DNA (deoxyribonucleic acid)


U.S. National Library of Medicine.


DNA (deoxyribonucleic acid) is the genetic material of most terrestrial organisms and one of two principal types of nucleic acid found in living cells (the other being RNA). DNA is a stable macromolecule consisting (usually) of two strands running in opposite directions. These strands twist around one another in the form of a double helix and are built up from components known as nucleotides. The double helix nature of DNA was established, among others, by Francis Crick, Rosalind Franklin, and James Watson.


There are 10 nucleotides per turn in the double helix (1 turn = 3.4 nanometer). Each nucleotide consists of a sugar (deoxyribose)-phosphate section attached to one of four organic bases – two purines, guanine and thymine, and two pyrimidines, adenine and cytosine. The bases on opposite strands project toward each other like the rungs of a twisted ladder.


Complimentary base pairing is highly specific: thymine always pairs with adenine (via 2 hydrogen bonds) and cytosine always pairs with guanine (via 3 hydrogen bonds). This specificity is a direct consequence of the shapes of the bases and is fundamental to terrestrial life since it underlies all aspects of inheritance and gene expression including DNA replication, DNA repair, transcription, RNA splicing, and translation.


The sequence of bases along the length of a DNA strand varies from species to species and from individual to individual, and determines the organism's development (see genetic code) by controlling protein synthesis. Although double-stranded DNA forms the genetic material for most terrestrial organisms, bacteriophages and viruses may use single-stranded DNA, single-stranded RNA, or double-stranded RNA.


That DNA might be able to survive for hundreds of thousands of years in the vacuum of space is suggested by the results of research announced in 1998 by Evan Williams and his colleagues at the University of California, Berkeley.1 The Berkeley group concluded that double-stranded DNA could maintain its structure in a vacuum for as long as 35 years at room temperature, and perhaps almost indefinitely in the very low temperatures of space.



1. Schnier, P. D., Klassen, J. S., Strittmatter, E. F., and Williams, E. R. "Activation Energies for Dissociation of Double Strand Oligonucleotide Anions: Evidence for Watson-Crick Base Pairing in Vacuo," Journal of the American Chemical Society, 120 (37), 9605 (1998).