cleotide) with a desired property, such as the ability to specifically recognize and bind a molecule associated with a particular disease, can be selected in a single experiment from a library containing approximately 1015 different compounds. First, a library of oligonucleotides is created in a machine called an oligonucleotide synthesizer. This apparatus can make oligonucleotides with either a defined or random sequence.

Oligonucleotides for SELEX are designed to have a central region containing random sequence and outer, flanking regions with defined sequences. These defined sequences will be used as primer-binding sites for the polymerase chain reaction (PCR). The oligonucleotide library is prepared as a mixture, usually containing about 1014 to 1015 different sequences. These specialized oligonucleotides, termed aptamers, are then exposed to target ligand molecules, which are typically attached to a solid support, such as a filter membrane. The unbound aptamers are then washed away, leaving only the rare aptamers that can bind the ligand adhering to the filter. These aptamers can then be recovered from the filter by washing it with a solution that disrupts the binding.

These binding candidate aptamers represent a minuscule fraction of the original library. Some may bind the target ligand tightly, but others may bind weakly. Since all the aptamers have defined primer-binding sites on the ends, this much-reduced population can now be amplified exponentially by PCR. After amplification, the aptamers can be subjected to another round of ligand binding, now using more stringent washing conditions, in which only the tightest-binding molecules will stay bound. These high-affinity binders can be recovered again subjected to still more cycles of PCR amplification, binding, washing, and recovery, until the population of aptamers consists exclusively of very tightly binding molecules.

For some applications, these molecules are useful directly. They can also be studied to design non-DNA molecules that have similar shapes but that will have more potential as drugs. see also DNA Libraries; DNA Microarrays; High-Throughput Screening.

Paul J. Muhlrad


Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Science, 2002.

Borman, Stu. "Combinatorial Chemistry." Chemical and Engineering News 75, Feb. 24, 1997.

PCR polymerase chain reaction, used to amplify DNA

Complex Traits

Complex traits are those that are influenced by more than one factor. The factors can be genetic or environmental. This is in contrast to simple genetic traits, whose variations are controlled by variations in single genes. Examples of simple traits include Huntington's disease and cystic fibrosis. Each of these traits is caused by a mutation in a single gene that alters or destroys the function of that gene. There are several thousand disorders caused by single genes, but these are almost always quite rare in the population, often occurring in less than one in five thousand individuals.

mutation change in DNA sequence

Figure 1A. In this model two-gene system, the risk of developing a disease varies between 0 and 8. It is determined by adding risk (2) for each T a person has at both Gene 1 and 2.

Almost any trait that is not simple is considered complex. If there are just a few genes that affect a trait, it may be called oligogenic. If there are many different genes that affect a trait, it may be called polygenic. If other, nongenetic factors are involved, it may be called multifactorial.

Diseases inherited as complex traits are often much more common in the population and include heart disease, Alzheimer's disease, and diabetes. There are many factors that can affect a complex disease. Perhaps most commonly, these traits have multiple genes, where variations in those genes can influence the risk of developing the disease.

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