Since 1951, Watson and Crick have built three molecular models of DNA. In building their models, they did not just consider their structure, but always related the function and information of DNA. They required that the models they built should satisfy the latest facts of physics, chemistry and mathematics research, such as X-ray diffraction results and the mechanics of base pairing, as well as biochemical knowledge, such as ketone type, hydrogen bonding, and bond angles, and even more so that the DNA could explain the theories of genetics and metabolism, which is a very advanced idea.
The first model was a three-stranded structure. It was based on a misunderstanding of experimental data and ultimately failed. But they were not discouraged, and continued to collect materials and review information. Franklin's X-ray diffraction photographs of B-type DNA, and Chargaff's analysis of the chemical composition of DNA had been a great revelation to Watson and Crick. The second model they built was a double-stranded helix, with the sugar and phosphate backbones on the outside and the bases arranged in pairs on the inside, and the bases were paired in the same way, i.e., A to A, C to C, G to G, T to T. This model also failed due to the wrong way of pairing. Despite this further failure, they drew a number of useful lessons from it and laid the foundation for the successful establishment of a third model. On February 20, 1953, Watson had a flash of insight and abandoned the base homologation scheme and adopted the base complementary pairing scheme, which was finally successful. Watson and Crick and after three weeks of repeated checking and refinement, March 18 finally succeeded in establishing the DNA molecule double helix structure model, and on April 25 in the British journal "Nature" published. DNA molecule rules of the double helix structure model met with the world, the main points are as follows: the DNA molecule is formed by the two parallel long chain of deoxynucleotides to the right of the helix; DNA molecule in the Deoxyribose and phosphate alternately linked, arranged on the outside, constituting the basic skeleton, the bases are arranged on the inside; the bases on the two chains are linked by hydrogen bonds, forming base pairs, that is, A and T, C and C pairing; DNA molecule in the two deoxyribonucleotides in the long chain of the atoms in the direction of the opposite direction, one is 5'?á3?á towards, the other one is 3'→5'
A few weeks later, Watson and Crick further proposed a hypothesis of DNA molecular replication in the journal Nature -- the mechanism of semi-conservative replication, which provides a broad Prospects. As early as 1953, before the discovery of the structure of the DNA molecule, Watson made a prediction on the route of genetic information transmission from DNA to protein: "DNA → RNA → protein", but lacked in-depth analysis. 1958, Crick put forward the central law of genetic information, which draws a picture of the possible information flow of all the three kinds of substances, namely, DNA, RNA and protein. DNA, RNA and protein may have the information flow are drawn up. Later, after people figured out the existence and role of three kinds of RNA, namely mRNA, rRNA and tRNA, and knew that DNA can form mRNA through transcription, mRNA passes through the nuclear pore and enters into the cytoplasm, and protein is synthesized in the ribosomes mainly formed by rRNA, with mRNA as the template and tRNA as the carrier, Crick modified the central law again.
In 1965, scientists discovered RNA replicase, suggesting that RNA could replicate itself, and in 1970, H. M. Temin and D. Baltimore isolated a reverse transcriptase in an RNA virus-infested host cell that paradoxically shifted the RNA to DNA, thereby integrating it into the host cell. In light of these realities, Crick revised the central law again in 1970, in which he argued that the direct transfer of genetic information from DNA to proteins was merely a theoretical assumption.
The central law rationalized the connection and division of labor between the two types of macromolecules, nucleic acids and proteins: the function of nucleic acids is to store and transfer genetic information, and to direct and control the synthesis of proteins; the main function of proteins is to carry out metabolism as well as to serve as a constituent of cellular structure. When the double helix structure of the DNA molecule was made public, it was recognized that the arrangement of the four bases contained a tremendous amount of information. If it is a DNA composed of 100 deoxyribonucleotides, the maximum amount of information it contains will reach 4100, a number 1000 times larger than the total number of atoms in the solar system, and therefore aroused the great interest of scientists to come to decipher the genetic code. After reasoning, it is obvious that the arrangement of 4 bases determines the arrangement of 20 amino acids in proteins, which is simplified as a mathematical permutation can only be 4→20, and in order to satisfy the number of 20, the full arrangement of 4 can only be 43=64, which can provide enough information for coding 20 kinds of amino acids. The triplet coding scheme was initially established, that is, the three adjacent bases in the mRNA molecule are called triplets, which can determine an amino acid in a polypeptide, so also the triplets of mRNA are called codons.
Crick believed that there exists not only a dictionary of triplet codons, but also possibly a start codon, a stop codon, and a synonymous codon. Under the unremitting efforts of Crick and many scientists, the genetic code was all deciphered in 1966: ① all genetic codes are composed of three consecutive nucleotides; ② many amino acid codons are not one, but by many close nucleotides, that is, the existence of a simple code; ③ 64 combinations of the three bases, 61 can be used to encode a variety of amino acids, of which AUG, GUG or the start signal of translation, called the start codon; the other three combinations can not encode any amino acid, all of them are coded termination symbols, which are UAA, UAG, UGA, called the termination codon. From this we can see how accurate Crick's speculation, so that we can see a real grasp of the pulse of science scientists, how far-sighted he is, how clear his thinking ah!
In 1969, under the continuous efforts of Crick and other scientists, overcoming all kinds of difficulties, we finally linked the arrangement of bases in nucleic acids with protein synthesis, and formed a table of genetic code, so that people can understand at a glance, and can quickly grasp the triplet code of bases in amino acid synthesis. Often compared to Mendeleev's periodic table of elements, it was an important milestone in the history of biology.
The model of the double-helix structure of the DNA molecule created by Watson and Crick, which was further confirmed in later scientific studies, greatly contributed to the development of molecular biology.
They were both awarded the Nobel Prize in Medicine and Physiology in 1962.