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Nucleic Acids
Nucleic acids (noo-KLEE-ihk AS-ihdz) are huge, complex carbonbased molecules. They contain the information that cells use to make
proteins. These macromolecules are made of carbon, hydrogen,
and oxygen, as well as nitrogen and phosphorus. Each of the cells in
your body contains a complete set of nucleic acids. This means that
each cell has all of the instructions necessary for making any protein
in your body.
The illustration on page 358 shows part of a nucleic acid molecule
called DNA, which looks like a twisted ladder. The sides of the ladder
are made of sugar molecules and phosphate groups. Each rung of the
ladder is composed of two nitrogen-containing molecules called bases.
DNA has four types of bases, represented by the letters A, C, T, and G.
The order of the bases in a DNA molecule is the way in which DNA
stores the instructions for making proteins. How do just four
molecules—A, C, T, and G—carry all of this important information?
Recall that a protein is composed of amino acids that have to be
linked in a certain order. Each of the 20 amino acids is represented
by a particular series of three DNA bases. For example, the sequence
T–A–C corresponds to, or is a code for, the amino acid tyrosine. There
are 64 different three-base sequences in DNA, each of which has a
specific meaning. This genetic code works in the same way in every
living thing on Earth. It provides a complete set of instructions for
linking amino acids in the right order to make each specific protein
molecule. The DNA code is only one part of making proteins, though.
Other types of nucleic acids, called RNA, are responsible for reading
the code and assembling a protein with the correct amino acids.
Check your reading
reading tip
The NA in DNA stands for
“nucleic acid.” The D
stands for “deoxyribose,”
which is the type of sugar in
the molecule.
RESOURCE CENTER
CLASSZONE.COM
Find out more about
carbohydrates, lipids,
proteins, and nucleic
acids.
How many different types of bases make up the genetic code
in DNA?
KEY CONCEPTS
CRITICAL THINKING
1. How does the function of a lipid
depend on its structure? (8.3.c)
4. Synthesize Give two
examples of carbon-based
molecules in living things that
are based on a chain structure.
Explain.
2. What determines the structure
of a protein? (8.3.a)
3. What role does DNA perform in
the making of proteins? (8.3.c)
5. Compare and Contrast
How are carbohydrates and
lipids similar? How are they
different?
CHALLENGE
6. Infer Suppose the order of
bases in a DNA molecule is
changed. What do you think
will happen to the structure of
the protein that is coded for by
that region of DNA? Why?
Chapter 11: Chemistry of Living Systems 359
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