HUMAN BIOLOGY

406 Chapter 21

Some genetic defects result in “inborn

errors of metabolism.” These disorders

arise when an enzyme vital to some

aspect of metabolism is faulty. A fairly

common example is alkaptonuria. In

people with this condition, the urine

turns black when it is exposed to the air.

Use Web or library resources to learn

more about inborn errors of metabolism.

Why does alkaptonuria make urine turn

black? What are some other inborn

errors of metabolism?

thiNk outsiDe the book

base-pair substitution

Type of gene mutation in

which the wrong nucleotide

is paired with a base during

DNA replication.

DNA polymerases

Enzymes that function in

DNA replication and repair.

DNA repair The repair of

DNA that has been mis-

copied during replication.

DNA replication The copy-

ing of DNA in a cell before

it divides.

gene mutation Change in a

gene’s nucleotide sequence.

semiconservative

replication Mechanism of

DNA replication, in which

one strand of an existing

DNA molecule is the tem-

plate for the formation of

a new second strand.

As replication occurs, the newly formed double-stranded

molecule twists back into a double helix. Because one

strand is from the starting molecule, that strand is said

to be conserved. Only the second strand has been freshly

assembled, so each DNA molecule is really half new and

half “old.” The DNA replication mechanism is thus called

semiconservative replication (Figure 21.4).

Every time a cell replicates its DNA, at least three billion

nucleotides must be assembled properly. DNA is copied

rapidly—between ten and twenty nucleotides per second

are added at a replication site. It’s no wonder, then, that

mistakes happen. By some estimates a human cell must fix

breaks in a strand of its DNA up to two million times every

hour! Fixing broken or otherwise damaged DNA is called

DNA repair. It is another task carried out by DNA polymer-

ases and other enzymes. Certain genetic disorders, includ-

ing the premature aging disorders progeria and Werner’s

syndrome, result when DNA repair is not effective.

a mutation is a change in the sequence

of a gene’s nucleotides

DNA can be damaged by various chemicals, ionizing radia-

tion, and ultraviolet light (as in sunlight or the rays of tanning

lamps). Every so often, DNA is altered in a way that changes a

gene. Sometimes one base gets substituted for another in the

nucleotide sequence. At other times, an extra base is inserted

into the sequence or a base is deleted from it.

Such a small-scale change in the nucleo tide

sequence of a gene is a gene mutation.

Section 2.13 introduced ribonucleic acid,

or RNA. When a cell makes a protein, the

DNA instructions for the order of its amino

acids are first rewritten, or transcribed, into

a form of RNA called mRNA (Figure 21.5A).

Section 21.3 explains this rewriting process.

For now, the point to remember is that when

a gene is mutated, the change alters mRNA

instructions for building the protein the nor-

mal gene encodes.

n each DNA molecule must be faithfully copied so parents

may pass on their traits to offspring.

n Links to aging 17.13, Cell cycle 18.2, Genes on

chromosomes 20.1

how is a Dna molecule

copied?

Chapter 19 noted that DNA must

be copied before a cell divides. This

process is called DNA replication.

During replication, enzymes called

DNA polymerases and other pro-

teins unwind the DNA molecule,

keep the two unwound strands sep-

arated, and assemble and seal a new

strand on each one. The unwinding

exposes stretches of the molecule’s

nucleotide bases. Cells contain unat-

tached nucleotides that can pair

with the exposed bases. A pairs

with T and G pairs with C; they are

linked by hydrogen bonds. Each

parent strand stays intact while a

new strand is assembled on it, nucle-

otide by nucleotide.

A

T

C

G

T

A

G

C

A

T

C

G

T

A

G

C

A

T

C

G

T

A

G

C

A

T

C

G

T

A

G

C

A

T

C

G

T

A

G

C

A

T

C

T

A

G

G C

C Each “old” strand

serves as a structural

pattern (a template)

for the addition of

bases according to

the base-pairing rule.

B Replication begins;

the two strands

unwind and separate

from each other at

specific sites along

the length of the DNA

molecule.

D Bases positioned

on each old strand are

joined together into a

“new” strand. Each

half-old, half-new DNA

molecule is just like

the parent molecule.

A Parent DNA molecule;

two complementary

strands of base-paired

nucleotides.

Figure 21.4 Animated!
One strand stays the
same when DNA is
replicated. Here the
original two-stranded
DNA molecule is shown
in blue. A new strand
(yellow) is assembled
on each parent strand.
(© Cengage Learning)

21.2 passing on Genetic instructions

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