Explain how a range of mutagens operate, including but not limited to: electromagnetic radiation sources, chemicals, naturally occurring mutagens

What this dot point is asking

NESA wants you to explain how different mutagens damage DNA at the molecular level. Mutagens are grouped into physical (radiation), chemical and biological agents, and you should be able to give a named example and a mechanism for each.

The answer

A mutagen is any agent that increases the rate of mutation above the spontaneous background. Mutagens are usually classified into three groups.

Physical mutagens (electromagnetic radiation)

These deliver energy that physically damages DNA.

Ultraviolet (UV) radiation. Non-ionising, short-wavelength light in the UV-B and UV-C bands. UV photons are absorbed by adjacent pyrimidine bases (especially thymine) on the same strand, causing them to covalently bond as a thymine dimer (a pyrimidine dimer). The dimer distorts the double helix. If nucleotide excision repair does not remove it before replication, DNA polymerase misreads the template and a mutation is fixed. UV exposure is the primary cause of basal cell carcinoma, squamous cell carcinoma and melanoma.

Ionising radiation (X-rays, gamma rays). Short-wavelength, high-energy electromagnetic radiation. Ionises water in the cell to produce hydroxyl radicals and other reactive species, which break the sugar-phosphate backbone. Causes single-strand and double-strand breaks that are difficult to repair accurately and often produce deletions, translocations and aneuploidy. Linked to leukaemia, thyroid cancer and germline mutations in irradiated populations (e.g. Hiroshima, Chernobyl).

Chemical mutagens

These react directly with DNA or its building blocks.

Base analogues

Molecules structurally similar to normal bases that are incorporated during replication and mis-pair. Example: 5-bromouracil resembles thymine but pairs with guanine, producing T to C transitions.

Alkylating agents

Add alkyl (methyl or ethyl) groups to bases. Methylated guanine mis-pairs with thymine instead of cytosine, fixing a G to A transition. Examples: mustard gas (used in chemical warfare, the first chemical mutagen identified, by Charlotte Auerbach), ethylmethanesulfonate (EMS), and many alkylating chemotherapy drugs.

Intercalating agents

Flat, planar molecules that wedge between adjacent base pairs, distorting the helix. During replication, DNA polymerase often inserts or deletes a base opposite the intercalator, causing a frameshift mutation. Examples: acridine orange, ethidium bromide, and aflatoxin B1 from Aspergillus moulds (a potent natural carcinogen linked to liver cancer).

Deaminating agents

Remove an amino group from a base. Example: nitrous acid converts cytosine to uracil; after replication this fixes a C to T transition.

Biological mutagens (naturally occurring)

These are living agents or biological molecules that cause mutations.

Viruses

Some viruses insert their DNA (or a reverse-transcribed DNA copy of their RNA) into the host genome. The insertion can disrupt a host gene or activate a nearby proto-oncogene. Example: human papillomavirus (HPV) integrates near tumour suppressor genes and causes cervical cancer. Hepatitis B virus integration is linked to liver cancer.

Transposons ("jumping genes")

DNA sequences that move within the genome, sometimes inserting into and disrupting other genes. They were discovered by Barbara McClintock in maize. Transposons are responsible for many spontaneous mutations in eukaryotes.

Reactive oxygen species (ROS)

Generated as by-products of normal aerobic metabolism. Oxidise guanine to 8-oxo-guanine, which mis-pairs with adenine and fixes a G to T transversion. These are responsible for much of the spontaneous mutation rate.

Summary table

Examples in context

Example 1. UVB radiation and Australian melanoma rates. Australia has the highest melanoma incidence in the world, with the Cancer Council reporting roughly 17 000 new cases per year. The molecular cause is UVB radiation (280-320 nm) penetrating skin keratinocytes and forming thymine dimers in DNA, where two adjacent T bases on the same strand bond covalently. If the dimer escapes nucleotide excision repair, DNA polymerase stalls at the lesion and may insert an incorrect base. Frequent UVB-induced mutations in the BRAF gene (V600E) drive uncontrolled melanocyte proliferation. NSW Health's SunSmart campaign targets exactly this mechanism by reducing total UVB exposure during peak hours (10 am to 3 pm) when Sydney's UV index regularly exceeds 11.

Example 2. Tobacco smoke and the p53 gene. Tobacco smoke contains benzo(a)pyrene, a chemical mutagen that forms bulky DNA adducts at guanine bases in lung epithelial cells. When DNA polymerase encounters these adducts, it preferentially mispairs the affected guanine with adenine instead of cytosine, producing a G to T transversion. NSW Cancer Institute data show that more than 70 percent of lung tumours from smokers carry G to T transversions in the TP53 tumour suppressor gene, compared with under 15 percent in non-smokers. The chemical fingerprint of the mutagen is literally written into the cancer genome, which is how researchers initially linked smoking causally to lung cancer at the molecular level.

Try this

Q1. Distinguish between a chemical mutagen and a physical mutagen, providing one example of each. [3 marks]

• Cue. Physical: ionising radiation such as gamma rays causing double-strand DNA breaks. Chemical: alkylating agents such as mustard gas adding methyl groups to bases.

Q2. A Sydney laboratory exposes cultured human fibroblasts to UVB for 0, 5, 10 and 20 minutes and counts thymine dimers per million bases as 2, 38, 75 and 144 respectively. Describe the relationship and predict the dimer count at 15 minutes. [3 marks]

• Cue. Approximately linear at about 7 dimers per minute; predicted value at 15 minutes is around 110.

Q3. A bushwalker is exposed to elevated background radiation from radon at a remote site over a weekend. (a) Identify the type of mutagen radon emits. (b) Describe the molecular damage it causes. (c) Explain why the resulting mutation rate is much higher in rapidly dividing tissues such as bone marrow than in post-mitotic neurons. [1+2+2 marks]

• Cue. (a) Alpha-particle ionising radiation. (b) Single- and double-strand DNA breaks. (c) Dividing cells must replicate damaged DNA, fixing the lesions as mutations; neurons rarely replicate so most damage is repaired in place.