What are pronuclei?

• A. Nucleus of the sperm or egg cell during the process of fertilization before fusion
• B. Nucleus of the sperm or egg before injection of a transgene
• C. Nucleus of the sperm or egg in which DNA has been removed
• D. Nucleus of the sperm or egg cell after fertilization and fusion

Answer: (A)

Pronuclei are the separate nuclei of the sperm and the egg during fertilization, before their genetic material fuses. After sperm entry, the paternal DNA forms the male pronucleus while the egg completes meiosis and forms the female pronucleus. These two pronuclei move toward each other, replicate their DNA, and then fuse to create the single diploid nucleus of the zygote. They exist as distinct nuclei at this stage rather than as a unified zygote nucleus, which happens only after fusion. This stage is distinct from a nucleus before a transgene is injected, from a nucleus in which DNA has been removed, or from the nucleus after fertilization and fusion. In lab contexts, pronuclear stages are key when studying fertilization and during techniques like pronuclear injection used to create transgenic animals.