Figure 222

Nucleotide subunits of DNA and RNA. Nucleotides are composed of a sugar, a base, and phosphate. (a) Deoxyribonucleotides present in DNA contain the sugar deoxyribose. (b) The sugar in ribonucleotides, present in RNA, is ribose, which has an OH at the position that lacks this group in deoxyribose.

Vander et al.: Human I. Basic Cell Functions 2. Chemical Composition of © The McGraw-Hill

Physiology: The the Body Companies, 2001 Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

Phosphate Sugar Bonds

FIGURE 2-23

Phosphate-sugar bonds link nucleotides in sequence to form nucleic acids. Note that the pyrimidine base thymine is only found in DNA, and uracil is only present in RNA. EQ]

FIGURE 2-23

Phosphate-sugar bonds link nucleotides in sequence to form nucleic acids. Note that the pyrimidine base thymine is only found in DNA, and uracil is only present in RNA. EQ]

two classes: (1) the purine bases, adenine (A) and guanine (G), which have double (fused) rings of nitrogen and carbon atoms, and (2) the pyrimidine bases, cy-tosine (C) and thymine (T), which have only a single ring (Figure 2-23).

A DNA molecule consists of not one but two chains of nucleotides coiled around each other in the form of a double helix (Figure 2-24). The two chains are held together by hydrogen bonds between a purine base on one chain and a pyrimidine base on the opposite chain. The ring structure of each base lies in a flat plane perpendicular to the phosphate-sugar backbone, appearing as steps on a spiral staircase. This base pairing maintains a constant distance between the sugar-phosphate backbones of the two chains as they coil around each other.

Specificity is imposed on the base pairings by the location of the hydrogen-bonding groups in the four bases (Figure 2-25). Three hydrogen bonds are formed between the purine guanine and the pyrimidine cyto-sine (G—C pairing), while only two hydrogen bonds can be formed between the purine adenine and the pyrimidine thymine (A—T pairing). As a result, G is always paired with C, and A with T. In Chapter 5 we shall see how this specificity provides the mechanism for duplicating and transferring genetic information.

RNA RNA molecules differ in only a few respects from DNA (Table 2-7): (1) RNA consists of a single (rather than a double) chain of nucleotides; (2) in RNA, the sugar in each nucleotide is ribose rather than de-oxyribose; and (3) the pyrimidine base thymine in

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Chemical Composition of the Body CHAPTER TWO

Chemical Composition of the Body CHAPTER TWO

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  • Gilly
    What is the basic function of sugars and starches in the body.?
    6 years ago

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