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Message: [QUOTE="Bombastus, post: 91915, member: 2178"] Somatic cell nuclear transfer copies the nuclear genome of one donor cell, while the egg still contributes most of the embryo’s mitochondrial DNA. A clone made this way would therefore be extremely close to the nuclear donor genetically, but “exact copy” is too tidy a description. No verified human has been born through reproductive cloning, so any claim about a human clone has to lean on cloned mammals and established human genetics. The distinction matters because [B][URL='https://goldmidi.com/community/threads/what-is-human-cloning.26162/']the basic human cloning process[/URL][/B] transfers a nucleus into an egg whose own nucleus has been removed, not the donor’s entire cell. The transferred nucleus contains almost all of the DNA people usually mean when they talk about a genome. The egg keeps its cytoplasm and mitochondria, which carry a small genome of their own. In some cloned animals, traces of mitochondrial DNA from the donor cell have also persisted alongside the egg’s mitochondrial DNA. [HEADING=2]The egg contributes DNA the nucleus cannot replace[/HEADING] A person created by SCNT would get the donor cell’s chromosomes from the transferred nucleus. Those chromosomes would carry the same inherited variants and the same basic nuclear genetic background as the cell that was copied. The mitochondrial genome is a separate piece of the picture. Mitochondria sit outside the nucleus and contain their own DNA. Because an enucleated egg keeps its mitochondria, the resulting embryo would usually inherit mitochondrial DNA mainly from the egg donor rather than the person whose nucleus was copied. Complete genetic identity would require matching both sources, not merely matching the chromosomes inside the nucleus. Even mitochondrial inheritance is not perfectly binary. Animal cloning has produced heteroplasmy, meaning more than one mitochondrial DNA population can exist in the same individual. A small amount from the transferred somatic cell can sometimes remain while the egg-derived population dominates. Calling two individuals “genetically identical” is still useful shorthand when their nuclear genomes are nearly the same. It becomes misleading when the phrase is taken literally. Nuclear DNA, mitochondrial DNA, and later mutations can all create differences that sequencing is able to detect. [HEADING=2]One donor cell can carry mutations of its own[/HEADING] Your body is not made from trillions of genetically frozen copies of the fertilized egg you began as. Cells pick up somatic mutations during development, aging, ordinary DNA replication, and exposure to damaging agents. Different patches of otherwise healthy tissue can therefore contain slightly different genetic lineages. SCNT selects one cell, or one cultured cell lineage, as the nuclear source. A skin cell chosen for cloning could carry variants that are absent from the donor’s blood cells or from another patch of skin. The clone would begin from the genome in that selected nucleus, not from some averaged master copy of the donor’s DNA. New changes can appear after the cloned embryo starts dividing as well. Every round of cell division creates another chance for a copying error, so genetic divergence continues after the transfer. Cloning does not lock a genome in place. Long serial cloning makes the point unusually clear. A 2026 [B][URL='https://www.nature.com/articles/s41467-026-69765-7']20-year serial-cloning experiment in mice[/URL][/B] followed repeated recloning from one original donor line and reported accumulating structural and lethal mutations across generations. One round of cloning is not equivalent to 58 generations of recloning, but the experiment shows why genetic copying should never be treated as permanent molecular stasis. [HEADING=2]Epigenetic resetting changes how copied DNA behaves[/HEADING] DNA sequence is only part of what cells inherit when they divide. Chemical marks on DNA and histone proteins help control which genes are active, quiet, or available for use. These epigenetic patterns can differ even when the underlying sequence is nearly unchanged. SCNT asks an adult cell nucleus to behave like the nucleus of a newly formed embryo. Egg cytoplasm has to erase much of the adult program and rebuild an embryonic one, and animal cloning repeatedly shows that this reset can be incomplete. Abnormal DNA methylation, genomic imprinting, chromatin structure, and X-chromosome regulation have all been linked to failed or abnormal clone development. Surviving clones can also show epigenetic differences without being genetically different in the everyday sense. Recent work in cloned cattle has measured divergent methylation patterns and chromatin accessibility after nuclear transfer, reinforcing the difference between carrying similar DNA and using it in precisely the same way. Gene activity can diverge without changing the letters of DNA, so appearance and development need not track sequence perfectly. A hypothetical human clone would therefore share an unusually close nuclear genetic relationship with the donor without being a molecular duplicate. The nuclear genomes could be extraordinarily similar, while mitochondrial inheritance, donor-cell mosaicism, new mutations, and epigenetic reprogramming create real biological differences from the beginning. [/QUOTE]

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