During cell division, the genetic information contained in the parental cells is completely transmitted to the two daughter cells, and the DNA is completely replicated into two copies during passage. This process is called DNA replication. Replication is performed in strict accordance with the A-T pairing and G-C pairing rules, usually with a high degree of completeness and accuracy. However, there are many factors in the internal and external environment of the organism that damage the DNA molecules, causing some errors in the replication. Therefore, the organism itself has a mechanism to repair the damage. Unrepaired errors are retained and become mutations. Mutations have the potential to improve genes and enhance the ability of organisms to adapt to the environment; they can also have adverse effects and can be fatal. Recessive mutations have no effect on biological traits. Mutation is a means of biological evolution.
(a) semi-reserved copy
To study the mechanism of replication, Meselson and Stahl designed an experiment in 1957. They first cultured E. coli in 15NH4Cl for 15 generations, so that all DNA was labeled with 15N, 15N-DNA (heavy DNA) was about 1% heavier than the usual 14N-DNA (light DNA); The culture was continued to be transferred to a medium containing 14NH4Cl, and as the replication progressed, 14N DNA was continuously produced. Samples were taken at different times during the incubation process, and E. coli cells were lysed with lysate and placed in CsCl solution for ultracentrifugation (140,000 r/min) for 20 hours. At this time, a gradient distribution was formed from the bottom of the tube to the CsCl density of the tube. The molecule stays at a level equal to its density, and an absorption band can be observed under UV detection. They found that the parents obtained in the culture medium containing 15NH4Cl were all heavy DNA (both chains contain a 15N heavy chain), and the DNA of the first progeny (F1) obtained in the culture medium containing 14NH4Cl was transferred. It is neither heavy DNA nor light DNA (both strands contain 14N light chains), but mixed-strand DNA with a density between the two; the second progeny (F2) also shows two DNAs. Absorption bands, one is DNA with a density between light and heavy DNA, and the other is light DNA; the DNA of the third generation (F3) also shows two absorption bands, which increases the proportion of light DNA; the fourth The proportion of light DNA in generation (F4) is even greater. Heavy DNA never appears in progeny DNA, indicating that DNA replication in E. coli is semi-reserved replication.
Their experimental results support the DNA replication patterns proposed by Watson and Crick. Since the DNA molecule consists of two polynucleotide strands, the bases on the two strands have a strict pairing law, so the two strands are complementary. That is, the order of nucleotides on any of the strands in the DNA molecule has determined the nucleotide sequence of the other strand complementary to it. Therefore, they proposed a semi-reserved replication mechanism of DNA: during the replication process, the hydrogen bonds between the bases are first broken, the double helices are separated, and the two chains are used as templates to synthesize new chains, resulting in two complementary new chains. The newly formed two DNA molecules have exactly the same nucleotide sequence as the original DNA molecules, except that one strand of the progeny DNA molecule is derived from the parental DNA and the other strand is newly synthesized.
The semi-reserved replication mechanism of DNA is a good illustration of the perfect unity of DNA as a genetic material.
(2) The beginning and direction of replication
DNA replication begins at a specific starting point in the chain and proceeds in the opposite direction to both sides, called bidirectional replication. Simple organisms like E. coli have only one starting point, eukaryotic DNA molecules are huge, and there are multiple origins of replication. The Alu repeats of mammalian cells may be related to the origin of replication. Most prokaryotic and eukaryotic cells, as well as viruses, are bidirectionally replicated and the replication rates in both directions are symmetrically equal.
The DNA double helix at the starting point of replication begins to unravel, and under the electron microscope, the vesicular shape can be seen, which is called copying or copying the eye; the loosened two strands of DNA single strand and the unsplitting double helix are shaped like a The fork is called the copy fork; the copy start point and the copy forks on both sides together form a unit called a replicon. E. coli forms only one replicon, whereas eukaryotic cells have multiple replicons because they have multiple origins of replication.
Since the two strands of the DNA double helix are antiparallel, when the copying, the two parent strands are loosened apart as a template synthesizing strand, and if one strand is in the direction of 5'-3', the other strand is The direction is 3'-5'. The catalytic synthesis of DNA polymerase can only be 5'-3', so one sub-chain can be synthesized continuously in the 5'-3' direction, called the leader chain; the other sub-chain can only be 5'-3' The direction is not continuous to synthesize many small fragments. This chain is called the follower chain. These small fragments are called Gangzi fragments. The small fragments of the follower chain are finally connected by DNA ligase into a complete sub-chain.
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