Complementary DNA
Complementary DNA
Complementary deoxyribonucleic acid (DNA) is DNA in which the sequence of the constituent molecules on one strand of the double-stranded structure chemically matches the sequence on the other strand.
A useful analog is to picture a key and a lock. While there are many different types of keys, only one design matches the contours of the lock and so will fit into the lock. The different chemical molecules that make up DNA also do not pair up nonspecifically. A lock-in-key fit operates at the molecular level.
The chemical molecules that make up DNA are known as nucleotide bases. There are four common types of bases in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). In the chemical lock-and-key fit, an A on one strand is able to form a bond only with a T; for the two strands of DNA to intertwine together, the A’s on one strand of DNA must pair with T’s on the other strand. Similarly, C on one strand always pairs with a G on the other strand. Thus, the sequence of one DNA strand determined the sequence of the other strand, which is described as being complementary.
Complementary DNA (cDNA) is a copy of a region of a strand of DNA. For example, if the original DNA strand had a sequence of ATT, the complementary sequence will be TAA. The cDNA will bind to the complementary site on the DNA strand.
Complementary DNA is important naturally, in the manufacture of new copies of DNA, and has become an important experimental tool. In DNA replication, the two strands are unwound from one another. A molecule called DNA polymerase runs the length of each strand, making a complementary copy of each strand. In other words, each strand acts as a blueprint to produce a complementary strand. The two new strands are complementary to one another, and so can join together in a process called annealing. The old strands also anneal. The result is two complete copies of DNA.
Complementary DNA has been exploited to develop research techniques and to produce genetically altered commercial products. A classic example of cDNA is the technique of polymerase chain reaction (PCR). PCR mimics the process of DNA manufacture in a test tube. In a series of reactions, a target stretch of DNA is copied, and the copies themselves serve as templates for more copies. The original DNA sequence is amplified to make a billion copies within minutes.
Because ribonucleic acid (RNA) is made using DNA as the blueprint, the phenomenon of complementary strands also extends to RNA. Like DNA, RNA is made of four bases; three of the bases are the same as found in DNA (A, C, and G). But, instead of T, RNA contains a base called uracil (U). In RNA, A pairs with U and C pairs with G. Complementary RNA (cRNA)is a copy of a strand of RNA that will bind to the appropriate region of the original molecule. If the original RNA stand had a base sequence of AUU, for example, the sequence of the cRNA strand would be UAA.
The association of a DNA or RNA strand to its complement is one of the basic research tools of the molecular biologist. Binding of a complement can identify target regions of DNA or RNA, and can be used to disrupt the process of DNA manufacture. If the complementary DNA is labeled with a compound that fluoresces, then the binding of the fluorescent probe can actually be visualized using a microscope. This permits the so-called real time examination of DNA synthesis.
Resources
Books
Drlica, Karl. Understanding DNA and Gene Cloning: A Guide for the Curious. New York: Wiley, 2003.
Gerlach, Neil. The Genetic Imaginary: DNA in the Canadian Criminal Justice System. Toronto: University of Toronto Press, 2004.
Synder, L., and W. Champness. Molecular Genetics of Bacteria, 2nd ed. Washington, DC: American Society for Microbiology Press, 2002.
Periodicals
Volchkov, V.E., V.A. Volchkova, E. Möhlberger, et al.“Recovery of Infectious Ebola Virus From Complementary DNA: RNA Editing of the GP Gene and Viral Cytotoxicity.” Science 229 (March 2001): 1965–1969.