Biology 12

Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 219

to call the nucleic acid containing this sugar
“deoxyribose nucleic acid” (deoxyribonucleic acid
or DNA). Figure 7.2 shows the structures of ribose
and deoxyribose sugars. Levene is pictured in
Figure 7.3.

Figure 7.2The structure of (A) ribose, found in RNA, and
(B) deoxyribose, found in DNA. In ribose, the 2 ′carbon is
bonded to a hydroxyl group. In deoxyribose, this carbon
is bonded to a single hydrogen molecule.

During the period when Levene was conducting
his studies on nucleic acids, other experimenters
demonstrated that Mendel’s factors of inheritance
were associated with the nuclein substance first
isolated by Miescher. By that time, nuclein had
been shown to be made up of individual structures
known as chromosomes, strand-like complexes of
nucleic acids and protein tightly bound together.
Thus, the finding that the factors of inheritance
were associated with nuclein drew increased
attention to both the protein component and the
properties of nucleic acids.

After distinguishing between DNA and RNA,
Levene went on to show that nucleic acids are
made up of long chains of individual units he
termed nucleotides. Both DNA and RNA contain
a combination of four different nucleotides. As
shown in Figure 7.4, each nucleotide is composed

of a five-carbon sugar, a phosphate group, and one

of four nitrogen-containing (nitrogenous) bases.

The bases found in DNA nucleotides are adenine

(A), guanine(G), cytosine(C), and thymine(T). In

RNA, the base uracil(U) is found instead of thymine.

The only difference between the nucleotides in

each nucleic acid is in their bases. As a result,

scientists studying nucleic acids soon began to

identify the nucleotides simply by their bases or,

more commonly, by their initials: A, G, C, T, and U.

Figure 7.4The general structure of a nucleotide. In DNA,

the sugar is deoxyribose, and the nitrogenous base is one

of the following: adenine (A), guanine (G), cytosine (C), or

thymine (T). In RNA, the sugar is ribose and the nitrogenous

base uracil (U) appears instead of thymine.

While A, G, C, T, and U are the major bases

found in nucleic acids, there are also some minor

ones. These are usually slightly altered forms of the

major bases. In many cases the minor bases serve

as specific signals involved in programming or

protecting genetic information.

At this point, the results of Levene’s work led

him to conclude incorrectly that nucleic acids

contained equal amounts of each of these

nucleotides. Based on this finding, he suggested

that DNA and RNA were made up of long chains

in which the nucleotides appeared over and over

again in the same order; for example, ACTGACTG

ACTG and so on. This, in turn, caused most

scientists to conclude that DNA could not be the

material of heredity because it was not complex

enough to account for the tremendous variation in

inherited traits. It was generally accepted that DNA

could be a structural component of hereditary

material, but scientists thought the primary

instructions for inherited traits must lie in the

proteins that are also found in chromosomes.

P

S

O

C

4 ′

5 ′

3 ′^2 ′

1 ′

pentose sugar

nitrogen-

containing

base

phosphate

thymine

CH 3

O

H

O H

H

N

N

uracil

O

H

O H

HH

N

N

Figure 7.3

Phoebus Levene

made some

important

discoveries about

the properties of

nucleic acids.

3 ′

OH

H

2 ′

H

H

1 ′

OH

H

4 ′

5 ′ O

deoxyribose

DNA

HOCH 2

RNA

3 ′

OH

H

2 ′

OH

H

1 ′

OH

H

4 ′

5 ′ O

ribose

HOCH 2

A B