The great news of Dolly was precisely the discovery that a differentiated mammalian somatic cell, such as the skin cell, the heart cell, etc. it could be reprogrammed to the initial stage and become totipotent again (ie being able to give rise to any cell type again).
This was achieved through transfer of the nucleus from a somatic cell of the sheep mammary gland that originated Dolly to an anucleated egg (egg without nucleus). Surprisingly, it began to behave like a newly fertilized egg by a sperm. This is probably because the egg, when fertilized, has mechanisms, for us as yet unknown, to reprogram DNA to make all its genes active again, which occurs in the normal process of fertilization.
Illustration of how Dolly's cloning went
Illustration of what human cloning would be like
We already know that this is not an easy process. Dolly was born only after 276 failed attempts. Moreover, of the 277 "mother of Dolly" cells that were inserted into a nucleus-free egg, 90% did not even reach the blastocyst stage. Further attempts to clone other mammals such as mice, pigs, calves, a horse and a deer have also shown very low efficiency and a very large proportion of misformed abortions and embryos. Penta, the first Brazilian heifer cloned from a somatic cell, died adult in 2002, just over a month.
Dolly and the DNA donor sheep
Also in 2002, the cloning of the copycat The first pet cat cloned from an adult somatic cell. For this, 188 eggs were used that generated 87 embryos and only one live animal. In fact, recent experiments with different types of animals have shown that this reprogramming of genes into the embryonic stage, which originated Dolly, is extremely difficult.
The group led by Ian Wilmut, the Scottish scientist who became famous for this experiment, claims that virtually all animals that have been cloned from non-embryonic cells in recent years are in trouble. Among the different defects observed in the very few animals born alive after numerous attempts are: abnormal placentas, gigantism in sheep and cattle, heart defects in pigs, lung problems in cows, sheep and pigs, immunological problems, failure in leukocyte production. , muscle defects in sheep.
Recent advances in reproductive cloning allow four important conclusions:
- most clones die early in pregnancy;
- cloned animals have similar defects and abnormalities, regardless of donor cell or species;
- these abnormalities are likely to occur due to faulty genome reprogramming;
- cloning efficiency depends on the differentiation stage of the donor cell. In fact, reproductive cloning from embryonic cells has been shown to be ten to twenty times more efficient, probably because the genes that are central to early embryogenesis are still active in the donor cell genome.
Interestingly, among all mammals that have already been cloned, efficiency is slightly higher in calves (about 10% to 15%). On the other hand, an intriguing fact is that there is still no news of a cloned monkey or dog. Perhaps this is why English scientist Ann McLaren has stated that failures in reprogramming the somatic nucleus may constitute an insurmountable barrier to human cloning.
Still, people like the Italian doctor Antinori or the Raelian sect advocate human cloning, a procedure that has been banned in every country. Indeed, a document signed in 2003 by science academies in 63 countries, including Brazil, calls for a ban on human reproductive cloning. The fact is that the mere possibility of cloning humans has sparked ethical discussions in all segments of society, such as: Why cloning? Who should be cloned? Who would decide? Who will be the clone's father or mother? What to do with clones that are born defective?
In fact, the current major ethical problem is the enormous biological risk associated with reproductive cloning. In my view, it would be the same as discussing the pros and cons of releasing a new medication, the effects of which are devastating yet totally uncontrollable.
Despite all these arguments against reproductive human cloning, experiments with cloned animals have taught us a lot about cell function. On the other hand, the core transfer technology for therapeutic purposes, the so-called therapeutic cloning, can be extremely useful for obtaining stem cells.
Text adapted from Zatz, Mayana. "Cloning and stem cells". Cienc. Cult., jun. 2004, vol. 56, no. 3, pp. 23-27, ISSN 0009-6725.