Polymerase chain reaction (PCR) is an in vitro nucleic acid amplification technique developed in the mid-1980s. It has the outstanding advantages of specificity, sensitivity, high yield, rapidity, simplicity, good repeatability and easy automation. It can amplify the target gene or a DNA fragment to100000 times or even a million times within a few hours in a test tube, and can be directly observed and judged by naked eyes; Enough DNA can be amplified from a hair, a drop of blood or even a cell for analysis, research and identification. What can only be done in the past few days and weeks can be done in a few hours by PCR. PCR technology is a revolutionary innovation and milestone in the field of biomedicine. Second, the basic principle of PCR is similar to the natural replication process of DNA, and its specificity depends on oligonucleotide primers complementary to both ends of the target sequence. PCR consists of three basic reaction steps: denaturation-annealing-extension: 1, template DNA denaturation: after the template DNA is heated to about 93℃ for a certain time, the double-stranded DNA of the template DNA or the double-stranded DNA formed by PCR amplification dissociates, so that it can be combined with primers to prepare for the next round of reaction; 2. Annealing (renaturation) of the template DNA and the primer: after the template DNA is denatured into a single strand by heating, it is cooled to about 55℃, and the primer is paired with the complementary sequence of the single strand of the template DNA; 3. Primer extension: Under the action of TaqDNA polymerase, DNA template-primer conjugate takes dNTP as the reaction raw material and target sequence as the template. According to the principle of base pairing and semi-conservative replication, three processes of cyclic denaturation-annealing-extension are repeated, so that a new semi-conservative replication chain complementary to the template DNA chain can be synthesized, and more "semi-conservative replication chains" can be obtained, which can be used as the template for the next cycle. It takes 2 ~ 4 minutes to complete a cycle, and the target gene can be amplified by millions of times in 2 ~ 3 hours. The number of cycles required to reach the platform depends on the number of copies of the template in the sample. At the same time, it is recognized that protein is synthesized by receiving the genetic information of RNA. In 1950s, Zamecnik et al. found that microsomes were the sites of intracellular protein synthesis in the experiments of morphology and separation of subcellular components. 1957, Hoagland, Zamek nick and Stephenson isolated tRNA, and put forward the hypothesis of the function of amino acids in the synthesis of protein. 196 1 year, Brenner and Gross observed the combination of mRNA and ribosome during the synthesis of protein. 1965, Holley first determined the primary structure of yeast alanine tRNA; Especially in the 1960s, with the joint efforts of several groups of scientists such as nirenberg, ochoa and Khorana, the genetic code encoding the synthesis of protein on RNA was deciphered. Subsequent studies showed that this genetic code was universal in the biological world, thus understanding the basic process of translation and synthesis in protein. From the development course of molecular biology, we can see that it is the fastest-growing frontier field in life science for half a century, which has promoted the development of the whole life science. Today, molecular biology is still developing rapidly, and new achievements and technologies are constantly emerging, but the history of molecular biology is still short and the accumulated data is not enough. For example, all kinds of creatures on the earth carry huge life information. So far, human beings only know a very small part, but they have not known many basic laws of nucleic acid and protein. Another example is that even though we have obtained the complete sequence of human genome DNA of 3× 109 BP by 2005, and determined the primary structure of 565,438+million genes, we still have a long way to go to thoroughly understand the functions, regulation, interrelation and coordination of these gene products, and understand the functions of more than 80% non-protein coded sequences. It can be said that the development prospect of molecular biology is brilliant, and the road will be difficult and tortuous. It is not difficult to flatten or amplify a single copy gene of eukaryotic cells by PCR. 4. The correctness of oligonucleotide with strong specificity as primer and template combination is the key to determine whether the reaction product is specific or not. 5. The crude product or total RNA with low requirements on the quality of original materials and containing a small amount of (pg, ng) target DNA can be used as the starting material for the reaction to obtain the target product. Mainly applicable to life science, medicine, agricultural science, environmental science, archaeology, historical event interpretation and health and safety.