What occurs during fertilisation?

During fertilization, the defining event is the fusion of the nuclei of two gametes, namely the sperm and the egg. This process is crucial as it results in the formation of a zygote, which contains a complete set of genetic material with half derived from each parent. The gametes carry haploid sets of chromosomes, and when they come together, they restore the diploid state necessary for developing a new organism.

This fusion initiates a series of developmental processes that lead to fertilization's successful outcome, including the combination of parental genetic traits and the establishment of a unique genetic identity for the new organism. Other statements, such as mitochondria entering the egg or sperm undergoing mitosis, do not accurately represent the fertilization process. Typically, the sperm contributes its genetic material upon fusion, but its mitochondria do not contribute to the egg. Mitosis does not occur within the egg until after fertilization has taken place and the zygote begins cellular division.

Therefore, the correct understanding of fertilization centers around the fusion of the gamete nuclei, which is foundational to the creation of new life.