Organic Chemistry

I

n previous chapters, we studied

two of the three major kinds of

biopolymers—polysaccharides and

proteins. Now we will look at the

third—nucleic acids. There are two types

of nucleic acids—deoxyribonucleic acid (DNA)

and ribonucleic acid (RNA). DNA encodes an organism’s entire hereditary informa-

tion and controls the growth and division of cells. In most organisms, the genetic infor-

mation stored in DNA is transcribed into RNA. This information can then be translated

for the synthesis of all the proteins needed for cellular structure and function.

DNA was first isolated in 1869 from the nuclei of white blood cells. Because this

material was found in the nucleus and was acidic, it was called nucleic acid. Eventual-

ly, scientists found that the nuclei of all cells contain DNA, but it wasn’t until 1944

that they realized that nucleic acids are the carriers of genetic information. In 1953,

James Watson and Francis Crick described the three-dimensional structure of DNA—

the famed double helix.

27.1 Nucleosides and Nucleotides

Nucleic acidsare chains of five-membered-ring sugars linked by phosphate groups

(Figure 27.1). The anomeric carbon of each sugar is bonded to a nitrogen of a hetero-

cyclic compound in a linkage. (Recall from Section 22.10 that a -linkage

is one in which the substituents at C-1 and C-4 are on the same side of the furanose ring.)

Because the heterocyclic compounds are amines, they are commonly referred to as

bases. In RNA the five-membered-ring sugar is D-ribose. In DNA it is 2-deoxy-D-ribose

(D-ribose without an OH group in the 2-position).

Phosphoric acid links the sugars in both RNA and DNA. The acid has three

dissociable OH groups with values of 1.9, 6.7, and 12.4. Each of the OH groups

can react with an alcohol to form a phosphomonoester,a phosphodiester, or a

phosphotriester. In nucleic acids the phosphate group is a phosphodiester.

pKa

b-glycosidic b

an RNA catalyst

032

27

Nucleosides, Nucleotides,

and Nucleic Acids

Studies that determined the structures
of the nucleic acids and paved the
way for the discovery of the DNA
double helix were carried out by
Phoebus Leveneand elaborated by
Sir Alexander Todd.

Phoebus Aaron Theodor Levene
(1869–1940)was born in Russia.
When he immigrated to the United
States with his family in 1891, his
Russian name Fishel was changed to
Phoebus. Because his medical school
education had been interrupted, he
returned to Russia to complete his
studies. When he returned to the
United States, he took chemistry
courses at Columbia University.
Deciding to forgo medicine for a
career in chemistry, he went to
Germany to study under Emil
Fischer. He was a professor of
chemistry at the Rockefeller Institute
(now Rockefeller University).