HSC Biology practice exams with worked answers

What Is a Mutation?

A mutation is any heritable change in the nucleotide sequence of DNA or in the structure/number of chromosomes. Mutations occur at two scales:

• Gene (point) mutations — changes to one or a few nucleotide bases within a single gene. Subtypes include substitution (one base swapped for another), insertion (extra base/s added) and deletion (base/s removed).
• Chromosomal mutations — large-scale alterations: deletion, duplication, inversion or translocation of chromosome segments, or changes in whole-chromosome number (aneuploidy, e.g. trisomy) or whole genome number (polyploidy).

It is essential to distinguish a somatic mutation (occurring in body cells — not passed to offspring, but can lead to cancer) from a germline mutation (occurring in gametes or cells that will form gametes — heritable and the basis of inherited disease and evolution).

Mutations arise either spontaneously (errors during DNA replication, roughly 1 in 10⁹ base pairs per replication cycle before repair) or are induced by external agents called mutagens.

Mutagens: Physical, Chemical and Biological

A mutagen is any agent that increases the rate of mutation above the spontaneous baseline. The NSW HSC syllabus requires you to classify mutagens into three categories.

Worked example — UV and thymine dimers: A skin cell's DNA is exposed to UV-B radiation. UV energy causes two adjacent thymine (T) nucleotides on the same strand to form a covalent bond, creating a T–T dimer. During the next round of replication the polymerase cannot read the dimer accurately and may insert incorrect bases opposite it. If the cell's nucleotide excision repair (NER) system fails to remove the dimer first, a point mutation is incorporated into one daughter cell. This is the molecular explanation for why chronic UV exposure causes most non-melanoma skin cancers (squamous and basal cell carcinomas).