Final answer:
Upon fertilization, the secondary oocyte finishes meiosis II, sheds a second polar body, and becomes a diploid zygote. The zygote travels down the fallopian tube, undergoes division, implants in the uterus, and gestation begins. Mechanisms prevent polyspermy to ensure that only one sperm fertilizes the egg.
Step-by-step explanation:
When a secondary oocyte is fertilized by a sperm, it completes the second meiotic division, which was previously arrested at metaphase II. This results in the formation of a second polar body and the zygote, which contains a full set of chromosomes - half from the mother and half from the father. The zygote proceeds through the fallopian tube, divides mitotically, implants itself into the endometrium of the uterus, and begins gestation.
During fertilization, the secondary oocyte transforms into an ovum, and the genetic materials from both the sperm and the ovum combine. The two haploid nuclei (pronuclei) replicate their DNA, migrate towards each other, their nuclear envelopes disintegrate, and the genetic material intermingles. This step completes fertilization and produces a single-celled diploid zygote with the necessary genetic instructions for human development.
It's essential for a successful pregnancy that no more than one sperm fertilizes the oocyte. Special mechanisms in the egg membrane release proteins to prevent polyspermy, ensuring only one sperm successfully contributes genetic material to the zygote.