2007-9-17

In the first half of the twentieth century, there were three great scientists whose name started with the first letter M of their family name, that is, the well-known Mendel G.J., Morgan T. H. and McClintock B.

McClintock B. is a woman scientist and was born in Hartford, Connecticut, the USA on June 16, 1902. She enrolled at Cornell University in 1919 and from this institution received the B.S. degree in 1923 and the Ph.D. in 1927. In the 1920s and 1930s, McClintock B. was mainly engaged in the research of maize genetics in Cornell University. It may be said that the maize genetics research team led by Emerson and Morgan's drosophila research team were two powerful forces engaged in the flourishing genetics research. McClintock B. was the core member of the maize research team and remained unmarried all her life, but special love of maize. Many important findings concerning genetic variability of maize chromosome such transposal, inversion, absence, ring chromosome, dicentric chromosome, breakage-fusion-bridge cycle, and nucleolus organizer as had a direct tie with her. In addition, she also successfully illustrated the whole course of neurospora meiosis. We may say that she impelled and promoted the establishment of sub-discipline of the cytogenetics with her research results in maize genetics. Nevertheless, the thing that really made her be listed in the history of science was her research on the movable gene in the maize- transposon (jumping gene).

In June 1941, she joined in the Cold Spring Harbor Laboratory, USA, and initiated her famous research. Before this, she early found that seeds and laminae of Indian colorful maize have many patches, whose size or the time of appearance is controlled by some unstable genes or "mutable genes". In addition, she found that the existence of the color of seeds (or laminae) is controlled by the genes on 9# chromosome, such as gene C for controlling the pigmentation. For the existence of gene C, the seeds (or laminae) were colored, vice versa. But, near the gene C, there is a Ds gene (called as dissociation factor) controls the expression or representation of gene C. For the existence of Ds gene, the gene C cannot make the seeds colored, i.e. the pigment fails to be synthesized, so the seeds are still discolored. If the gene Ds leaves away gene C, that is, rupturing or exfoliated from the previous position, the gene C may be expressed again and the seeds are colored. Nevertheless, the action of gene Ds, i.e. releasing from the chromosome, is dominated by the third gene Ac (called as activating factor). For the existence of gene Ac, the gene Ds releases from the chromosome to release the restraint against the gene C. So the gene C may be expressed and the seeds are colored, vice versa. This is¡¡McClintock B.'s "Ds Ac system". Although her concept of "transposon" was advanced in 1938, this system cost her 6 years from 1944 to 1950.

In this system, the genes Ds and C are located at the adjacent position of the same chromosome, exception to genes Ac and Ds very far, even not on the same chromosome. Nevertheless, it still has the function of activating the gene Ds. After it dissociates, the gene Ds may leave away gene C and move to other positions, and may be recombined near gene C, i.e. "jumping".

Because of the uncertain dissociation time and duration of the gene Ds, the patches on the seeds are different in size. In other words, the reason why the maize seeds (laminae) have patches and the size of patches determines the expression of pigment gene C is that they are regulated and controlled by other one or multiple genes. It is the result that McClintock B. traced the gene at the cell level, although the previous people did not know the DNA.

On the chromosome, genes may move, i.e. having the function of "transposition" and "jumping". At that time, it was strange to geneticists, because according to the traditional concept, the genes on the chromosome are fixed and have their certain position, distance and sequence, but only can change their relative position through exchange and recombination, and change their relative nature through mutation; nevertheless, scientists never thought that genes may jump to another position from the one on the chromosome, even to another chromosome. Therefore, after they read McClintock B.'s Origin and Behavior of Maize's Easy Mutation Points published in 1950, and Chromosome Structure and Gene Expression published in 1951, and had an intensive understanding of her works done, they could hardly believe it, but thought that this woman maybe go crazy.

Although she wasn't be understood, she still insisted on her original intention and test results. Soon, she also found out another transposition mutation regulating system called as Spm. Because this went against the traditional genetics concept, she was isolated and helpless. People remained skeptical of her claims. This woman scientist who enjoyed a good reputation in the circle of American genetics (in 1944, chosen as the Member of the National Academy of Sciences, USA; in 1945, served as the chairman of Genetics Society of America, and won many national prizes) experienced quite long lonely and depressed time in her life. Friends stood off her gradually, and she had no choice but to live a solitary existence, nearly became a loner.

In 1953, when James D.Watson and Francis Harry Compton Crick found out the duplex structure of genetic material DNA, the genetics entered into a new stage of molecular genetics from the microbial genetics. In early 1960s, French scientist Jacob and Monod used bacilluscoli for tests and advanced lactose operon model and unveiled the mechanism of gene regulation in organism. It encouraged McClintock B. greatly and she thought that the lactose operon model was so similar with her Ds-Ac system. For this, she especially wrote an article named Comparison of Maize and Bacilli Gene Control System in order to catch the attention of the science circle. Soon, the science circle accepted the theory of Jacob and Monod, and both of them won the Nobel prize in 1965. Nevertheless, they still ignored McClintock B.'s transposable element and regarded her and her theory as heterodoxy.

Yet, a scientific theory is true at all, like what is true cannot become false, and what is false cannot become true. In the context of further development of molecular biology and molecular genetics, scientists gradually found out many phenomena same as or similar with McClintock B.'s transposable element in the bacilli, epiphyte, even other higher animals and plants. For example, in 1963, R.E.Taylor found that the bacteriophage Mu may be inserted into the genome of bacterial chromosome; in 1966, Spijkers and other scientists found out the F(sex factor) in the bacilluscoli, which may be integrated on the chromosome and may be dissociated outside the chromosome; In later 1960s' scientists found out the so-called insertion sequence (IS) in the bacilluscoli; later, in the salmonella, the gene's mobility (transposon) and antibotics resistant genes etc. were also found out. The series of problems made people review the McClintock B.'s researches on maize. Especially, through the efforts of several scientists who had an intensive understanding of McClintock B.'s works, the people gradually knew McClintock B.'s research results and were surprised at here scientific findings transcending times and her perseverance. In 1976, on the Symposium on "DNA Insertion Element, Plasmid, and Episome" held in the Cold Spring Harbor, the term of "transposable element" of McClintock B. was definitely acknowledged to explain the DNA segment which can be inserted into the genome. At this time, the people looked at McClintock B. with new eyes. Now, as we look back the history, we may say that: McClintock B. is the really pioneer of "operon theory" of gene regulation. In early1940s', she herself concluded the concept of "transposable element" by means of researching the traditional genetics and cytology, and solved the problem that can be solved only with the methods of molecular biology and molecular genetics. Therefore we may say that she is the person before time. Her maize transposable element has been proved at the molecular level. Scientists have separated transposable elements in many kinds of prokaryotes and eukaryotes, and conducted the researches on DNA level. McClintock B.'s transposable element theory advanced a half-century ago was of great significance to the development of later molecular biology and molecular genetics, and the implementation of genetic engineering (DNA recombination technology), transgene research, cancer research and human genome plan.

In 1983, the Nobel Prize Appraisal Committee of Royal Swedish Academy of Sciences granted the Nobel Prize for Physiology and Medicine to this persevering woman scientist of 81 years old. She is the first woman scientist independently awarded the Nobel Prize in the field of genetics research as well as the third woman scientist independently awarded the Nobel Prize in the world (the first one is Polish woman scientist Marie Curie and the second one is Dorothy Mary Crowfoot Hodgkin. Both ones are a chemist). Although the prize was late for 15 years, McClintock B. finally won the acknowledgement by the circle of science in her life.

On September 2, 1992, McClintock B. passed away at the age of 90 in Cold Spring Harbor.