Any kind of computer, be it mechanical, electrical or biological, needs two basic capabilities in order to function: to store information and to perform operations on it. We need to address both issues: how can information be stored in DNA strands, and what molecular biology techniques could potentially be used for computations . In order to distinguish between ordinary mathematical operations and bio-molecular procedures performed on DNA strands, I will use the term bio-operations to refer to the latter.
DNA (deoxyribonucleic acid) is found in every cellular organism as the storage medium for genetic information. It is composed of units called nucleotides, distinguished by the chemical group, or base, attached to them. The four bases, are adenine, guanine, cytosine and thymine , abbreviated as A, G, C, and T. (The names of the bases are also commonly used to refer to the nucleotides that contain them.) Single nucleotides are linked together end--to--end to form DNA strands. A short single-stranded polynucleotide chain, usually less than 30 nucleotides long, is called an oligonucleotide . The DNA sequence has a polarity : a sequence of DNA is distinct from its reverse. The two distinct ends of a DNA sequence are known under the name of the 5' end and the 3' end, respectively. Taken as pairs, the nucleotides A and T and the nucleotides C and G are said to be complementary . Two complementary single--stranded DNA sequences with opposite polarity will join together to form a double helix in a process called base-pairing or annealing . The reverse process -- a double helix coming apart to yield its two constituent single strands -- is called melting .