Nucleic Acids Store Information in Their Sequence of Chemical Units
March 14 and 17, 2013
Read the following passage. Highlight the key terms and underline the main
ideas. Answer the questions that follow.
Once most biologists were convinced that DNA is the genetic material, new
questions arose. What is it about the specific arrangement of atoms that gives
DNA its unique properties? How is DNA able to store genetic information, copy
it, and pass it from generation to generation? The challenge became one of
determining how the structure of this molecule could account for its role in
heredity.
The Building Blocks of DNA
The heritable genetic information of an organism is stored in the molecule called
deoxyribonucleic acid (DNA). DNA is a kind of nucleic acid, a polymer built
from monomers called nucleotides. Another group of nucleic acids, called
ribonucleic acids (RNAs), also plays a key role in cells. You will learn about the
function of RNAs later in this chapter.
Nucleotides are the building blocks (the monomers) of nucleic acid polymers.
Only four types of nucleotides make up DNA. Examine the chemical structure of
a single nucleotide in Figure 11-5. Notice that each nucleotide has three parts:
1. A ring-shaped sugar called deoxyribose
2. A phosphate group (a phosphorus atom surrounded by four oxygen
atoms)
3. A nitrogenous base (ny TRAW juhn us): a single or double ring of
carbon and nitrogen atoms with functional groups (nitrogenous means
"nitrogen-containing")
Figure 11-5
A nucleotide has three
components: a sugar, a phosphate
group, and a nitrogenous base.
Nitrogenous Bases The four nucleotides found in DNA differ only in their
nitrogenous bases, called bases for short (Figure 11-6). The bases thymine (T)
and cytosine (C) are single-ring structures called pyrimidines (py RIM uh deenz).
Adenine (A) and guanine (G) are larger, double-ring structures called purines
Nucleic Acids Store Information in Their Sequence of Chemical Units
March 14 and 17, 2013
(PYOOR eenz). The one-letter abbreviations stand for both the bases alone and
for the nucleotides containing them.
DNA Strands Nucleotides are joined to one another by covalent bonds that
connect the sugar of one nucleotide to the phosphate group of the next. This
repeating pattern of sugar-phosphate-sugar-phosphate is called a sugar-phosphate
"backbone." The nitrogenous bases are lined up along this backbone (Figure 117).
Figure 11-6
DNA contains four different nitrogenous bases. Thymine and cytosine
have single-ring structures. Adenine and guanine have double-ring
structures.
Just as amino acid monomers combine and form a polypeptide, the nucleotides
of a nucleic acid polymer can combine in many different sequences. For
example, the part of a nucleic acid shown in Figure 11-7 has nine nucleotides
arranged in the order CTGCTATCG. This arrangement is only one of many
possible. Since nucleotide chains also vary in length, from only a few hundred
nucleotides to millions of nucleotides, the number of possible nucleotide
sequences is essentially unlimited.
Nucleic Acids Store Information in Their Sequence of Chemical Units
March 14 and 17, 2013
Figure 11-7
Nucleotide monomers join together by covalent bonds
between the sugar of one nucleotide and the phosphate of
the next, forming a sugar-phosphate backbone.
For years, people wondered how strings of nucleotides could serve as the
hereditary material. As you will read next, it turns out that a string of nucleotides
is just one key feature of DNA's structure.
DNA's Structure
In the early 1950s, scientists Rosalind Franklin and Maurice Wilkins produced
some intriguing photographs of DNA using a method called X-ray
crystallography. This technique provides clues to the shapes and dimensions of
complex molecules. The photographs showed the basic shape of DNA to be a
helix, and revealed the basic dimensions of the helix.
The Double Helix Meanwhile, scientists James Watson and Francis Crick
modeled DNA's structure with tin and wire. Their early models failed to explain
DNA's chemical properties. Then one day, Watson saw one of Franklin's X-ray
crystallography photos of DNA. Using the clues provided by Franklin's work,
Watson and Crick created a new model in which two strands of nucleotides
wound about each other. This formed a twisting shape called a double helix
(Figure 11-8). Their model placed the sugar-phosphate backbones on the outside
of the double helix and the nitrogenous bases on the inside. They hypothesized
that the nitrogenous bases that aligned across the two strands formed hydrogen
bonds. This new model successfully represented DNA's structure.
Nucleic Acids Store Information in Their Sequence of Chemical Units
March 14 and 17, 2013
Figure 11-8
The bases pair up between
the two intertwined sugarphosphate backbones,
forming the double helix
discovered by Watson and
Crick. A pairs with T, and
G pairs with C.
Complementary Base Pairs Watson and Crick realized that the individual
structures of the nitrogenous bases determine very specific pairings between the
nucleotides of the two strands of the double helix. These pairings are due to the
sizes of the bases and their abilities to form hydrogen bonds with each other. The
purine adenine pairs with the pyrimidine thymine, and the purine guanine pairs
with the pyrimidine cytosine. In the biologist's shorthand, A pairs with T, and G
pairs with C. A is also said to be "complementary" to T, and G is complementary
to C. So, while the sequence of nucleotides along the length of one of the two
DNA strands can vary in countless ways, the bases on the second strand of the
double helix are determined by the sequence of the bases on the first strand. Each
base must pair up with its complementary base. Base-pairing rules set the stage
for understanding how the information in DNA is passed through generations.
In 1953, Watson and Crick described their model in a two-page article in the
science journal Nature. Few milestones in the history of biology have been as
important as the discovery of the double helix and the pairing of complementary
bases.
Concept Check
1. What are the three parts of a nucleotide? Which parts make up the backbone of
a DNA strand?
2. List the two base pairs found in DNA.
3. If six bases on one strand of a DNA double helix are AGTCGG, what are the
six bases on the complementary section of the other strand of DNA?