HUMAN BIOLOGY

DNa, GeNes, aND BioteChNoloGy 405

What is a DNa molecule?

• A DNA molecule consists of two strands of nucleotides held
  together at their bases by hydrogen bonds. The two strands
  run in opposite directions and twist into a double helix.


• DNA’s nucleotides are built of the sugar deoxyribose, a
  phosphate group, and one of the nitrogen-containing bases
  adenine (A), guanine (G), thymine (T), and cytosine (C).


• In a DNA molecule, two kinds of base pairs occur: A—T
  and G—C.

taKe-home messaGe

base pair Two bases (A—T

or G—C) held together by

hydrogen bonds.

gene A sequence of nucle-

otides in a DNA molecule.

nucleotide sequence The

order of nucleotides in a

gene; the sequence codes

for a specific polypeptide

chain.

The pattern of base

pairing (A with T,

and G with C) is

consistent with the

known composition

of DNA (A—T,

and G—C).

a gene is a sequence of nucleotides

As you already know, genes are the units of heredity.

Chemically, a gene is a sequence of nucleotides in a

DNA molecule. The nucleotide sequence of each gene

codes for a specific polypeptide chain. Polypeptide chains,

remember, are the basic structural

units of proteins. We’ll see how they

form later on. Figure 21.3 shows one

way researchers can visualize small

fragments of DNA.

We now turn to a key feature of

each DNA molecule—how it can

be copied, or replicated, so that the

information it contains is faithfully

passed on to new generations.

Figure 21.2 Animated! This diagram
shows how nucleotide bases are arranged
in the DNA double helix. Three different
models are combined here. Notice that
the two sugar–phosphate backbones run
in opposite directions. It may help to think
of the sugar units of one strand as being
upside down. By comparing the numerals
used to identify each carbon atom of the
deoxyribose molecule (1 9 , 2 9 , 3 9 , and so
on), you can see that the strands run in
opposing directions. (© Cengage Learning)

Figure 21.3 DNA fragments can be visualized through a

process called gel electrophoresis. Above: DNA obtained

from human cells. The extraction process has given the DNA

a “cotton candy” appearance.

NIH Custom Medical Stock Photo/Newscom

Patrick Landmann/Science Source

Human DNA

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