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From what I understand, one of the challenges of cloning is that the DNA methylation that is supposed to occur during embryonic development does not occur normally and this prevents natural gene expression in the cloned zygote. However, the typical descriptions of these processes are highly technical and beyond my monkey-level intelligence. I was wondering if someone could explain in simple terms why DNA methylation does not occur normally in naive cloning processes.

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Your description sounds a bit confused. Perhaps the best way to think about it is that a natural zygote's DNA has undergone epigenetic "programming" that places different genes in different expression states appropriate for that stage of development (for example, you wouldn't want to turn the cell into a cartilage cell at that point, so those genes should be off). DNA methylation is one method of such programming, there are others known, and likely additional unknown ones as well.

If you replace the nucleus of a zygote with a non-zygote nucleus, much of the programming won't be appropriate for the zygotic stage and development won't proceed properly. Cloning technology involves developing processes and reagents to reprogram the nucleus into a state that is compatible with normal development once it is inserted into a zygote.

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