1932, the famous quantum physicist NielsBohr gave a speech entitled "Light and Life" at an international optical medical conference. He believes that the material structure is not enough to fully explain the phenomenon of life, so explaining the mystery of life on the basis of physics may lack some basic factors.
1935, one of Bohr's students, delbruck, wrote a paper entitled "About Gene Mutation and Gene Structure", arguing that genetics is just a discipline that Bohr thinks lacks some basic factors, because neither physics nor chemistry can explain the mystery of genetics. Delbruck once worked with geneticists to induce mutation by treating fruit flies with radiation, so as to calculate the size of genes. They found that the size of a gene is similar to that of the largest molecule, but it has a high degree of stability different from ordinary molecules.
The discussion about gene stability attracted the attention of another theoretical physicist, Irving Schrodinger (1887 ~ 196 1).
It turns out that a single cell of bacteria can't be examined with naked eyes, but hundreds of millions of cells obtained by multiple cell division of a single bacteria on the surface of solid medium can gather together to form colonies visible to the naked eye, and colonies can show various relativity. For example, on the medium containing some dyes and lactose, the colonies that can ferment lactose are dark purple with metallic luster, while the colonies of mutant bacteria that can not ferment lactose are almost colorless; For example, bacteria sensitive to streptomycin cannot form colonies on the medium containing streptomycin, but mutant bacteria resistant to streptomycin can form colonies on it; For example, bacteria that can synthesize an amino acid by themselves cannot form colonies on mutant bacteria that do not contain this amino acid, and so on. There are thousands of genes that have been located on the chromosome of Escherichia coli, most of which can make bacteria show relative characteristics in suitable media.
In genetic research, locating genes on chromosomes is a basic work, but it is not the most important work. Mutants can be used to study various biochemical reactions in organisms, such as the synthesis of amino acids and vitamins. More complex metabolic processes, such as protein synthesis and DNA replication, are partly stored by mutants. In the process of using mutants to carry out these studies, if we wait for mutants to appear by chance, as Bedell and Tatton did, molecular genetics could not develop so fast. Here, the means of directional screening of mutants plays a very important role. Here is a brief introduction to a common method.
Penicillin specifically inhibits the synthesis of cell wall substances of Escherichia coli, but does not inhibit the synthesis of other substances such as protein. Therefore, add the same amount of penicillin to the same two test tubes cultured with Escherichia coli, one test tube is put into the refrigerator and the other test tube is put into the incubator. After two hours, it will be found that the number of live bacteria in the previous test tube neither increases nor decreases, but the number of live bacteria in the latter test tube is greatly reduced. This is because the bacteria in the incubator can't synthesize cell wall substances, but other substances in the cells are constantly synthesized, and finally the cells swell and rupture: the bacteria in the refrigerator are not suitable for growth because of the temperature, so the substances in the cells and the cell wall substances are no longer synthesized, and the cells can keep balance and avoid death. Using this principle, mutants can be screened effectively. To put it simply, the specific method is to cultivate the wild type treated with mutagens such as radiation in a culture solution containing penicillin instead of organic matter, in which wild-type Escherichia coli is killed in large quantities because it can grow, and mutations that cannot synthesize amino acids themselves or vitamins are not killed because it cannot grow, so that mutant bacteria can be "concentrated" and easily separated. In order to further improve the efficiency of isolating mutants, some auxiliary measures can be taken, such as inoculating the "concentrated" bacterial liquid on a solid culture medium without any amino acids, marking the colonies one by one after the colonies are formed, and then adding a culture medium containing only various amino acids for culture, so that the colonies that grow for the second time are mutants that cannot synthesize a certain amino acid. Other special mutants can also design special methods to "concentrate". One advantage of using E.coli for genetic research is that it is convenient to obtain various mutants. Imagine that if so many mutants were discovered by accident, I am afraid that the arrival of the era of molecular genetics will be delayed for at least several decades. There are many advantages in using E.coli as a genetic research material: the culture medium is simple, and biotin needs to be added to the culture medium of Neurospora, and the culture medium of E.coli is all inorganic salts except a sugar; Escherichia coli proliferates rapidly, and many experiments can't see the results until the next night; It is also an advantage to facilitate large-scale culture. For the chemical analysis of compound eye pigment or its synthetic intermediate, it is easy to get dozens or hundreds of grams of Escherichia coli cells. The cells of Escherichia coli are easy to preserve, which is beneficial to the genetic research of a large number of strains (called strains in bacteria). Put the strains in the low-temperature refrigerator when not in use, and take them out of the refrigerator when necessary, so that the bacteria can be revived. So who first used such good research materials for genetic research? The story might as well start from Wright's boyhood.
While the genetic research of bacteria is still deepening, the characters in the drama of molecular genetics have been on stage one after another. Molecular genetics has penetrated into all fields of life science and will continue to develop. However, as a script, the content cannot be infinite. The description here will be limited to four aspects as the basic work of molecular genetics, which are also the basis of genetic engineering: genetic code, gene expression, gene regulation and gene mutation.