(b) : A $DNA $ molecule has two unbranched complementary strands which are spirally coiled. The two chains are antiparallel $i.e.$, they run parallel but in opposite direction. One chain has the polarity $5'→ 3'$ whereas, other has $3' →5'$. Both are heldtogether by hydrogen bonds between their bases $i.e., A = T$ and $G \cong C$ and the amount of adenine is equal to thymine and guanine equals to cytosine. The base ratio $A + T / G + C$ may vary from one species to another but is constant for a given species. The purine and pyrimidines are always in equal amount $(A + G = T + C)$ but $A + T$ is not necessarily equal to $G + C$.