Question 1

Which of the following best describes the difference between oncogenes and tumor-suppressor genes in the context of cancer development?

Accepted Answer

Oncogenes are mutated forms of proto-oncogenes with dominant, gain-of-function mutations that promote cell division, whereas tumor-suppressor genes are recessive, loss-of-function mutations that normally inhibit cell division.

Question 2

A patient is diagnosed with chronic myelogenous leukemia (CML). Cytogenetic analysis reveals a shortened chromosome 22, known as the Philadelphia chromosome. Which genetic event is responsible for this abnormality, and how does it contribute to cancer?

Accepted Answer

A chromosomal translocation fuses the bcr and abl genes, creating a BCR-ABL fusion protein with constitutive kinase activity that drives cell proliferation.

Question 3

Which of the following mechanisms can convert a proto-oncogene into an oncogene? Select the option that lists all correct mechanisms.

Accepted Answer

Missense mutation, gene amplification, chromosomal translocation, and viral integration.

Question 4

A missense mutation in the H-ras gene changes the 12th amino acid from glycine to valine. What is the most likely consequence of this mutation at the protein and cellular level?

Accepted Answer

The Ras protein will have decreased GTPase activity, remaining in the active GTP-bound state and continuously signaling for cell division.

Question 5

According to Knudson's two-hit hypothesis for retinoblastoma, which of the following best explains why inherited retinoblastoma typically presents earlier and more frequently than the sporadic form?

Accepted Answer

Individuals with inherited retinoblastoma inherit one defective copy of the rb gene, so only one additional somatic mutation is needed for tumor development.

Question 6

The p53 protein is often called the 'guardian of the genome.' Which of the following is NOT a function of p53 in response to DNA damage?

Accepted Answer

Directly phosphorylating cyclin-dependent kinases to advance the cell cycle.

Question 7

A patient with xeroderma pigmentosum is highly susceptible to skin cancer due to a defect in which cellular process?

Accepted Answer

Nucleotide excision repair, leading to accumulation of DNA mutations.

Question 8

Which of the following best describes the molecular function of the Rb protein in regulating the cell cycle?

Accepted Answer

Rb binds to E2F, inhibiting its activity; phosphorylation of Rb releases E2F, allowing transcription of genes required for cell cycle progression.

Question 9

A woman inherits one mutated allele of the BRCA-1 gene. According to the concept of loss of heterozygosity (LOH), what must occur for her to develop breast cancer related to BRCA-1?

Accepted Answer

A somatic mutation or chromosomal loss must inactivate the remaining normal BRCA-1 allele in a breast cell.

Question 10

Which of the following epigenetic changes is most likely to silence a tumor-suppressor gene and promote cancer?

Accepted Answer

Hypermethylation of CpG islands in the promoter region of the tumor-suppressor gene.

Question 11

A new chemotherapy drug is found to inhibit topoisomerase I. Based on its mechanism, which phase of the cell cycle is most directly affected, and why?

Accepted Answer

S phase, because topoisomerase I is essential for relieving DNA supercoiling during DNA replication.

Question 12

Which of the following best explains how the drug imatinib (Gleevec) treats chronic myelogenous leukemia (CML)?

Accepted Answer

It binds to the BCR-ABL fusion protein, blocking ATP binding and preventing phosphorylation of downstream signaling proteins required for cell division.

Question 13

A cancer cell is found to have a mutation in the gene coding for a histone acetyltransferase. Which of the following is a plausible consequence of this mutation?

Accepted Answer

Decreased acetylation of histones, potentially resulting in silencing of tumor-suppressor genes and promotion of cancer.

Question 14

A patient is treated with tamoxifen for breast cancer. What is the molecular mechanism by which tamoxifen inhibits tumor growth?

Accepted Answer

Tamoxifen binds to the estrogen receptor, preventing estrogen from activating gene transcription required for cell division.

Question 15

Which of the following environmental agents is associated with both cancer risk and epigenetic changes, and is commonly found in tobacco smoke and automobile exhaust?

Accepted Answer

Polycyclic aromatic hydrocarbons

Question 16

A researcher observes that a tumor cell line has lost a chromosome that carries a tumor-suppressor gene. Which genetic mechanism of tumor-suppressor gene inactivation does this represent?

Accepted Answer

Aneuploidy

Question 17

Which of the following best describes the role of checkpoint proteins in maintaining genome integrity?

Accepted Answer

They sense DNA damage or chromosome missegregation and can halt cell cycle progression to allow repair or trigger apoptosis.

Question 18

A patient with a family history of colon cancer is found to have a mutation in the APC gene. According to the multistep model of colorectal cancer, what is the likely sequence of genetic events leading to malignancy?

Accepted Answer

Loss of APC → activation of ras oncogene → loss of DCC → loss of p53 → carcinoma and metastasis.

Question 19

A new cancer therapy is being developed that inhibits DNA methyltransferase. Which of the following is the most likely therapeutic effect of this drug?

Accepted Answer

It will decrease DNA methylation, potentially reactivating silenced tumor-suppressor genes.

Genetic Basis of Cancer: Study Notes

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