Figure 220

Formation of a disulfide bond between the side chains of two cysteine amino acids links two regions of the polypeptide together. The hydrogen atoms on the sulfhydryl groups of the cysteines are transferred to another molecule, X, during the formation of the disulfide bond.

three components: a phosphate group, a sugar, and a ring of carbon and nitrogen atoms known as a base because it can accept hydrogen ions (Figure 2-22). The phosphate group of one nucleotide is linked to the sugar of the adjacent nucleotide to form a chain, with the bases sticking out from the side of the phosphate-sugar backbone (Figure 2-23).

DNA The nucleotides in DNA contain the five-carbon sugar deoxyribose (hence the name "deoxyribonucleic acid"). Four different nucleotides are present in DNA, corresponding to the four different bases that can be linked to deoxyribose. These bases are divided into

OH H

Typical deoxyribonucleotide

FIGURE 2-21

Hemoglobin, a multimeric protein composed of two identical a chains and two identical ¡3 chains. (The heme groups attached to each globin chain are not shown.)

FIGURE 2-21

Hemoglobin, a multimeric protein composed of two identical a chains and two identical ¡3 chains. (The heme groups attached to each globin chain are not shown.)

Phosphate O

Base (cytosine)

O

H

H/

C-c'

OH

Typical ribonucleotide

Base (cytosine)

Sugar (ribose)

Typical ribonucleotide

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Essentials of Human Physiology

Essentials of Human Physiology

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