Biological Systems MCAT Practice Exam

During oogenesis, what occurs in the primary oocytes before fertilization?

1. They are activated to complete meiosis.

2. They remain frozen in prophase I.

3. They divide equally to produce ova.

4. They produce multiple polar bodies.

The correct answer is: They remain frozen in prophase I.

During oogenesis, primary oocytes indeed remain frozen in prophase I of meiosis until the female reaches sexual maturity and begins the menstrual cycle. This pause in development is a critical defining feature of the oocyte's maturation process. The primary oocytes, which are formed during fetal development, enter prophase I and become arrested in this stage, often for many years, until hormonal signals initiate the resumption of meiosis. Once the oocyte is stimulated to continue development, typically during ovulation, it will complete the first meiotic division to produce a secondary oocyte and a polar body. However, until that point, the primary oocyte remains in that arrested state. In contrast to this process, the other options do not accurately describe what occurs before fertilization. For instance, activation to complete meiosis and dividing equally to produce ova would not occur prior to fertilization as these processes are involved in the maturation and fertilization phases. Similarly, while polar bodies are produced, it is not the primary function of the arrested primary oocyte, and the overall production mechanism does not result in multiple polar bodies until later stages of meiosis.