Chapter 10
Chemical-Induced
Mutagenesis
DNA and Mutations
• A mutation is a permanent change in the DNA.
• DNA is in our chromosomes and it codes for
all the information that makes us unique.
• It codes for all cellular proteins (e.g., enzymes)
that are critical to life.
DNA and Mutations
• Human DNA is packaged in 23 pairs of
chromosomes: 22 homologous pairs and 2 sex
chromosomes (XX in females and XY in males).
– Each chromosome is composed of a DNA
molecule that is complexed with numerous
proteins.
• DNA must be able to replicate, segregate, and
maintain its integrity from replication to
replication.
Figure 10-1 DNA: the informational
molecule of life
Courtesy of Oak Ridge National Laboratory, U.S. Dept. of Energy.
Structure of DNA
• DNA is organized into genes, of which 30,000
or so encode information that is critical to
maintain human life.
• The DNA is a base sequence that codes the
information necessary for cellular growth,
differentiation, and replication.
Structure of DNA
• DNA is composed of the following bases:
– Purines: adenine (A), guanine (G)
– Pyrimidines: cytosine (C), thymine (T)
• Bases are organized into the DNA nucleotide,
which contains a purine or pyrimidine base +
sugar (deoxyribose) + a phosphate group.
Figure 10-2
Molecular
components and
their arrangement
in DNA.
DNA and Mutations
• The pairing of bases is A-T and C-G.
• Incorrect base pairing can result in the potential
alteration of information that may be important to
the normal physiology of the individual.
• The deletion or replacement of a single nucleotide
can result, for example, in an altered protein, such as
an enzyme.
• If these changes occur in an informational portion of
the DNA, then an abnormal protein may result.
Did You Know?
Experimentally, choosing the right mutagen means
selecting the right combination of mutagen efficiency
and mutagen specificity. Ethane methyl sulfonate (EMS)
for example is a chemical mutagen that most commonly
causes transitions by methylation of G residues but can
also yield a variety of other types of mutations and has
become an important experimental tool for routine
genetic analysis. Other studies that need to produce
large multigene deletions, gamma irradiation, UV
irradiation, and formaldehyde are commonly used
mutagens.
Figure 10-3 Deletion of a nucleotide
Modified from Genetics Home Reference. A Service of the National Library of Medicine. (2010). Deletion Mutation.
http://ghr.nlm.nih.gov/handbook/illustrations/mutationtypes?show=missense. Accessed January 24, 2013.
Figure 10-3 Replacement of a
nucleotide
Modified from Genetics Home Reference. A Service of the National Library of Medicine. (2010). Deletion Mutation.
http://ghr.nlm.nih.gov/handbook/illustrations/mutationtypes?show=missense. Accessed January 24, 2013.
DNA and Mutations
• Damage to DNA occurs both spontaneously
and as a result of exposures to environmental
agents.
• We have the capacity to both recognize and
repair mutated DNA.
• This may well be the only biological
macromolecule that can be repaired.
Figure 10-5 Messenger RNA is the cytoplasmic
informational molecule transcribed from DNA
Adapted from
National Institute of
General Medical
Sciences. (2010).
Central Dogma
Illustrated. Available
at:
http://images.nigms.ni
h.gov/imageRepositor
y/2547/Translation.
jpg. Accessed
January 24, 2013.
Figure 10-6 Consequences of
damaged DNA
Mutations
• Mutations are acquired at some time during a
person’s life, or they can be inherited. They can be:
– Induced as a result of exposure of the DNA to
environmental mutagens
– Spontaneous as a result of “normal” cellular
processes
– Acquired (= somatic) some time during the life of
an individual. Unless these occur in the gametes,
they cannot be passed onto offspring.
– Hereditary (= germline) acquired from a parent
through the union of the gametes at fertilization
and can be present in all the cells of the offspring.
Mutations and Apoptosis
• Apoptosis is a form of planned or programmed cell
death. It is a normal process that occurs during:
– embryological development
– normal cellular replacement
– in response to physical, biological, or chemical
stressors
• Normal cell turnover occurs, w/o necrosis and
inflammation.
