of that gene. The entire com­plement of DNA within the chromosomes of an organism is called the genome. The more complex organisms, that is, eukaryotes, contain much more DNA in their genomes than is found in genes. This nongene DNA has often been called “junk DNA,” as scientists have yet to find a specific function for it. The junk DNA can amount to 90 to 99 percent of the to­tal DNA in the cell nucleus.
Within the nucleus the DNA is part of the chromosomes. The number of chromosomes varies with species but is generally about twenty to forty pairs. However, there are exceptions: The round worm Ascaris megalocephala has but one pair of chromosomes, while the fern Ophioglossum reticulatum has six hundred thirty. Humans have twenty-three pairs. In prokaryotes, such as bacteria, the DNA is found in a single chromosome, and this con­stitutes the bacterial genome.
The concept of a genome can be extended. Mitochondria, the cellular organelles found in all eukaryotes, as well as plastids such as the chloro- plast found in plants, originally evolved from bacteria-like ancestors that took up residence within the primitive eukaryotic cell. These are called endosymbionts. Mitochondria and chloroplasts retain some of the genes of these ancestral endosymbionts, and one can then speak of the mitochondr­ial or chloroplast genome. In addition, many bacteria harbor plasmids, small circular pieces of DNA containing a few genes that form a plasmid genome.
Genomes do not have to consist of double-stranded DNA. Indeed, it is among the viruses that one finds a wide variety of genome forms. These genomes may be composed of double-stranded or single-stranded DNA. The DNA molecules may be linear or form a circle. Other viruses use RNA as their genetic material. These RNA genomes may be single-stranded or double-stranded. Viroids are another interesting group. Viroids are disease- causing entities in plants, such as the tomato stunt viroid or the avocado sunblotch viroid. Viroids resemble viruses, but unlike viruses they lack a coat protein(s) and consist of a genome of only approximately 240 to 400 bases of RNA.
The study of genomes has been made possible by the development of automated DNA sequencers and high-powered computers that can overlap pieces of genome sequence to derive the entire DNA base sequence. This led to the development in the late 1990s of a new field of study called ge­nomics. Genomics uses genome sequence data to identify genes, to predict the structure of gene products, to study the evolution of individual genes, or to examine the genetic relationships among species. With this technol­ogy, genome sequencing is progressing rapidly. The National Institutes of Health maintains a genome database (www.ncbi.nlm.nih.gov). As of May 2001, more than six hundred complete genomes have been deposited in the database. Most of these are viruses, along with four eukaryotes and almost fifty prokaryotes. Several hundred more partial sequences are also available. A first draft of the entire three-billion-plus bases of the human genome was completed in early 2000 and announced on June 26 of that year. The work is expected to be completed in 2003.  
References
National Institutes of Health. <http://www.ncbi.nlm.nih.gov>.
Singer, Maxine, and Paul Berg. Genes and Genomes. Mill Valley, CA: University Sci­ence Books, 1991.