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. When Watson and Crick proposed the
DNA
double helix structure model, they speculated that DNA was first copied by breaking the hydrogen bond between the two chains, and then using each chain as a template to form a new DNA strand, so that one of the newly synthesized child DNA molecules came from parent DNA, and the other chain was semi-synthetic, which is semi-replicated.In 1958, Meselson and Stahl first confirmed semi-reserved replication of DNA in E. coli using nitrogen marker technology, and they bred E. coli in a 15N-labeled NH4Cl
culture
base for 15 generations, allowing all E. coli DNA to be labeled at 15N and 15N NA. The bacteria were then transferred to a 14N-labeled NH4Cl
medium
for culture, where bacteria, cleavage cells were collected, and DNA locations were observed using CsCl density gradient centrifugation. Because 15N NA is denser than normal DNA (14N-DNA), two dnas with different densities are distributed in different zones at the density gradient centrifugation of chlorinated radon.results show that 15N NA in all 15N-labeled mediums is shown as a heavy density band located at the bottom of
centrifuge tube
tube. When transferred to a 14N-labeled medium, the first generation was given a medium density band, a hybrid molecule of 15N NA and 14N-DNA. The second generation has two zones, the medium density band and the low density band, which indicate that they are 15N14N-DNA and 14N14N-DNA, respectively. With the increase of culture algelus in 14N medium, the low density band is enhanced, and the medium density band is gradually weakened, and after centrifugation, from the bottom of the tube to the mouth of the tube, the density distribution of CsCl solution forms a density gradient from high to low, and the DNA molecules of different weights stay at their equivalent ClCs density, and the zone formed by the DNA molecules can be seen in ultraviolet light. To confirm that the first generation of hybrid molecules were indeed half 15N-DNA-half 14N-DNA, the hybrid molecules were denatured by
-heated
, and the DNA before and after denaturation was centrifuged by CSCl density gradients, resulting in a medium density band for pre-denatured hybrids and two zones, heavy density bands (15N-DNA) and low density bands (14N-DNA). Their experiments can only be successfully explained by the theory of semi-reserved replication (Figures 16-2 and 16-3).
half-preserved copy of 16-2 DNA
The first generation of molecules contains a progeny chain (shown in melanin) paired with another newly synthesized chain (in white). In the later continuous replication process, the original pro-generation two chains remain intact, so there are always two molecules each have a pro-generation chain.
. The semi-reserved replication of the 16-3 DNA
the location of the DNA after the centrifugation of the Meslson Stahl experimental density gradient: the left three tubes are the control, and the right three tubes are the results
.