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: adenine (A), cytosine (C), guanine (G), and thymine (T). In the chemical "lock and key" fit, an A on one strand always pairs with a T on the other strand. As well, a C on one strand always pairs with a G on the other strand. The two strands are described as complementary to one another.

Complementary DNA (cDNA) is a copy of a region of a strand of DNA. For example, if the original DNA stand 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. RNA is made of four bases; adenine (A), cytosine (C), guanine (G), and uracil (U; instead of the thymine found in DNA). In the lock in key scenario, an A pairs with the U) on the other strand, and a C always pairs with a 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 compliment 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 "real time" examination of DNA synthesis.