polymerase chain reaction (PCR), laboratory process in which a particular DNA segment from a mixture of DNA chains is rapidly replicated, producing a large, readily analyzed sample of a piece of DNA; the process is sometimes called DNA amplification.

The Process

In PCR, DNA (see nucleic acid) is immersed in a solution containing the enzyme DNA polymerase, unattached nucleotide bases (the subunits that DNA is composed of), and "primers," short sequences of nucleotides designed to bind with an end of the desired DNA segment. Two primers are used: one primer binds at one end of the desired segment on one of the two paired DNA strands, and the other primer binds at the other end but on the other strand. The solution is heated to break the bonds between the strands of the DNA. When the solution cools, the primers bind to the separated strands, and DNA polymerase quickly builds a new strand by joining the free nucleotide bases to the primers. When this process is repeated, a strand that was formed with one primer binds to the other primer, resulting in a new strand that is restricted solely to the desired segment. Thus the region of DNA between the primers is selectively replicated. Further repetitions of the process can produce billions of copies of a small piece of DNA in several hours.

Development and Applications

PCR was developed in 1985 by Kary B. Mullis, who was awarded the 1993 Nobel Prize in chemistry for his work. It is used in DNA fingerprinting and in medical tests to identify diseases from the infectious agent's DNA. In forensic use, the test can be used to compare two samples of DNA, usually by looking at matches (or mismatches) of six inherited traits (e.g., hair curliness) from each of the samples. Each trait is controlled by a single gene, each gene having at least two forms, or alleles, resulting in 21 combinations of these alleles, some of them very rare. A nonmatch conclusively excludes a suspect. PCR also is used in taxonomic classification to help show evolutionary relationships between organisms on the molecular level. It has the advantage of being able to be used even when only very small samples, such as tiny pieces of preserved tissue from extinct animals, are available.

chain reaction, self-sustaining reaction that, once started, continues without further outside influence. Proper conditions for a chain reaction depend not only on various external factors, such as temperature, but also on the quantity and shape of the substance undergoing the reaction. A chain reaction can be of various types, but nuclear chain reactions are the best known. A line of dominoes falling after the first one has been pushed is an example of a mechanical chain reaction; a pile of wood burning after it has been kindled is an example of a chemical chain reaction. In the latter case each piece of wood, as it burns, must release enough heat to raise nearby pieces to the kindling point. The wood, therefore, must be piled close enough together so that not too much heat is lost to the surrounding air. The conditions for a nuclear chain reaction can be understood by analogy. In the case of the fission of a nucleus, the reaction is begun by the absorption of a slow neutron. Each fission produces two or three fast neutrons. In order to sustain a chain reaction, a sample must be large enough to slow the neutrons so that one can be captured by another nucleus and produce a second fission. The sample must also be compact to prevent neutrons from escaping. The minimum quantity of a fissionable material necessary to sustain a nuclear chain reaction is called the critical mass. In a nuclear fission bomb, a chain reaction is started by forcing together two or more samples of fissionable material, each of less than critical mass, to form one sample of supercritical mass. The number of subsequent fissions produced by a single fission is always greater than one. The total number of fissions increases rapidly (exponentially) with time. In a fission reactor, the number of subsequent fissions for each fission must be exactly one. If the rate is less, the chain reaction will stop; if greater, it will soon grow out of control. In one type of fission reactor, a combination of fuel rods and control rods is moved in or out of a solid block of moderating material to control the reaction rate. In another type of reactor, the temperature of a liquid moderator controls the reaction. See also nuclear energy; nuclear reactor.

Laboratory technique used to make numerous copies of specific DNA segments quickly and accurately. These are needed for various experiments and procedures in molecular biology, forensic analysis (DNA fingerprinting), evolutionary biology (to amplify DNA fragments found in ancient specimens), and medicine (to diagnose genetic disease or detect low viral counts). Invented by Kary Mullis, PCR requires a DNA template (as little as one molecule) to copy, nucleotides to build the copies, and the enzyme DNA polymerase to catalyze the formation of bonds between the nucleotide monomers. Each three-step cycle (separating the two strands of the DNA double helix, marking the ends of the segment to be copied, and catalyzing the formation of bonds), which takes only minutes to complete, doubles the number of DNA strands present in the reaction medium. Repetition of this cycle many times results in an exponential increase in the amount of DNA.

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Process yielding products that initiate further processes of the same kind. Nuclear chain reactions are a series of nuclear fissions initiated by neutrons produced in a preceding fission. A critical mass, large enough to allow more than one fission-produced neutron to be captured, is necessary for the chain reaction to be self-sustaining. Uncontrolled chain reactions, as in an atomic bomb, occur when large numbers of neutrons are present and the reactions multiply very quickly. Nuclear reactors control their reactions through the careful distribution of the fissionable material and insertion of neutron-absorbing materials.

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