Please answer the following essay questions Describe what is

Solution

Answer : Semi conservation DNA replication

Semi-conservative replication is a process in which during replication, Each DNA strand serves as a template for the synthesis of a new strand, producing two new DNA molecules, each with one new strand and one old strand. The Watson and Crick hypothesis of semi – conservative DNA replication was confirmed by Meselson–Stahl using cesium chloride density gradient centrigugation.

Meselson and Stahl grew E. coli cells for many generations in a medium in which the sole nitrogen source (NH4Cl) contained 15N, the “heavy” isotope of nitrogen, instead of the normal, more abundant “light” isotope, 14N. The DNA isolated from these cells had a density about 1% greater than that of normal [14N]DNA. Although this is only a small difference, a mixture of heavy [15N]DNA and light [14N]DNA can be separated by centrifugation to equilibrium in a cesium chloride density gradient.

Later, The E. coli cells grown in the 15N medium were transferred to a fresh medium containing only the 14N isotope, where they were allowed to grow until the cell population had just doubled. The DNA isolated from these first-generation cells formed a single band in the CsCl gradient at a position indicating that the doublehelical DNA molecules of the daughter cells were hybrids containing one new 14N strand and one parent 15N strand which confirms semi conservative mode of replication.