(b) : Taylor et al $(1957)$ conducted experiment on Vicia faba (broad bean) to prove semi­conservative replication of $DNA$. He fed dividing cells of root tips of Vicia faba with radioactive $^3H$ containing thymine instead of normal thymine and found that all the chromosomes became radioactive. Labelled thymine was then replaced with normal one. Next generation came to have radioactivity in one of the two chromatids of each chromosome while in subsequent generation radioactivity was present in $50\%$ of the chromosomes. This is possible only if out of the two strands of a chromosome, one is formed a fresh while the other is conserved at each replication.