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Unit 1 0 1 2 3 4 5
Module 5
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Nucleic Acids

LEARNING GOALS LG 5.1 Nucleotides are the building blocks of nucleic acids.

LG 5.2 Phosphodiester bonds join nucleotides to form nucleic acids.

LG 5.3 Cellular DNA takes the form of a double helix.
LG 5.4 DNA and RNA have similarities and differences.

n the last few modules, we introduced three of the four length. Some of the information in DNA encodes proteins
Iorganic molecules—carbohydrates, lipids, and proteins. that provide structure and do much of the work of the cell.
We have seen that the structures of these biological macro- Second, DNA transmits genetic information to other mole-
molecules are closely related to their functions. Proteins, cules and from one generation to the next.
discussed in Module 4, are a good example. Composed of In this module, we will focus on DNA. As we did with
long, linear strings of 20 different kinds of amino acids in carbohydrates and proteins, we will begin by discussing its
various combinations, each protein folds into a specific three- subunits. Then we will examine how these subunits are
dimensional shape due to chemical interactions between the joined to form nucleic acids, and how information is stored
amino acids along the chain. The three-dimensional struc- in DNA. We will then discuss the structure of DNA and
ture of the protein determines its functional properties and how the structure gives clues to its functions. Finally, we
enables the protein to carry out its job in the cell. will end with a comparison of DNA and its close molecular
Another notable example of the close relationship relative—ribonucleic acid, or RNA.
between structure and function can be seen in the molecule
deoxyribonucleic acid (DNA), which we briefly introduced PREP FOR THE AP EXAM
®
in Module 0. DNA molecules from all cells and organisms FOCUS ON THE BIG IDEAS
have a very similar three-dimensional structure, reflect-
ing their shared ancestry. This structure allows it to carry INFORMATION STORAGE AND TRANSMISSION: look
out its two major functions, which we briefly introduced for the properties of nucleotides and nucleic acids that allow
them to store genetic information and to pass it accurately
in Module 0. First, DNA stores genetic information. This from cell to cell and parent to offspring.
information is encoded in the sequence of subunits along its

5.1 Nucleotides are the building and oxygen. In DNA, there are four different bases, which
blocks of nucleic acids are shown in FIGURE 5.2 . Two of the bases are double-

ring structures known as purines ; these are the bases

Just as proteins are polymers made up of amino acids and
carbohydrates are built from simple sugars, nucleic acids Ionized O – 5’
hydroxyl
such as DNA and RNA are polymers of nucleotides. In this groups – O P O H 2 C O Base (A, G, T, or C)
section, we will examine the structure of nucleotides, the O 4’ H H H H 1’
building blocks of nucleic acids. Phosphate 3’ 2’
group
OH
H
Nucleotides consist of three basic components: a base, a Deoxyribose
5-carbon sugar, and a phosphate group. These three compo- sugar
nents are shown in FIGURE 5.1 . Each component plays an FIGURE 5.1 DNA nucleotide structure
important role in the overall structure of DNA. Let’s consider Nucleotides consist of a nitrogenous base, a 5-carbon sugar, and a
each one in turn. phosphate group. This figure uses the convention of not labeling the
The first component is a nitrogenous base , which is carbon atoms in the ring structure. They are understood to be at the

a cyclic molecule that contains nitrogen, carbon, hydrogen, intersection where two lines meet.

78 UNIT 1 cHeMisTRY OF liFe

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