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During normal
development (Panel A), a haploid (1n) sperm cell fertilizes a haploid oocyte
to form a diploid (2n) zygote that undergoes cleavage to become a blastocyst
embryo. Blastocysts are implanted in the uterus and ultimately give rise to
an animal. During reproductive cloning (Panel B), the diploid nucleus of an
adult donor cell is introduced into an enucleated oocyte, which after
artificial activation divides into a cloned blastocyst. On transfer into
surrogate mothers, a few of the cloned blastocysts will give rise to a clone.
In contrast, therapeutic cloning (Panel C) requires the explantation of
cloned blastocysts in culture to yield a line of embryonic stem cells that
can potentially differentiate in vitro into any type of cell for therapeutic
purposes.
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