Model 3: Base pairing of nucleotides creates secondary structures. The most important secondary structure of DNA is the famous double helix. In this by hydrogen bonds between pairs of nitrogen-containing bases, or base pairs. Figure 5 structure, two sugar-phosphate backbones spiral around each other, and are held together illustrates how the hydrogen bonding partnership works. Note that bases almost never pair with the Figure 5: Hydrogen bonds hold AT and CG (complementary) base pairs together in DNA. The shaded "R" indicates where the bases are attached to the sugar- phosphate backbone. In RNA, uracil replaces thymine. N-H N: HIN: b. There must be a CH Adenine (A) Critical Thinking Questions: 14. Review: In general, two conditions are required for hydrogen bonding to occur: a. There must be a hydrogen atom bonded to a Oxygen Nitrogen atom (to donate the hydrogen bond). lone Pair Thymine (T) (to accept the hydrogen bond). Guanine (G) H or Cytosine (C) 15. Use dashed lines to draw hydrogen bonds between A and T in the appropriate locations in Figure 5. Do the same for the CG pair. 16. DNA double helices with more CG content are more heat stable than those with more AT content. Explain why, this is so. (See Figure 5.) single stranded has higher absorbance than double stranded. The higher the melting point, the more the CGS 17. Words or phrases that read the same forwards and backwards are called palindromes. Examples are "radar" or the phrase "A man, a plan, a canal: Panama!" Suppose that one strand of DNA is palindromic (such as ATGGTA). Would the complementary strand also be palindromic? (Hint: Write the complementary sequence.) Explain.