Compare and contrast purines and pyrimidines and provide examples of each. Answer: Purines contain two carbon-nitrogen rings and four nitrogen atoms, while the pyrimidines contain one carbon-nitrogen ring and twonitrogen atoms. Adenine and guanine are purines, and thymine and cytosine are pyrimidines (See section 4.1 in module). Describe the structure and arrangement of DNA. Answer: DNA is a double helix, which means it is composed of two helicalstrands. The two strands are held together by the ‘rungs’, which are madeof complementary pairs. The pairs are held together by weak hydrogen bonds.