USA Biology Olympiad (USABO) Practice Exam

Which of the following transitions occurs during the reproductive process?

1. Tube nucleus becomes a sperm nucleus.

2. Egg nucleus divides into two haploid nuclei.

3. Diploid zygote undergoes meiosis.

4. Sperm cell fertilizes multiple egg cells.

The correct answer is: Tube nucleus becomes a sperm nucleus.

The transition where the tube nucleus becomes a sperm nucleus is correct because, during fertilization in plants, the pollen grain germinates and forms a pollen tube that delivers the sperm cells to the ovule. Within the pollen tube, one of the nuclei, typically referred to as the tube nucleus, helps in the growth and guidance of the tube towards the ovule. Upon reaching the ovule, the tube nucleus is involved in the process that results in the fusion with the egg cell, ultimately contributing to the formation of the sperm nucleus. In contrast, the other options describe processes that do not accurately reflect key events during reproduction. The division of the egg nucleus into two haploid nuclei does not occur as part of fertilization; instead, the egg remains a single entity until fertilization occurs. The diploid zygote undergoing meiosis is a misrepresentation of zygote development, as the zygote typically undergoes mitotic divisions to develop into an embryo rather than meiosis, which reduces chromosome number. Lastly, sperm cells fertilizing multiple egg cells is not a standard reproductive strategy; usually, one sperm fertilizes one egg, leading to the formation of a zygote, while polyspermy—fertilization by multiple sperm—is