In the middle of last century (1950s), james watson and Francis Crick put forward the famous DNA double helix and double strand base pairing model. According to this model, they further proposed a semi-conservative model of DNA replication. Although this model looks much simpler than the completely conservative model that coexisted at that time, it still lacks convincing data. Finally, in 1957, MatthewMeselson, then a graduate student of California Institute of Technology, and FranklinStahl, then a postdoctoral fellow, designed and implemented this group of famous experiments to prove the semi-retention mechanism of DNA replication. In the experiment, they first cultured E.coli cells in a culture medium with 15NH4Cl as the sole nitrogen source for a long time (14th generation), so that all the nitrogen atoms in the cells existed in the form of 15N (including those in DNA molecules). At this time, a large number of excessive nucleotide molecules of 14N4cl and 14N were added, and bacteria began to ingest14n, so all existing "old" DNA molecules should be labeled as 15N, while new DNA should be unlabeled. Next, they let the cells continue to grow happily, while extracting DNA molecules at different times and separating them by CsCl density gradient centrifugation. Finally, each line consists of an experimental code (this number is useless to us), a small picture, a peak spectrum and a number called generation from left to right. The last number actually reflects how many times the cell divides (that is, how many times it is copied) after nitrogen 14 is added, and the black band pattern on the left reflects the corresponding position of DNA in the centrifuge tube. You can think of this series of small pictures as a set of stacked centrifugal tubes, each of which is aligned to the left (in the density gradient separation method, the denser molecules should be closer to the bottom of the tube, that is, to the right in the above picture). In this way, it is not difficult to see that with the division of cells, the density of DNA in the above picture decreases and thus migrates to the left. A cell divides once and only has one DNA band, which denies the so-called total retention mechanism, because according to the total retention mechanism, DNA replication should generate a brand-new DNA double-stranded molecule by completely copying an "old" DNA double-stranded molecule, so when a replication is over, half of the DNA molecules should be brand-new (both double-stranded molecules only contain 14n). The other half should be "completely old" (both double-stranded molecules only contain 14N), so that the density of two black bands should be between the two, that is, one chain is 14N and the other chain is 15N. However, the DNA sample (generation = 1.9) after about two repetitions showed two black bands with the same intensity in the centrifuge tube, one with the same density as the generation = 1 and the other with the same DNA of 14N. This result is completely consistent with the semi-reservation mechanism: in this way, the debate on DNA replication mechanism was finally solved perfectly by meselson and Starr, and a series of major breakthroughs were made in genetics and genomics in the next 50 years.

Watson and Crick proposed the DNA double helix model. It is speculated that in the process of DNA replication, first of all, the hydrogen bond of DNA breaks and the double strands separate from each other. Then each strand can be used as a template, on which new polynucleotide chains can be synthesized by pairing according to the principle of base complementarity. In this way, the base sequence of the two offspring DNA is exactly the same as that of the parent DNA. One strand in each progeny DNA comes from the parent DNA, and the other strand is newly synthesized. This kind of replication is called semi-conservative replication. The experimental results showed that 15Nna obtained in all media labeled with15n showed dense bands at the bottom of the centrifuge tube. When transferred to the medium labeled with 14N, a medium density band was obtained, which was a hybrid molecule of 15N NA and 14N-DNA. There are two bands in the second generation: medium density band and low density band, which indicate that they are 15N 14N-DNA and14n-DNA respectively. With the increase of culture generation in 14N medium, the low-density bands are enhanced, while the medium-density bands are gradually weakened. After centrifugation, the density distribution of CsCl solution from high to low forms a density gradient, and DNA molecules with different weights stay at the same CsCl density, and bands formed by DNA molecules can be seen under ultraviolet light. In order to confirm that the first generation hybrid molecule is indeed half 15n-DNA- half 14n-DNA, this hybrid molecule is denatured by heating, and the DNA before and after denaturation is subjected to CsCl density gradient centrifugation respectively. The results showed that the hybridized molecules before denaturation were medium density bands, and after denaturation, they were divided into two bands, namely, heavy density band (15n-DNA) and low density band (. Their experiment can only be satisfactorily explained by semi-conservative replication theory. The first generation molecule contains a parent chain (shown by melanin), which is paired with another newly synthesized chain (shown by white). In the process of continuous replication in the future, the two chains of the original matrix remain intact, so there are always two molecules with one chain of the original matrix. Melson? The position of DNA after density gradient centrifugation in Stahl experiment: the left three tubes as control; The third tube on the right is the experimental result.

In order to verify the semi-conservative replication of DNA, scientists designed the following experiment: the root tip cells of Vicia faba (2n= 12) were treated with a culture medium containing 3H label until it was completed. After the second division, the cells were transferred to the common culture medium containing colchicine, and then the cells were divided twice continuously. Radioactivity of chromosomes in splinter cell was detected by autoradiography. According to this experimental analysis, the answer is: (The effect of colchicine does not affect replication, but it can inhibit the formation of spindle) Transfer the root tip cells labeled with 3H into the common culture medium containing colchicine and divide it twice continuously, so there are 24 chromosomes in the cells before the second division. If the hypothesis of semi-conservative replication of DNA is established, the experimental results should be: (1) one shows radioactivity and the other shows no radioactivity.

(2) Only half of chromosomes showed radioactivity, and only one chromatid per chromosome showed radioactivity.

(1) One DNA molecule on each chromosome formed after the first generation division, one strand is radioactive and the other strand is non-radioactive. If it is semi-conservative replication, the two strands formed in ordinary culture medium with the two strands after unwinding as templates (one is radioactive and the other is non-radioactive) are not radioactive. In this way, each chromosome after replication has a radioactive chromatid (formed with radioactive mother chain as template) and a non-radioactive chromatid (formed with non-radioactive chain as template), so each chromosome is radioactive in the first metaphase, while one chromatid on each chromosome is radioactive and the other is not.

(2) The second generation division is based on the first generation division. Because one chromatid on each chromosome is radioactive and the other is not in the metaphase of the first generation division, only half of the chromosomes formed after centromere division are radioactive, and only one DNA strand in the radioactive chromosomes is radioactive. If it is semi-conservative replication, two non-radioactive sub-chains are formed in common culture medium with the two strands after melting as templates, so that only one chromatid is formed after radioactive chromosome replication.