• Apoptosis is also an important way that genetically
altered cells can be removed from the body if DNA
repair does not occur.
Figure 10-7
Apoptosis
Reproduced from Genetics Home
Reference. A Service of the National
Library of Medicine. (2010). The
Process of Apoptosis. Available at:
http://ghr.nlm.nih.gov/handbook/illust
rations/apoptosisprocess. Accessed
December 20, 2012.
Apoptosis
• Unlike necrosis, in which large numbers of
cells have died and which is frequently
accompanied by inflammation and cellular
replacement by connective tissue, the
programmed cell death of apoptosis is a
selective destruction of the cell.
• As the cell dies, one can observe “apoptotic
bodies” that in the final stage of apoptosis are
digested by phagocytic cells.
Apoptosis
• The triggering of apoptosis is complex and
involves the cell receiving chemical
messengers to “turn on” those genes involved
in the self-destruction process.
• If these genes become mutated and apoptosis
is compromised, then the cell is at greater risk
of becoming one that may transform into a
cancerous cell.
Figure 10-8 Final stage of apoptosis
Reproduced from Genetics Home Reference. A Service of the National Library of Medicine. (2010). Final Stage of Apoptosis.
Available at: http://ghr.nlm.nih.gov/handbook/illustrations/apoptosismacrophage. Accessed January 12, 2013.
Tests for DNA Damage and
Mutagenicity
•
•
•
•
•
•
The Ames test
Tests for chromosome aberrations
Micronuclei assays
Sister chromatid
Exchanges in populations of proliferating cells
DNA repair studies (unscheduled DNA
synthesis assay)
• And others that detect “changes” in the DNA.
Figure 10-9 The Ames mutagenicity test
Table 10-1 Not All Carcinogens Are Mutagens
Examples of Chemical Mutagens
• Some chemical mutagens directly react to
disrupt the base pairing within the DNA
macromolecule.
– Nitrous acid can deaminate the pyrimidine and
purine bases.
– This can convert adenine to hypoxanthine.
– Hypoxanthine now pairs with cytosine.
Examples of Chemical Mutagens, cont.
• The scientific literature contains numerous examples
of chemical agents shown to be mutagenic.
– Some of these chemicals, such as bromouracil,
resemble the purine and pyrimidines bases of DNA.
– Bromouracil was synthetically created and has
been used extensively in research because it
resembles the pyrimidine thymine but has a
bromium atom instead of a methyl group.
– Bromouracil is an example of a DNA base analogue.
Examples of Chemical Mutagens, cont.
• DNA base analogues can be incorporated into
the DNA and pairs with adenine during DNA
replication.
• DNA analogues may be of some value to the
chemotherapy because the rapidly dividing
cancerous cells may preferentially incorporate
them into the DNA resulting in cellular death.
– they are not specific for cancerous cells so other
dividing cells may be affected
Examples of Chemical Mutagens, cont.
• Intercalating agents tend to wedge in between the
bases along the DNA molecule, thus preventing the
functions of DNA polymerase and other normally
present chemicals (e.g., binding proteins).
– This results in the inability of the DNA to properly
transcribe information or prevent DNA synthesis.
– Examples include the dyes ethidium bromide and
acridine orange
– These chemicals can produce base frameshifts by
molecular stretching of the DNA double helix
– This may trick the DNA polymerase into placing an
additional base into the DNA.
Examples of Chemical Mutagens, cont.
• Other chemicals such as methyl or ethyl
methanesulfonate, mustard gas, and
nitrosoguanidine, can add methyl or ethyl
groups onto the bases.
– These types of chemicals are referred to as
alkylating agents.
– They can cause the replacement of a pyrimidine
base with a purine base.
Examples of Chemical Mutagens, cont.
• Some large molecules, such as benzo[a]pyrene
and N-acetoxy-2-cetylaminofluorene, can bind
to the DNA bases
– This can cause chemical adducts formation.
– The adducts may form noncoding regions within
the affected DNA,
– Peroxides (and other chemicals) may actually
cause breaks within the DNA strand.