HUMAN BIOLOGY

DNa, GeNes, aND BioteChNoloGy 407

hoW is DNa replicateD?

• In DNA replication, enzymes and other proteins unwind the
  double helix, assemble a new, complementary strand on it,
  then seal the strands together.


• DNA replication is said to be semiconservative because each
  newly formed molecule contains one of the parent strands.


• Enzymes active in DNA replication also may repair damage
  in DNA.


• A gene mutation is a change in one or more bases in the
  nucleotide sequence of DNA.


• A mutation may be harmful, neutral, or beneficial depending
  on how it affects body structures and functions.

taKe-home messaGe

Figure 21.5 Animated! There are

several types of gene mutations.

A Part of a gene (blue), the mRNA

(brown), and the specified amino acid

sequence. B A base-pair substitution.

C A deletion in DNA. (© Cengage Learning)

reonine Proline Glutamate Lysine

A

T

C

G

U

A

C

G

C

G

U

A

G

C

G

C

G

C

G

C

G

C

A

T

A

T

A

T

A

T

Valine

A

T

C

G

U

A

C

G

C

G

U

A

U

AA

G

C

G

C

G

C

G

C

G

C

A

T

A

T

A

T

A

T

C

G

U

A

C

G

C

G

U

A

G

C

G

C

G

C

G

C

G

C

A

T

A

T

A

T

part of DNA

mRNA transcribed

from DNA

resulting amino

acid sequence

base substitution

in DNA

altered mRNA

altered amino

acid sequence

deletion

in DNA

altered mRNA

altered amino

acid sequence

Glutamate

reonine Proline Glutamate

reonine Proline Glycine Arginine

Lysine

A

B

C

occur in the DNA at any time in any cell, they are inherited

only when they take place in germ cells that give rise to gam-

etes. Whether a mutation turns out to be harmful, neutral, or

beneficial depends on a variety of factors, including how the

resulting protein affects body functions.

One common kind of gene mutation is base-pair
substitution, in which the wrong nucleotide is paired with
an exposed base while DNA is being replicated (Figure
21.5B). Proofreading enzymes may fix the error. But if they
don’t, a mutation will be established in the DNA in the next
round of replication. As a result of this mutation, one amino
acid might replace another during protein synthesis. This is
what has happened in people who have sickle-cell anemia
(Section 19.4). In another common type of mutation, a dele-
tion, a base is lost (Figure 21.5C). This loss is smaller than
the deletion of part of a chromosome described in Section
20.8, but both may have major impacts on body functioning.
In an expansion mutation, a nucleotide sequence is repeated
over and over, sometimes many hundreds of times. Expan-
sion mutations cause a variety of genetic disorders, includ-
ing Huntington’s disease, spinal muscular atrophy (Fig-
ure 21.6A), and fragile X syndrome (Figure 21.6B). In this
disorder, a sequence of three nucleotides is repeated in an
X chromosome gene. The result is an abnormal protein
needed for proper brain development. While mutations can

A

Figure 21.6 Gene mutations cause numerous disorders.

A This man was born with a form of spinal muscular atrophy

(SMA), a degenerative disorder of the nervous system caused

by an expansion mutation. The progressive loss of motor

neurons results in muscle wasting throughout the body and is

eventually fatal. B An X chromosome showing the constriction

that occurs with fragile X syndrome (arrow). Boys tend to be

more seriously affected because, unlike girls, they have only

one X, so there is no “backup” X with a normal gene to code

for a normal version of the affected protein.

C. J. Harrison

B

Lea Paterson/Science Source

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).