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DNA Molecules

Dateline: 06/15/98

By Alan Bruzel

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The above continuous hyperlink represents a molecule of DNA. Use the mouse to move slowly along its length from left to right. The radio buttons act as indicator lights, denoting the increase in genetic complexity, as measured by longer DNA lengths. Note the progression from simple genes to simple, freely replicating concatenations of genes (called viruses) to yet more complex structures.

DNA molecules are linear, unbranched polymers composed of nucleic acid molecules – deoxyribonucleic acid molecules, to be exact – so that DNA may be properly called a polyDeoxyriboNucleicAcid. One nucleic acid molecule consists of a purine or pyrimidine base covalently bound to deoxyribose phosphate. The length of a single-stranded DNA molecule is measured in units of these bases. Because most DNA molecules exist as a complementary pair of polymer strands held together by interstrand hydrogen bonds, the length of these double-stranded DNAs is measured in base pairs (bp). One thousand base pairs is represented by the abbreviation kb, and one million base pairs by Mb.

A, C, G, and T are the four bases present in DNA. These letters represent adenine, cytosine, guanine, and thymine, respectively.

A genome is the total content of DNA in one virus or in one cell. The genome of viruses and bacteria contains one molecule of DNA. In the examples provided here, this molecule may range in length from 7,800 bp (Commelina yellow mottle virus) to 8,000,000 bp (Streptomyces coelicolor). In eukaryotes (cells with nuclei), the genome is one cell's collection of chromosomes, each chromosome presumably consisting of one molecule of DNA. The human genome contains 23 chromosomes whose DNA lengths total about 3 X 10⁹ bp.

This article concerns itself not with the chemical makeup of DNA, but with its relationship to life. How much DNA is necessary to produce a self-perpetuating entity? The answer is: very little if the entity is a virus (some have only a few genes), but much more (millions of times more) if the entity is more enterprising and requires tens of thousands of genes for it to function. But why do lilies, butterflies, and lungfish (among others) need so much more DNA than we, self-described, complicated humans? This question remains unanswered.

DNA length (approximate):

Escherichia coli (bacteria) alanine transfer RNA 1B gene = 76 bp
E. coli ferredoxin gene = 333 bp
Bacillus subtilis (bacteria) glutathione peroxidase gene = 480 bp
Bacillus subtilis ATP synthase (subunit a) gene = 732 bp
E. coli biotin synthetase gene = 1038 bp
Human ß-globin gene = 1.5 kb
Human insulin gene = 1.7 kb
E. coli ß-galactosidase gene = 3.5 kb
B. subtilis bacillopeptidase F gene = 4.3 kb
Commelina yellow mottle virus = 7.8 kb
Sulfolobus virus 1 = 15.5 kb
Human mitochondrial DNA = 16 kb
Human adenovirus 2 = 35.9 kb
Lambda virus = 48.5 kb
Reclinomonas americana (zooflagellate) mitochondrial DNA = 69 kb
Smallpox virus = 186 kb
Human thyroglobulin gene = 300 kb
Mosquito iridescent virus = 440 kb
Zea mays (corn) mitochondrial DNA = 570 kb
Mycoplasma genitalium (bacteria) genome = 580 kb
Borrelia burgdorferi (bacteria) (Lyme disease) genome = 935 kb
Drosophila melanogaster (fruit fly) chromosome 4 = 1.75 Mb
Muskmelon mitochondrial DNA = 2.5 Mb
Schizosaccharomyces pombe (yeast) chromosome III = 3.5 Mb
E. coli genome = 4.6 Mb
S. pombe chromosome II = 4.6 Mb
S. pombe chromosome I = 5.7 Mb
Streptomyces coelicolor (bacteria) genome = 8 Mb
S. pombe genome = 13.8 Mb
Plasmodium falciparum (malarial parasite) genome = 25 Mb
Mus musculus (mouse) chromosome 11 = 45 Mb
D. melanogaster chromosome 3 = 50 Mb
Human chromosome 20 = 90 Mb
Caenorhabditis elegans (nematode) genome = 100 Mb
D. melanogaster genome = 165 Mb
Human chromosome 6 = 225 Mb
Oryza sativa (rice) genome = 441 Mb
Musa sp. (banana) genome = 873 Mb
Spinacia oleracea (spinach) genome = 989 Mb
Gallus gallus (chicken) genome = 1,200 Mb
Zea mays (corn) genome = 2,500 Mb
Homo sapiens (human) genome = 3,000 Mb
Nicotiana tabacum (tobacco) genome = 4,434 Mb
Vanilla planifolia (vanilla) genome = 7,672 Mb
Avena sativa (oat) genome = 11,315 Mb
Triticum aestivum (wheat) genome = 15,966 Mb
Triturus cristatus (crested newt) genome = 18,600 Mb
Necturus maculosus (mudpuppy) genome = 50,000 Mb
Lilium longiflorum (Easter lily) genome = 90,000 Mb
Fritillaria assyriaca (butterfly) genome = 124,900 Mb
Protopterus aethiopicus (lungfish) genome = 139,000 Mb

Recommended Web resources for additional information:

Database of Genome Sizes
Material supplied by the Technical University of Denmark.

Institut Pasteur
Information on E. coli and B. subtilis genes and gene products.

Plant Genome Information
Collection supplied by the US Department of Agriculture.

Virus Databases Online
Information on viral genomes from the Australian National University.

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