Semi-reserved replication of DNA

the name
Watson and Crick, in proposing a model of the
DNA
double helix structure, 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 chain in the newly synthesized subgenre DNA molecule came from parent DNA and the other chain was newly synthesized, and that the replication was 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.
then transfers the bacteria to a 14N-labeled NH4Cl
medium
for culture, collecting bacteria, rifting cells, and observing the location of DNA using the CsCl density gradient centrifugation method when cultured with different algehes. 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 later increase in culture algetrobats in 14N medium, the low density band is enhanced, and the medium density band gradually decreases, after centrifugation, from the bottom of the tube to the tube mouth, the density distribution of CsCl solution from high to low formation density gradient, different weight DNA molecules stay at the same CSCl density, in ultraviolet light can be seen in the DNA molecular formation zone.
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 of hybrid molecules prior to denaturation, which was divided into two zones, the heavy density band (15N-DNA) and the low density band (14N-DNA). Their experiments can only be successfully explained by the theory of semi-reserved replication.
first-generation molecule contains a progeny chain (shown in melanin) paired with another newly synthesized chain (indicated 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.
.


.