Day 61 MCAT Practice Question

Passage 1: Nucleic acid structure and genetic regulation

Retinoblastoma is a rare eye cancer that primarily affects young children. It is

caused by mutations in the RB1 gene, which is a tumor suppressor gene responsible

for regulating cell division in the retina. The RB1 gene encodes the retinoblastoma

protein (pRB), which plays a crucial role in the control of the cell cycle.

In normal cells, pRB is phosphorylated by cyclin-dependent kinases (CDKs) during

the G1 phase of the cell cycle, allowing the cell to progress to the S phase. However,

when the RB1 gene is mutated, it produces a non-functional pRB that cannot

be phosphorylated, leading to uncontrolled cell division and the formation of

retinoblastoma tumors.

Recent studies have explored the potential of using recombinant DNA technology

to introduce a functional RB1 gene into retinoblastoma cells. One approach involves

the use of adenoviral vectors to deliver the RB1 gene to the affected cells. The

introduced RB1 gene is placed under the control of a strong promoter, ensuring high

levels of expression. Once the functional pRB is produced, it can regulate the cell

cycle and suppress tumor growth.



Another area of research focuses on the development of small molecules that

can mimic the function of pRB. These molecules, known as CDK inhibitors, can

prevent the phosphorylation of pRB by CDKs, effectively halting the cell cycle and

preventing the proliferation of retinoblastoma cells. Several CDK inhibitors, such

as palbociclib and abemaciclib, have shown promise in preclinical studies and are

currently being investigated in clinical trials.



Understanding the molecular basis of retinoblastoma has also led to the

identification of biomarkers that can aid in the early detection and monitoring of

the disease. One such biomarker is the expression level of the RB1 gene, which can

be assessed using quantitative reverse transcription-polymerase chain reaction

(qRT-PCR). Low levels of RB1 expression may indicate the presence of mutations

and an increased risk of developing retinoblastoma.

Which of the following best describes the function of the retinoblastoma

protein (pRB) in normal cells?

A) pRB phosphorylates cyclin-dependent kinases (CDKs) during the G1 phase

B) pRB promotes the progression of cells from the G1 phase to the S phase

C) When pRB is phosphorized by CDKs, it allows cell cycle progression from G1

to S phase

D) pRB is a proto-oncogene that stimulates cell division when activated