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The DNA of this single cell contains the genetic information needed to specify construction of an entire multicellular animal—in this case, a frog, Xenopus laevis. (Courtesy of Tony Mills.)

This molecule is a complex of two identical heavy chains and two identical light chains. Each heavy chain contains four similar, covalently linked domains, while each light chain contains two such domains. Each domain is encoded by a separate exon, and all of the exons are thought to have evolved by the serial duplication of a single ancestral exon.

(A) RNA contains the sugar ribose, which differs from deoxyribose, the sugar used in DNA, by the presence of an additional -OH group. (B) RNA contains the base uracil, which differs from thymine, the equivalent base in DNA, by the absence of a -CH₃ group. (C) A short length of RNA. The phosphodiester chemical linkage between nucleotides in RNA is the same as that in DNA.

Hemoglobin is an abundant protein in red blood cells that contains two copies of α globin and two copies of β globin. Each of these four polypeptide chains contains a heme molecule (red), which is the site where oxygen (O₂) is bound. Thus, each molecule of hemoglobin in the blood carries four molecules of oxygen.