All the interactions between nucleic acid molecules that help express genetic information involve base‐pairing between complementary sequences. Complementarity is sometimes defined as selective stickiness. Complementary molecules fit together. In the case of nucleic acids, complementarity generally involves base pairing. For example, mRNA is complementary to one strand of DNA, and the anticodon of tRNA is complementary to the codon in mRNA. Replication, transcription, and translation all involve base‐pairing at several levels.
The central dogma allows the controlled expression of genetic information. Consider an Escherichia coli
bacterium in its natural environment, the human gut. Its survival and replication would be favored by being able to use a variety of sugars to produce energy. On the other hand, making enzymes requires a large amount of energy. The conflict between these two demands is resolved by the bacterial genome's ability to synthesize the enzymes needed for digestion of sugars only when needed. Thus, for example, the enzymes involved in lactose digestion are made only when lactose is present in the environment. Usually, the synthesis of different proteins is controlled transcriptionally, that is, through regulating the synthesis of mRNA. When an E. coli
bacterium encounters lactose, it synthesizes the mRNA species encoding the enzymes that degrade lactose. These mRNAs are translated into protein and the proteins catalyze the reactions required to digest lactose. After the mRNAs are translated they are degraded in the cell, so the control system contains the means of shutting itself off, as well.
This arrangement allows the amplification of DNA information. One DNA sequence, if transcribed into 20 mRNAs, each of which is translated into 20 protein molecules, can encode 400 (20 × 20) enzymes, each of which can catalyze the breakdown of thousands of lactose molecules. All sorts of organisms use variations of this simple control model to control their growth and replication, the synthesis of macromolecular components, such as ribosomes, and a wide variety of anabolic and catabolic capabilities.