24-2 Base Pairing in DNA:The Watson- Crick Model Samples of DNA isolated from different tissues of the same species have the same proportions of heterocyclic bases Samples of DNA from different species often have greatly different proportions of bases Composition of human DNA =30%each of adenine and thymine 20%each of guanine and cytosine Composition of the bacterium Clostridium perfringens 37%each of adenine and thymine 13%each of guanine and cytosineSamples of DNA isolated from different tissues of the same species have the same proportions of heterocyclic bases Samples of DNA from different species often have greatly different proportions of bases ▪ Composition of human DNA ▪ 30% each of adenine and thymine ▪ 20% each of guanine and cytosine ▪ Composition of the bacterium Clostridium perfringens ▪ 37% each of adenine and thymine ▪ 13% each of guanine and cytosine 24-2 Base Pairing in DNA: The Watson￾Crick Model