SIX-MEMBERED RINGS WITH TWO HETEROATOMS
431
Box 11.5
Pyrimidines and nucleic acids
The storage of genetic information and the transcription and translation of this information are functions of the
nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). They are polymers whose building
blocks are nucleotides, which are themselves combinations of three parts, i.e. a heterocyclic base, a sugar, and
phosphate (see Section 14.1).
The bases are either monocyclic pyrimidines or bicyclic purines (see Section 14.1). Three pyrimidine bases
are encountered in DNA and RNA, cytosine (C), thymine (T) and uracil (U). Cytosine is common to both DNA
and RNA, but uracil is found only in RNA and thymine is found only in DNA. In the nucleic acid, the bases are
linked through an N-glycoside bond to a sugar, either ribose or deoxyribose; the combination base plus sugar is
termed a nucleoside. The nitrogen bonded to the sugar is that shown.
N
H
N
O
NH 2
cytosine, C
N
H
NH
O
O
thymine, T
H 3 C
N
H
NH
O
O
uracil, U
pyrimidine bases in nucleic acids
in DNA only
in RNA only
N
N
O
NH 2
N
NH
O
O
H 3 C
N
NH
O
O
sugar
sugar
sugar
nucleosides
We should note particularly that uracil and thymine are dioxypyrimidines, whereas cytosine is an aminooxypyrimidine. All three pyrimidines are thus capable of existing in several tautomeric forms (see Section 11.6.2).
N
N
O
NH 2
N
NH
O
NH
N
N
OH
NH
tautomeric forms of cytosine
preferred form
in nucleic acids
N
NH
O
O
H 3 C
preferred form
in nucleic acids
N
N
O
OH
H 3 C
N
N
O
OH
N
NH
O
O
preferred form
in nucleic acids
N
N
OH
O
N
N
OH
O
H 3 C
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
tautomeric forms of thymine
tautomeric forms of uracil
The number of possible forms is reduced somewhat by the fact that one of the nitrogens is bonded to the sugar in
the nucleic acid; it no longer carries a hydrogen to participate in tautomerism. The tautomeric forms indicated are
found to predominate in nucleic acids. The oxygen substituents exist almost entirely as carbonyl groups, whereas