How Does Pet Cloning Work?

Cloning is the creation of two peas of the same organism as the original organism. It originally refers to gene duplication, but now it is widely used, and has already exceeded this meaning. In biology, cloning mainly refers to DNA replication in molecular biology, culturing a group of cells with the same genome in cytology, and obtaining biological individuals with the same genome in zoology. 

In principle, there are two methods for artificial cloning of higher organisms: embryo splitting and cloning into egg cells or embryonic cells with removed genetic material by nuclear transfer.

Embryo Splitting Cloning

Although sexual reproduction is widely used in higher animals, does cloning exist naturally in them? The answer is yes. For example, identical twins originate from the division of the same fertilized egg and have the same genome. This is also an idea of artificial cloning animals. They divide early embryos into multiple small embryos and then develop into multiple individuals. This approach is based on the plasticity of early embryonic cells. At this time, the fate of cells is still reversible and totipotent, while later cells, especially mature cells, do not have this characteristic.

Pet cloning methods include embryo splitting, removing blastomeres from very early animal embryos and separating them before differentiation to produce two or more independent organisms. When embryo division is used, pet cloning must be carried out before the pet is born, and the clone grows at the same time as the original pet (in a similar way to monozygotic twins).

This method is not only technically difficult, but also more important because early embryos must be used, which greatly limits the scope of cloning. Many characteristics cannot be known until the individual is mature. If you want to clone an individual at this time, you need to use somatic cell nuclear transfer technology.

Somatic Cell Nuclear Transfer Cloning

Because many characteristics of animal individuals are controlled by genes, individuals with the same characteristics as the transplanted individuals can be obtained as long as the nucleus of the genome is transferred to enucleated oocytes or early embryonic cells for further development. In the narrow sense, animal cloning refers to cloning using somatic cell nuclear transfer technology. At present, most animal cloning adopts this method.

Two Chinese cloning monkeys, Zhongzhong and Huahua.

In this process, the oocytes are removed from the surrogate mother, and the oocytes are enucleated as nuclear receptors by cell disassembly or cell recombination technology, which is a process of removing the nucleus from the oocytes. Then, somatic cells are taken from cloned animals and transferred to blank oocytes. Somatic cells or nuclei containing a small amount of cytoplasm, i.e., nucleoplasts, are used as nuclear donors. The latter is transferred to enucleated oocytes to provide genetic material, and fused with oocytes with current to construct recombinant embryos. The oocytes are then activated and reinserted into the surrogate mother. Donor nuclei reprogram in the cytoplasm of enucleated oocytes, start cleavage, and begin the process of embryonic development. The result is the generation of an animal that is almost genetically identifical as the animal from which somatic cells are taken.

Summary

Embryo division and nuclear transfer are essentially different in technology and the degree of genetic characteristics of the final embryo. Embryo division does not change the age of cells or the effectiveness of cells. Embryos from the division are at the same developmental stage, with exactly the same age as undivided embryos, and are genetically identical. Clonal nuclear transfer technology uses different methods to transfer genetic programs (nuclei with required genetic material) from totipotent mother cells or non-totipotent cells (embryos, fetuses, and even differentiated somatic cells) to unfertilized oocytes whose nuclei have been removed. This technology provides the possibility of replicating adult individuals and their genetic programs. The result is a new individual whose existence does not come from the fertilization of egg cells by sperm cells.

Cloning technology has changed the way we look at the uniqueness of life and genetics. In the future, scientists can use cloning technology to provide a large number of special animals that serve humans and apply them to the preservation of the genes of animals with excellent bloodlines. 

References

1. Vajta G. Cloning: A Sleeping Beauty Awaiting the Kiss? Cell Reprogram. 2018 Jun;20(3):145-156.
2. Saini M, et al. An update: Reproductive handmade cloning of water buffalo (Bubalus bubalis). Anim Reprod Sci. 2018 Oct;197:1-9.
3. Liu Z, et al. Cloning of Macaque Monkeys by Somatic Cell Nuclear Transfer. Cell. 2018 Feb 8;172(4):881-887.e7. 
4. Keefer CL. Artificial cloning of domestic animals. Proc Natl Acad Sci U S A. 2015 Jul 21;112(29):8874-8.