What is Genomic DNA?

Genomic DNA constitutes the total genetic information of an organism. The genomes of almost all organisms are DNA, the only exceptions being some viruses that have RNA genomes. Genomic DNA molecules are generally large, and in most organisms are organized into DNA–protein complexes called chromosomes. The size, number of chromosomes, and nature of genomic DNA varies between different organisms (see table Sizes and molecular weights of various genomic DNAs). Viral DNA genomes are relatively small and can be single- or double-stranded, linear, or circular. All other organisms have double-stranded DNA genomes. Bacteria have a single, circular chromosome. In eukaryotes, most genomic DNA is located within the nucleus (nuclear DNA) as multiple linear chromosomes of different sizes. Eukaryotic cells additionally contain genomic DNA in the mitochondria and, in plants and lower eukaryotes, the chloroplasts. This DNA is usually a circular molecule and is present as multiple copies within these organelles.

Sizes and molecular weights of various genomic DNAs

Genomic DNA contains genes, discrete regions that encode a protein or RNA. A gene comprises the coding DNA sequence, as well as the associated regulatory elements that control gene expression. Nuclear eukaryotic genes also contain noncoding regions called introns. The number of genes varies widely between different organisms. Coding DNA represents only a small fraction of eukaryotic genomic DNA: the bulk of the DNA is noncoding, much of which is made up of repetitive sequences. Some noncoding DNA has structural and regulatory functions; however, the function of most of this DNA is largely unknown. The number of copies of each genetic locus present in a cell, or ‘ploidy’, also varies between organisms. The somatic (body) cells of organisms that reproduce sexually are usually diploid, having two sets of homologous chromosomes and hence two copies of each genetic locus, while the germ (reproductive) cells are haploid and have only one copy of each chromosome. Prokaryotic cells are haploid. Some plants are polyploid, for example, modern wheat, which is hexaploid (six copies of each chromosome).