Describe DNA replication. Is this process conservative?

DNA Nucleotides are monomers which form phosphodiester bonds during a condensation reaction to become polymer stands. Two polymer strands twist together into a double helix to form the final DNA molecule structure. Each nucleotide has a nitrogen containing base which is either adenine, guanine, cytosine or thymine. During DNA replication, the enzyme DNA helicase unwinds the double helical structure by breaking the hydrogen bonds between adjacent bases of either strand. This provides one template strand for new nucleotides to be added to their complementary bases on the template strand. The purine base adenine and the pyrimidine base thymine form two hydrogen bonds between them, whereas the purine bas guanine and its complementary pyrimidine base cytosine form three hydrogen bonds. The enzyme DNA polymerase anneals these bases to their complementary bases on the template strand. In eukaryotes DNA replication occurs during S phase of Interphase in the nucleus. DNA replication can be done and controlled in the laboratory also by PCR (polymerase chain reaction). Repeated thermal cycles exponentially amplify a small DNA sample into a larger usable sample. Initially, the double helix is denatured thermally at 95oC to separate the parent strands. Then pre- made primer strands are added at 55oC and anneal to the start of the sequence. Extension occurs at 72oC with the addition of nucleotides and Taq polymerase ( a thermostable DNA polymerase).DNA replication is not conservative but semi- conservative. The experiments of Meselson and Stahl demonstrate this by the use of E. coli grown in 15N media. These E. coli incorporate this heavier isotope into their nitrogen containing DNA bases and centrifugation reveals a lower band. After one generation is then grown on 14N media the centrifugation shows a 'medium' hybrid band, and after a second generation a light band and a hybrid band are seen.