Cloning
When Dolly was cloned,
the nucleus of an egg
was replaced with a nucleus
of the skin. Dolly is not so healthy because its genetic material is old.
Most cloned eggs die. Other
develop malformations, because they received old nuclei.
As embryo
develops, cells become committed to different lineages because gene-expression
programs change as cells differentiate. This commitment is represented in
the CA by state, and age. Each
new state and its age structure are
equivalent to lineage commitment. Morphologically, the nuclei in some cloned
embryos resemble those of the somatic cells that were used for cloning.
The cloned cells retained their (advanced) biological age of the donor nucleus,
like in our previous experiment.
Another obstacle to cloning is the egg cytoplasm, which is regarded as an inert container for carrying the fused nucleus. Yet the cytoplasm is very active. It carries mitochondria with their own DNA, which it inherited from the mother. Its protein will determine the axis of symmetry of the developing fetus, and the future site of its head. Other cytoplasmic factors control nuclear genes by a process called imprinting. All these factors were inherited form the mother!
Monozygotic twins
With all these obstacles one wonders whether cloning is really feasible? The answer is provided by Nature, when a fertilized egg divides into two identical twins. Both get the same genes, and cytoplasmic factors. They are the best clones one can hope for. Are they really identical? Or simply similar, like our two CA depicted above?
Further reading:
Cloning fallacy
Beware of the Gene
Iatrogenic Medicine
Setup