Exercises: Mutations

Explain what a mutation is and when mutations usually occur (errors in DNA copying or the action of external agents).

Distinguish between a mutation that occurs in a somatic cell (for example, a skin cell) and one that occurs in a germ cell: which of them can be passed on to offspring?

Name three mutagenic agents, physical or chemical, and briefly explain how they act on DNA.

Most mutations have no consequences. Explain two reasons why many mutations go unnoticed (DNA repair systems and features of the genetic code).

Explain the relationship between the accumulation of mutations in genes controlling the cell cycle and the development of cancer.

Mutations are said to be "the raw material for evolution". Reason why, without mutations, there would be no new genetic variation for natural selection to act upon.

A patient who has had a melanoma successfully removed from their back raises two concerns during a medical appointment:

1. "I am worried that I have passed this mutation on to my newborn child."

2. "Since they have already removed my tumor and my skin is tanned and weathered, I am now immune and no longer need to use sunscreen."

Refute both statements with well-founded arguments:

1. a) Justify why their child will not inherit this melanoma, distinguishing between mutations in somatic cells and in germ cells.
2. b) Explain why it is biologically false that skin becomes "immune" to cancer, detailing the relationship between cumulative ultraviolet radiation, mutations in proto-oncogenes or tumor suppressor genes, and the loss of cell cycle control.

Look at the diagram illustrating the genetic and cellular change in sickle cell anemia. Imagine that during genetic screening, two individuals are identified with mutations in that same codon (GAA):

• Person A: undergoes a silent point mutation where GAA changes to GAG (which also codes for glutamic acid).

• Person B: undergoes the deletion of the first base (the G is lost), altering the reading frame.

1. a) Reasonably predict the phenotype of person A at the cellular level, explaining which feature of the genetic code prevents them from developing sickle cell anemia.
2. b) Compare the expected cellular and functional impact on person B with the case of sickle cell anemia shown in the diagram. Justify why a single-nucleotide deletion is typically biologically more destructive to the protein than the point substitution shown.

A patient with bacterial tonsillitis stops their antibiotic treatment on the third day of the seven prescribed because they no longer have a fever or discomfort. Their reasoning is: "If I keep taking the antibiotic, the bacteria will learn to fight it and mutate to become resistant; it's better to stop now so as not to force them to mutate."

1. a) Identify the conceptual error in their reasoning and refute their statement, clearly distinguishing between the random, pre-existing nature of mutations and the selective role of the antibiotic.
2. b) Predict what will happen to the residual bacterial population in their throat if they discontinue the treatment halfway through, and reason why this misuse contributes to the proliferation of multidrug-resistant strains.