- A Genes that are understood by molecular biologists to actively promote and considerably accelerate tumour growth once they are expressed in a cell, functioning as dominant growth-promoting factors rather than as recessive brakes on cell division
- B Genes that normally prevent uncontrolled cell division; loss of function (recessive, both alleles must be lost - Knudson two-hit hypothesis) allows uncontrolled growth; examples: TP53 (guardian of genome), RB1 (retinoblastoma), BRCA1/2 (breast cancer)
- C Tumour suppressor genes are in fact more accurately classified by geneticists as generally being a specific subtype of oncogene found in tumours, requiring mainly a single activating mutation in one allele to drive uncontrolled proliferation
- D Tumour suppressor genes are understood to be expressed and biologically active mainly within liver tissue, and nowhere else in the body, playing no protective role whatsoever in tissues such as breast, colon, retina, or bone
Correct answer: B. Genes that normally prevent uncontrolled cell division; loss of function (recessive, both alleles must be lost - Knudson two-hit hypothesis) allows uncontrolled growth; examples: TP53 (guardian of genome), RB1 (retinoblastoma), BRCA1/2 (breast cancer)
Explanation: Tumour suppressor genes (TSGs): inhibit cell cycle, promote apoptosis, maintain genome integrity (guardians). Loss of function = cancer (recessive - both alleles must be inactivated). TP53 (most mutated gene in cancer) arrests cell cycle; triggers apoptosis on DNA damage. RB1 (first TSG discovered) controls G1/S transition. BRCA1/2 - DNA repair; BRCA mutation dramatically increases breast/ovarian cancer risk.
Concept context
Pathogens, infectious diseases, non-infectious diseases, drugs, cancer, and public health. High NEET importance.