Final answer:
Fertilization occurs A) when a sperm cell unites with an egg cell, typically in the fallopian tubes, resulting in a zygote that contains genetic material from both parents. This event completes the egg's meiosis and marks the beginning of a new human organism.
Step-by-step explanation:
Fertilization occurs when a sperm cell unites with an egg cell. This complex process results in the formation of a zygote, marking the beginning of a new human organism. Fertilization typically takes place within the fallopian tubes, where a haploid sperm successfully enters a haploid egg, causing the final completion of meiosis for the egg and resulting in a diploid cell. The newly formed zygote contains genetic material from both parents, with half of the chromosomes coming from the mother's egg and the other half from the father's sperm.
The transit of sperm is a journey involving overcoming various obstacles within the female reproductive system. Only a fraction of the millions of sperm released during ejaculation make it to the vicinity of the egg. After the sperm penetrates the egg, meiosis is completed, and the merging of sperm and egg nuclei occurs, creating the diploid zygote which will undergo multiple cell divisions and eventually implant in the uterus.
In summary, fertilization occurs when a sperm cell enters the egg in the fallopian tube. The zygote that is formed from this union undergoes several cell divisions and eventually implants in the uterus, continuing its development into a new human being.