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.• 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.

DNA: the informational molecule of life

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. – 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.

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.

Deletion of a nucleotide

Replacement of a nucleotide

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.

Messenger RNA

Consequences of damaged DNA

Mutations• Mutations are acquired at some time during a

person’s life, or they can be inherited from a parent.

• Mutations 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, without 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.

Apoptosis

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.

• 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.

Apoptosis

Tests for DNA Damage and Mutagenicity

• A number of toxicological tests can evaluate the effects of a chemical agent as being deleterious to DNA: – 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.

The Ames Test

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 that have been 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.