DNA is deoxyribonucleic acid (abbreviation of deoxyribonucleic acid), also known as deoxyribonucleic acid, which is the main component of chromosome and the substance of gene. It is sometimes called "genetic particle" because in the process of reproduction, the father copies and transfers part of his DNA to his offspring, thus completing the spread of traits. The chromosome of prokaryotic cell is a long DNA molecule. There are more than one chromosome in the nucleus of eukaryotic cells, and each chromosome contains only one DNA molecule. However, they are usually larger than DNA molecules in prokaryotic cells and bind to protein. The function of DNA molecules is to store all the genetic information of almost all protein and RNA molecules that determine species traits; Code and design all the procedures of orderly transcription of genes and expression of protein by biological organisms in a certain time and space, and complete directional development; The unique character and personality of the organism and all the stress reactions when interacting with the environment are preliminarily determined. In addition to chromosomal DNA, there are very few DNA with different structures in mitochondria and chloroplasts of eukaryotic cells. The genetic material of DNA virus is also DNA, and a few are RNA.
Molecular characteristics of DNA
The double helix structure of stable DNA molecules is relatively stable. This is because within the double helix structure of DNA molecules, two long deoxynucleotide chains are stably connected in parallel through base pairs formed by hydrogen bonds. In addition, the longitudinal interaction between base pairs further strengthens the stability of DNA molecules. This vertical interaction between base pairs is called base stacking force, which is caused by the interaction between π electrons of aromatic bases. At present, it is generally believed that base stacking force is the most important factor to stabilize DNA structure. Furthermore, the ionic bond formed between negatively charged phosphate groups and positively charged cations outside the double helix can reduce the electrostatic repulsion between the double strands, thus stabilizing the DNA double helix structure to some extent.
Diversity of DNA molecules Because of the different number of base pairs, the arrangement order of base pairs is ever-changing, which constitutes the diversity of DNA molecules. For example, a DNA molecule with 4 000 base pairs carries four kinds of genetic information, namely 10.
DNA molecules with different specificity have different sequences of base pairs, so each DNA molecule has its specific sequence, which contains specific genetic information, thus making DNA molecules specific.
Discover, develop
The discovery of DNA structure is one of the most legendary chapters in the history of science. The discovery of DNA structure is an epoch-making achievement, but the way to find it is model construction, just like the "patchwork" method of children's puzzles. Watson and Crick are the best performers in this group.
1928 Watson was born in Chicago, USA. 16 years old, graduated from the university of Chicago with a bachelor's degree in zoology, and began to show his talents in biology. At the age of 22, Watson received his Ph.D., then went to Cavendish Laboratory of Cambridge University in England, met Crick who worked here earlier, and started their legendary cooperative career. Crick was born in Northampton, England on June 8th, on 19 16, and graduated from London University at the age of 2 1. After World War II, he came to Cavendish Laboratory in Cambridge. Crick, like Watson, has a strong interest in DNA and has turned from physics to biology.
At that time, people already knew that DNA is a slender polymer compound, which consists of a series of deoxynucleotide chains, including deoxyribose, phosphoric acid and nitrogenous bases, with a total of four bases. 195 1 year, many scientists launched a competition to study the structure of DNA. At that time, there were two famous DNA molecular research groups. One is the Royal Academy research group headed by the famous physicist Wilkins and chemist Franklin. They mainly use X-ray diffraction to study the structure of DNA. One is the research team of California Institute of Technology, led by the famous chemist Pauling. They mainly studied the structure of DNA by the method of model construction, and found the protein A helix by this method.
195 1 In February, Wilkins showed a very beautiful DNA X-ray diffraction photograph taken by Franklin at the conference on biomacromolecule structure held in Italy. Watson, who has always been interested in DNA, was too excited to speak when he saw this picture, and his heart was pounding. According to this photo, he concluded that the structure of DNA is spiral. He is determined to make a DNA model. He told the idea to his collaborator Crick, who agreed.
The work of Watson and Crick to establish the molecular structure model of DNA began in the autumn of 195 1. They adopted the model construction method, modeled on the method of constructing protein's α helix model by the famous chemist Pauling, and matched deoxynucleotides with paper and iron wire according to the data of crystallography.
They built one model after another, and they were all rejected. But Watson insists that DNA molecules may be double-stranded structures. Because many things in nature are paired, so are chromosomes in cells. After that, they completed the double helix structure with deoxyribose and phosphoric acid alternately arranged as the basic skeleton, bases arranged outside (figure 1), and deoxyribose and phosphoric acid alternately arranged.
It is a double helix structure, with a basic skeleton, bases arranged inside and homotypic base pairs (Figure 2).
During the period of 1952, biochemist Chajiafu visited Cambridge University and reported his DNA analysis results of different organisms such as human, pig, cow, sheep, bacteria and yeast. Chagaff's results show that although there are great differences in the number and relative proportion of four deoxynucleotides in the DNA of different organisms, there are A=T and G=C in the DNA of any substance, which is called Chagaff's law of DNA chemical composition. 1in July, 952, when Chagav visited Cavendish's laboratory, he explained the law of A:T=G:C= 1: 1 to Crick in detail. Later, Crick's friend, theoretical chemist Griffith, calculated that among the four deoxynucleotides in DNA, A must bind to T and G must bind to C, which is consistent with the completion of Chagaff's law. Later, Pauling's former colleague Donovan told Watson that the A-T and G-C pairing was maintained by hydrogen bonds. The above work became the basis of A-T pairing and G-C pairing structure in Watson and Crick's DNA molecular model.
At this point, the DNA model appeared. On February 28th, Watson made a model with four bases out of cardboard, glued the cardboard to the skeleton and matched it to the center. Crick immediately pointed out that only two single strands with opposite directions can perfectly match bases, which is just consistent with the X-ray diffraction data. The complete DNA molecular structure model was completed on March 7, 1953/kloc-0. According to this model, DNA molecule is a double helix structure, and each helix unit contains 10 base pairs, with a length of 34 angstroms (1 angstrom = 10- 10 meter). The diameter of the spiral is 20 angstroms. On April 15, Watson and Crick published their first paper on this model in Nature.
The discovery of DNA molecular double helix model is a milestone in the history of biology, which provides a structural explanation for DNA replication, makes people no longer doubt that DNA is the material basis of genes, and lays the foundation of molecular genetics. The influence of DNA double helix model in science is far-reaching.
Some people say that Watson and Crick won the Nobel Prize for The Giant's Toes, but I don't think so. On the basis of X-ray diffraction photos, Watson and Crick, especially Watson, have a broader vision, learn from experts and get a new comprehensive result, and the part of this comprehensive result that is greater than it is those who can't get together.