DNA and RNA differ somewhat in their sugar groups, bases, and strand structure (Table 4.1). Four bases are found in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). RNA is also made up of four different monomers, but its nucleotides include uracil (U) instead of thymine.

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A key to understanding the structure and function of nucleic acids is the principle of complementary base pairing. In DNA, thymine and adenine pair (T-A), and cytosine and guanine pair (C-G). In RNA, the base pairs are A-U and C-G.

Base pairs are held together primarily by hydrogen bonds. As you can see, there are polar C═O and N—H covalent bonds in the bases; these can form hydrogen bonds between the δ^- on an oxygen or nitrogen of one base and the δ⁺ on a hydrogen of another base.

Individual hydrogen bonds are relatively weak, but there are so many of them in a DNA or RNA molecule that collectively they provide a considerable force of attraction, which can bind together two polynucleotide strands, or a single strand that folds back onto itself. This attraction is not as strong as a covalent bond, however. This means that individual base pairs are relatively easy to break with a modest input of energy. As you will see, the breaking and making of hydrogen bonds in nucleic acids is vital to their role in living systems.

RNA Even though RNA is generally single-stranded (Figure 4.3A), base pairing can occur between different regions of the molecule. Portions of the single-stranded RNA molecule can fold back and pair with one another (Figure 4.3B). Thus complementary hydrogen bonding between ribonucleotides plays an important role in determining the three-dimensional shapes of some RNA molecules. Complementary base pairing can also take place between ribonucleotides and deoxyribonucleotides. Adenine in an RNA strand can pair either with uracil (in another RNA strand) or with thymine (in a DNA strand). Similarly, an adenine in DNA can pair either with thymine (in the complementary DNA strand) or with uracil (in RNA).

Question

If a folded RNA molecule were heated, hydrogen bonds between bases in the RNA would break and the molecule would assume a random shape, losing its specific shape.

DNA Usually, *DNA is double-stranded; that is, it consists of two separate polynucleotide strands of the same length that are held together by hydrogen bonds between base pairs (Focus: Key Figure 4.4A). In contrast to RNA’s diversity in three-dimensional structure, DNA is remarkably uniform. The A-T and G-C base pairs are about the same size (each is a purine paired with a pyrimidine), and the two polynucleotide strands form a “ladder” that twists into a double helix (Focus: Key Figure 4.4B). The sugar–phosphate groups form the sides of the ladder, and the bases with their hydrogen bonds form the “rungs” on the inside. DNA carries genetic information in its sequence of base pairs rather than in its three-dimensional structure. The key differences among DNA molecules are their different nucleotide base sequences.

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