Which of the following describes the fusion event in fertilization?

• A. Cells divide
• B. Nuclei fuse
• C. Chromosomes duplicate
• D. Cytoplasm merges

Answer: (B)

The key idea is the joining of genetic material from both gametes. In fertilization, the sperm and egg each bring a haploid nucleus—the fusion event described is the merging of these nuclei, a process called karyogamy. When the male and female pronuclei come together, their nuclear membranes break down and their chromosomes combine to form a single diploid zygote nucleus. This is the moment that restores the species’ full chromosome number and sets up the zygote to begin DNA replication and the first mitotic division.

The other options describe events that happen at different times: cells dividing occurs after fertilization as the zygote begins to develop; chromosomes duplicating happens as part of preparing for mitosis after the zygote forms; cytoplasm merging refers to the merging of the cytoplasms (plasmogamy), which is related but not the same as the fusion of the genetic material into a single nucleus.