Quick questions on Causes of mutation: physical, chemical and biological mutagens: HSC Biology Module 6

These deliver energy that physically damages DNA.

These react directly with DNA or its building blocks.

These are living agents or biological molecules that cause mutations.

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.

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.

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.

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.

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).

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

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.

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.

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.

Two adjacent thymines absorbed UV photons and formed a thymine dimer. The dimer distorted the helix; during replication DNA polymerase inserted a wrong base opposite one of the thymines, fixing the mutation. Loss of TP53 function removes a key cell-cycle checkpoint, contributing to skin cancer.

Intercalators wedge between bases and cause frameshifts; base analogues are incorporated as bases and cause substitutions.

Many students list only chemicals and radiation. Include viruses, transposons or reactive oxygen species to score the full range mark.