Biology 12

Chapter 9 DNA Mutations and Genetic Engineering • MHR 289

the development of new species of organisms. As
Figure 9.5 indicates, stability and the variability
that can result from mutations are both important
features of the material of heredity

Causes of Mutations

Many mutations arise as a result of the molecular
interactions that take place naturally within the
cell. These mutations are known as spontaneous
mutations. One source of spontaneous mutations
is incorrect base pairing by DNA polymerase
during the process of DNA replication. The rate
of spontaneous mutations varies among organisms
and even among different genes within a single cell.
While every cell undergoes spontaneous
mutations, exposure to certain factors in the
environment can increase the rate of mutation.
Mutations that are caused by agents outside the cell
are said to be induced. A substance or event that
increases the rate of mutation in an organism is
called a mutagen. Mutagens fall into two general
categories, as described below.

Physical Mutagens

Over a period of more than 20 years in the early
1900s, American researcher Thomas Morgan
observed about 400 visible mutations in the tens
of millions of fruit flies with which he worked. In
1926, one of Morgan’s students (American Herman
Muller) bombarded a population of fruit flies with
X rays and produced several hundred mutants in a
single day. Muller went on to study the mutagenic
properties of X-ray radiation, and was awarded the
Nobel prize in 1946.

High-energy radiation, like that from X rays and

gamma rays, is known as a physical mutagen

because it literally tears through a DNA strand,

causing random changes in nucleotide sequences.

It may break one or both strands of the DNA

molecule, causing mutations ranging from the

deletion of just a few nucleotides to the loss of large

portions of chromosomes. High-energy radiation is

the most damaging form of mutagen known.

Ultraviolet (UV) radiation, which is present in

ordinary sunshine, has a lower energy level range

than X rays but is still a powerful mutagen. It is

most likely to affect the pyrimidine bases (C and T)

within the DNA molecule. Where two pyrimidines

are adjacent to each other, UV radiation can cause

a chemical change in the bases that bonds them

covalently to form a larger molecule called a dimer.

Figure 9.6 shows how the resulting dimer distorts

the DNA molecule. The distortion then interferes

with DNA replication. UV radiation damage as a

result of exposure to the Sun is one of the most

common causes of cancer (specifically melanoma,

a form of skin cancer) among light-skinned people.

The skin pigment melanin helps to absorb UV

radiation and offers some protection to the DNA

within skin cells. For this reason, people with dark

skin have a lower risk of developing cancer as a

result of exposure.

Chemical Mutagens

A molecule that can enter the cell nucleus and

induce mutations is called a chemical mutagen.

These mutagens react chemically with the DNA

molecule. Some chemicals, known as base analogues,

have molecular structures that are similar to those

For light-skinned people, a single sunburn can double their

risk of developing skin cancer.

BIO FACT

In mammals, spontaneous mutations are more than twice

as likely to arise in the DNA of males than in females.

BIO FACT

ultraviolet

light

thymine

dimer

kink

T

T A

A Figure 9.6Exposure to UV

radiation can cause a new

bond to form between adjacent

thymine bases. In turn, the

resulting dimer distorts the

DNA double helix and

interferes with replication.