and signals int o and out of the cell. Cell membrane proteins are also a point of attack for
infectious agents such as viruses, or may bind with drugs, leading to reactio ns that pro-
duce their characteristic effects.
3.6.4 Nucleic Acids
Nucleic acids do not form a large portion of the mass of living things, but make up for this
in importance by being central to reproduction and control of cell function (DNA and
RNA) and as the single most important compound in energy metabolism [adenosine tri-
phosphate (ATP)]. DNA and RNA are linear polymer s of nucl eotides; ATP is a single
nucleotide.
A nucleotide is a compound consisting of three parts:
pyrimidine or purine base þribose or deoxyribose sugar þ one or more phosphates
The pyrimidines are based on a six-membered ring containing two nitrogens. Only the
following three pyrimidines are found in DNA and RNA: thymine, cytosine, and uracil
(Figure 3.13). Purines have an additional five-membered ring fused to the pyrimidine.
Only two purines are used in DNA and RNA, adenine and guanine. Nucleotides
using these bases are labeled with their first letter: A, G, U, C, or T. The five-carbon
sugars are bonded to the base, and the phosphate(s) are connected to the sugar via an
ester.
A nucleoside is the same as a nucleotide, without the phosphate. The nucleoside
formed from adenine and ribose is called adenosine. The nucleoside formed from thy-
mine and deoxyribose is called thymidine.
Besides forming a chain of phosphates, the phosphate portion of the molecu les can
form ester bonds to two nucleotide sugars, forming a linear polymer with the phosphates
and sugars as a ‘‘backbone’’ and the bases as branches (Figure 3.14).
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are polymers formed
from nucleotides. In DNA the sugars are deoxyribose; in RNA the sugars are ribose.
Another important difference between these polymers is that DNA does not contain uracil,
while RNA never includes thymine. In other words, DNA includes only A, G, C, and
T; RNA has only A, G, C, and U.
RNA is present only as a single chain. However, two strands of DNA form a fascinat-
ing structure called the double helix, discovered by Watson and Crick in 1953. It happens
that thym ine and adenine on two different strands can form two hydrogen bonds in just the
right position relative to each other, and cytosine and guanine on two complementary
strands form three such bonds (Figure 3.14). Thus, the sequence of bases on one strand
determines the sequence on the other. The tw o complemen tary strands are held together
by a large number of hydrogen bonds, making the pairing very stable. The resulting struc-
ture resembles a ladder, with each sugar–phosphate backbone making one side of the lad-
der, and the base pairs forming the rungs. A purine is always opposite a pyrimidine, to
keep the lengths of the rungs all the same. In addition, the ladder has a right-hand twist to
it, making a complete turn every 10 ‘‘rungs’’ or base pairs, producing the famous double
helix structure.
Notice from Figure 3.14 that the point of attachment at one end of the chai n of nucleo-
tides is carbon 5 of the sugar, and at the other end it is carbon 3. This gives a direction to
58 THESUBSTANCESOFLIFE