Essentials of Anatomy and Physiology

NUCLEIC ACIDS

DNA and RNA

The nucleic acids, DNA(deoxyribonucleic acid) and
RNA(ribonucleic acid), are large molecules made of
smaller subunits called nucleotides. A nucleotidecon-
sists of a pentose sugar, a phosphate group, and one of

several nitrogenous bases. In DNA nucleotides, the

sugar is deoxyribose, and the bases are adenine, gua-

nine, cytosine, or thymine. In RNA nucleotides, the

sugar is ribose, and the bases are adenine, guanine,

cytosine, or uracil. DNA and RNA molecules are

shown in Fig. 2–10. Notice that DNA looks somewhat

like a twisted ladder; this ladder is two strands of

Some Basic Chemistry 41

Deoxyribose (DNA)

Ribose (RNA)

Phosphate

Adenine

Guanine

Thymine (DNA)

Uracil (RNA)

Cytosine

Chromatin in

the nucleus

Cell

Double helix

DNA strands

Hydrogen

bonds

RNA strand

Figure 2–10. DNA and RNA. Both molecules are shown, with each part of a nucleotide

represented by its shape and in a different color. Note the complementary base pairing of

DNA (A–T and G–C). When RNA is synthesized, it is a complementary copy of half the DNA

molecule (with U in place of T).

QUESTION:Why can’t adenine pair with guanine to form a rung of the DNA ladder?