HUMAN BIOLOGY

36 Chapter 2

What is a nucleic acid?

• A nucleic acid is a single or double-stranded molecule built
  of nucleotides.


• Nucleic acids include DNA and RNAs.

taKe-home message

ATP Adenosine triphos-
phate, a nucleotide that has
phosphate groups attached
and that serves as an
energy carrier in cells.

coenzyme Enzyme “helper”
molecule that moves hydro-
gen atoms and electrons to
the sites of chemical reac-
tions in cells.

DNA Deoxyribonucleic acid,
which contains the sugar
deoxyribose; the genetic
material.

nucleic acid Single- or
double-stranded molecule
built of nucleotides. DNA is
a double-stranded nucleic
acid.

nucleotide Molecule
built of a sugar (deoxyri-
bose or ribose), a nitrogen-
containing base, and one or
more phosphate groups.

RNA Any of several ribonu-
cleic acids, all of which con-
tain the sugar ribose; RNAs
help build cell proteins.

nucleotides and nucleic acids

n    The fourth and final class of biological molecules consists of

nucleotides and nucleic acids.

n Link to Life’s characteristics 1.1

nucleotides are energy

carriers and have

other roles

A nucleotide (noo-klee-oh-tide) is

composed of one sugar, at least one

phosphate group, and one nitrogen-

containing base. The sugar—ribose

or deoxyribose—has a five-carbon

ring structure. Ribose has two oxy-

gen atoms attached to the ring, and

deoxyribose has one. The bases

have a single or double carbon

ring structure.

The nucleotide ATP (for adenos-

ine triphosphate) has a row of three

phosphate groups attached to its

sugar (Figure 2.29). In cells, ATP

links chemical reactions that release

energy with other reactions that

require energy. This connection is

possible because ATP can transfer

a phosphate group to many other

molecules in the cell, providing the

acceptor molecules with the energy

they need to enter into a reaction.

Some nucleotides are part of coenzymes, or “enzyme

helpers.” They move hydrogen atoms and electrons from

one reaction site to another. Some other nucleo tides act

as chemical messengers inside and between cells. One of

these is a nucleotide called cAMP (for cyclic adenosine

monophosphate). It is extremely important in the action of

some hormones.

nucleic acids include Dna and the rnas

Nucleotides are building blocks for single- or double-

stranded molecules called nucleic acids. In a strand’s

backbone, covalent bonds join each nucleotide’s sugar

to a phosphate group of the neighboring nucleotide

(Figure 2.30A). In this book you will read often about the

nucleic acid DNA (deoxy ribonucleic acid), which contains

the sugar deoxyribose. DNA consists of two strands of

nucleotides, twisted together in a double helix (Figure

2.30B). Hydrogen bonds between the nucleotide bases

hold the strands together, and the sequence of bases

encodes genetic information. Unlike DNA, RNA (short for

ribonucleic acid) is usually a single strand of nucleo tides.

There are several kinds of RNA, but all have the sugar

ribose. RNAs have crucial roles in processes that use

genetic information to build proteins in cells.

Figure 2.29 ATP is the energy-carrying nucleotide in cells.

O–

• O P

OH OH

H

O

O

CH 2

NH 2

CH

O–

P

O

O

O–

PO

O

H

O

HH

C

C

C

HC

N

N N

N

sugar (red)

three phosphate groups

oxygen phosphorus

base (blue)

Figure 2.30 Chains of nucleotides form nucleic acids.

A Bonds between the bases in nucleotides. B Model of DNA,

a nucleic acid with two strands of nucleotides joined by

hydrogen bonds and twisted into a double helix.

2.13

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A B

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