During transgenic mouse creation, which developmental stage is typically used for DNA injection to create the transgene?

During transgenic mouse creation, which developmental stage is typically used for DNA injection to create the transgene?

• A. Mature oocyte
• B. Sperm
• C. Zygote pronucleus
• D. Morula

Answer: (C)

Injecting the transgene into the pronuclei of a fertilized mouse egg (the zygote pronucleus) is standard because this early, single-cell stage places the foreign DNA into a genome that will be copied as the embryo divides, so the transgene is present in all cells of the animal and, importantly, can be transmitted through the germline. The pronuclear environment is accessible for microinjection and allows the DNA to integrate into the host genome before the first cell division, producing a founder animal whose offspring can inherit the transgene. Injecting into a mature oocyte wouldn’t involve a fertilized genome suitable for stable, heritable integration. Sperm isn’t the right target for creating these transgenic embryos because the goal is to introduce exogenous DNA into the developing embryo, not into gametes in isolation. Injecting at later stages like the morula can lead to mosaic animals where not all cells carry the transgene, reducing effectiveness of germline transmission.

Injecting the transgene into the pronuclei of a fertilized mouse egg (the zygote pronucleus) is standard because this early, single-cell stage places the foreign DNA into a genome that will be copied as the embryo divides, so the transgene is present in all cells of the animal and, importantly, can be transmitted through the germline. The pronuclear environment is accessible for microinjection and allows the DNA to integrate into the host genome before the first cell division, producing a founder animal whose offspring can inherit the transgene. Injecting into a mature oocyte wouldn’t involve a fertilized genome suitable for stable, heritable integration. Sperm isn’t the right target for creating these transgenic embryos because the goal is to introduce exogenous DNA into the developing embryo, not into gametes in isolation. Injecting at later stages like the morula can lead to mosaic animals where not all cells carry the transgene, reducing effectiveness of germline transmission.