Question

explain semi- conservative replication of eukaryotic DNA

Answer 1

Dear Student, Semiconservative replication describes the mechanism by which DNA is replicated in all known cells. It derives its name from the fact that this mechanism of transcription was one of three models originally proposed for DNA replication. The deciphering of the structure of DNA by Watson and Crick in 1953 suggested that each strand of the double helix would serve as a template for synthesis of a new strand. However, there was no way of knowing how the newly synthesized strands might combine with the template strands to form two double helical DNA molecules. The semiconservative model seemed most reasonable since it would allow each daughter strand to remain associated with its template strand. Messelson and Stahl performed the experiment to prove that DNA replicates semi-conservatively. The salient features of their experiment include E.coli was grown in a medium containing heavy isotope 15N as the nitrogen source. 15N was incorporated into newly synthesised DNA as well and the DNA became heavy DNA. Heavy DNA molecule can be differentiated from normal DNA by density gradient centrifugation using cesium chloride as the gradient. Then, cells were again transferred into a medium with 14N as nitrogen source. Samples were taken from this media and their DNA was extracted. E .coli divides every 20 minutes. Therefore, the DNA extracted after 20 minutes had a hybrid density. DNA extracted after 40 minutes had equal amount of hybrid and light intensities. This implies that the newly synthesised DNA obtained one of its strands from the parent. Regards