What is the primary method for producing live birth mammals?

• A. Somatic Cell Nuclear Transfer (SCNT): extract nucleus from egg; replace egg's nucleus with donor nucleus from adult to be cloned; stimulate cell with chemicals or electricity; transfer cloned embryo.
• B. In vitro fertilization followed by natural gestation.
• C. Embryo splitting within the uterus.
• D. Genetic modification of zygotes using CRISPR.

Answer: (A)

Cloning by somatic cell nuclear transfer is the method used to produce live birth mammals that are genetically identical to a donor. In this process, the nucleus from an adult body cell is removed and inserted into an egg cell that has had its own nucleus removed. The egg’s cytoplasm reprograms the donor nucleus, reactivating embryonic development. With a stimulus such as an electric pulse or chemical treatment, the cell begins to divide and form an embryo, which is then implanted into a surrogate mother. If development succeeds, a live birth results that mirrors the donor’s genetic material. This approach is famous for producing Dolly the sheep and remains the primary cloning method for creating cloned mammals.

In vitro fertilization followed by gestation produces live births but does not create a clone of a specific donor. Embryo splitting can yield identical offspring but is a more limited, situational cloning method. Genome editing with CRISPR changes genes in a zygote or embryo, which is not the cloning process itself.