nucleotides called a double helix (two coils). Alterna-
ting phosphate and sugar molecules form the uprights
of the ladder, and pairs of nitrogenous bases form the
rungs. The size of the bases and the number of hydro-
gen bonds each can form the complementary base
pairing of the nucleic acids. In DNA, adenine is always
paired with thymine (with two hydrogen bonds), and
guanine is always paired with cytosine (with three
hydrogen bonds).
DNA makes up the chromosomes of cells and is,
therefore, the genetic codefor hereditary characteris-
tics. The sequence of bases in the DNA strands is actu-
ally a code for the many kinds of proteins living things
produce; the code is the same in plants, other animals,
and microbes. The sequence of bases for one protein is
called a gene. Human genes are the codes for the pro-
teins produced by human cells (though many of these
genes are also found in all other forms of life—we are
all very much related). The functioning of DNA will
be covered in more detail in the next chapter.
RNA is often a single strand of nucleotides (see Fig.
2–10), with uracil nucleotides in place of thymine
nucleotides. RNA is synthesized from DNA in the
nucleus of a cell but carries out a major function in the
cytoplasm. This function is protein synthesis, which
will also be discussed in the following chapter.
ATP
ATP (adenosine triphosphate) is a specialized
nucleotide that consists of the base adenine, the sugar
ribose, and three phosphate groups. Mention has
already been made of ATP as a product of cell respira-
tion that contains biologically useful energy. ATP is
one of several “energy transfer” molecules within
cells, transferring the potential energy in food mole-
cules to cell processes. When a molecule of glucose is
broken down into carbon dioxide and water with the
release of energy, the cell uses some of this energy to
synthesize ATP. Present in cells are molecules of ADP
(adenosine diphosphate) and phosphate. The energy
released from glucose is used to loosely bond a third
phosphate to ADP, forming ATP. When the bond of
this third phosphate is again broken and energy is
released, ATP then becomes the energy source for cell
processes such as mitosis.
All cells have enzymes that can remove the third
phosphate group from ATP to release its energy,
forming ADP and phosphate. As cell respiration con-
tinues, ATP is resynthesized from ADP and phos-
phate. ATP formation to trap energy from food and
breakdown to release energy for cell processes is a
continuing cycle in cells.
The structure and functions of the nucleic acids are
summarized in Table 2–6.SUMMARY
All of the chemicals we have just described are consid-
ered to be non-living, even though they are essential
parts of all living organisms. The cells of our bodies
are precise arrangements of these non-living chemi-
cals and yet are considered living matter. The cellular
level, therefore, is the next level of organization we
will examine.42 Some Basic Chemistry
Table 2–6 NUCLEIC ACIDSName Structure Function
DNA (deoxyribonucleic acid)RNA (ribonucleic acid)ATP (adenosine triphosphate)A double helix of nucleotides;
adenine paired with
thymine, and guanine
paired with cytosine
A single strand of nucleotides;
adenine, guanine, cytosine,
and uracil
A single adenine nucleotide
with three phosphate
groups- Found in the chromosomes in the nucleus of a cell
- Is the genetic code for hereditary characteristics
- Copies the genetic code of DNA to direct protein
synthesis in the cytoplasm of cells - An energy-transferring molecule
- Formed when cell respiration releases energy from
food molecules - Used for energy-requiring cellular processes