Final answer:
Sperm entry into an oocyte occurs after the oocyte has completed meiosis, which is triggered by sperm penetration allowing the oocyte to transform into a mature ovum, and ultimately leading to the formation of a diploid zygote.
Step-by-step explanation:
In most species, sperm entry into an oocyte (egg) typically occurs after the oocyte has completed meiosis. Upon ovulation, the oocyte released by the ovary is still not fully matured; it remains arrested at metaphase II of meiosis until fertilization takes place. The successful passage of a sperm through the layers surrounding the oocyte, such as the corona radiata and the zona pellucida, triggers the completion of meiosis. The fusion of the sperm's plasma membrane with that of the oocyte allows for the release of the sperm's nucleus into the oocyte’s cytoplasm, which then signals the oocyte to complete meiosis.
The oocyte disposes of the excess genetic material in a structure called a second polar body and transforms into a mature ovum, a female haploid gamete, at this point. Only then do the two haploid nuclei from the sperm and oocyte come together, eventually forming a diploid zygote with all the genetic instructions necessary to develop into an organism. The intricate journey of sperm involves capacitation, acrosomal reaction, and eventual fertilization, during which the oocyte progresses from a paused state in meiosis to complete maturation prompted by sperm penetration. This detailed process illustrates the remarkable interaction between male and female gametes during the moment of conception.