Molecular Basis Of Inheritance

Semiconservative replication, proposed by Watson and Crick and experimentally proven by Meselson and Stahl, means each daughter DNA molecule retains one original parental strand and one freshly synthesised strand. The parental double helix unwinds, and each single strand serves as a template on which complementary nucleotides are added by DNA polymerase following base-pairing rules. Consequently, every daughter duplex is half old and half new. The option suggesting two parental strands describes conservative replication, where the original helix stays intact and a wholly new helix forms separately, a model disproved experimentally. The choice of two new strands is also conservative-model logic applied to the daughter molecule and is incorrect. Three new strands is biologically impossible since a DNA duplex contains only two strands in total. As recorded in NCERT Class 12, Chapter 6 (Molecular Basis of Inheritance), the Meselson-Stahl experiment using nitrogen isotopes confirmed the semiconservative mechanism by density gradient centrifugation. As a sanity check, conserving one template strand per daughter molecule ensures faithful transmission of genetic information, which matches the one-old-one-new composition of each helix.