236 Green Chemistry, 2nd ed
rock and extract nutrients from it. The algae are photosynthetic, so they produce the
biomass required by the system, which is utilized in part by the fungi. Another important
symbiotic relationship is that in which nitrogen-fixing bacteria grow in nodules on
leguminous plant roots. The bacteria receive nutrients from the plants in exchange for
chemically fixed nitrogen required for plant nutrition.
9.7. DNA and the Human Genome
In Chapter 5, Section 5.10, deoxyribonucleic acid, DNA, was discussed and it was
noted that this macromolecule stores and passes on genetic information that organisms
need to reproduce and synthesize proteins. Recall that DNA is composed of repeating
units called nucleotides each consisting of a molecule of the sugar 2-deoxy-β-D-
ribofuranose, a phosphate ion, and one of the four nitrogen-containing bases, adenine,
cytosine, guanine, and, thymine (conventionally represented by the letters A, C, G, and
T, respectively). DNA is one of two nucleic acids, the other one of which is ribonucleic
acid, RNA. Like DNA, RNA consists of repeating nucleotides but the sugar in RNA is
β-D-ribofuranose and it contains uracil instead of thymine in its bases. The structural
formulas of segments of DNA and RNA are shown in Figure 5.10.
The structure of DNA is a key aspect of its function, and its elucidation by Watson
and Crick in 1953 was a scientific insight that set off a revolution in biology that is going
on to this day. The huge DNA molecules consist of two strands counterwound with each
other and held together by hydrogen bonds (discussed in Section 7.1 and illustrated for
water molecules in Figure 7.1). A representation of this structure is shown in Figure 9.5.
In this structure, the hydrogen bonds connecting complementary bases on the two
Figure 9.5. Representation of the double helix structure of DNA. Hydrogen bonds between complementary
bases on the two strands are shown by dashed lines.