One reason why a sheep, a less well-understood experimental subject than
the laboratory mouse, should have proved easier to clone may stem from
differences in the initial stages of the two species' embryonic development.
After reaching maturity in the ovary of the mother, the unfertilized eggs of all
(5) mammals accumulate a supply of proteins, and the means of producing fresh
protein. In this way, the mammalian egg brings with it a larder for the embryo
to make use of until its own genes activate and supply this requirement
themselves. The sheep embryo disposes of its store properly and need not
depend on its own genes until the sixteen-cell stage, four cell divisions
(10) successive to fertilization, while in contrast, the mouse embryo commences this
process more precociously, becoming reliant on the activity of its own genes
after just the first division when the fertilized egg becomes two cells.
Therefore, a foreign nucleus introduced into a sheep egg exploits a respite in its
host's biological development, allowing it to adapt to its new role before
(15) assuming genetic control.
Concomitantly, a nucleus introduced into a mouse egg must acclimatize
quickly for its genes to be able to direct embryonic development within a single
cell division, so perhaps there is insufficient time for the extensive re-
programming of compulsory gene activity. The human embryo is thought to rely
(20) on its own genes after three cellular divisions, which might or might not
provide time enough for a foreign nucleus to acclimate. However, were
scientists to comprehend the nature of the indispensable re-programming then
there is every likelihood that both mice and humans could be cloned.
Despite the long-standing availability of this technology, there has until
(25) recently been little interest in it. Some people suffering from infertility as a
result of rare hereditary diseases could produce offspring, but cloned individuals
may be at risk given scientists' limited knowledge of the long term effects of
allowing an "old" adult cell nucleus to commence life again in an egg. The
nucleus of a skin cell could have accumulated a multitude of genetic mistakes of
(30) no consequence to its role in the skin, but the same cell could prove deleterious
in other tissues, or immensely increase the probability of the affliction with
cancer. The threat to general human health posed by cloning, as opposed to the
individual, is difficult to determine, but the risks are almost certainly lower
than those encountered in the effective inbreeding of consanguine marriages,
(35) and thus there are no scientific grounds per se for banning cloning. Like other
practices inconsequential to the physical well being of humanity, but generally
deemed undesirable on moral or social grounds, the prohibition of human
cloning will ultimately rest with only a simple pragmatic decision.
In this passage, the author is primarily concerned with
A.advancing an argument against human cloning based on evidence from cloning experiments performed on lower life forms
B.measuring the technical and ethical limitations of the embryonic cloning process in mammals
C.refuting arguments against human cloning through the use of hard scientific evidence
D.illuminating the critical ethical distinctions in process between human cloning and sheep cloning
E.describing how the ethical issues in the quest for human cloning have given rise to new models of embryonic development
第2题
13 .下列不能用于治疗室性心律失常的是
A .奎尼丁、利多卡因
B .维拉帕米、胺碘酮
C .普罗帕同、利多卡因
D .普萘洛尔、奎尼丁
E .普萘洛尔、普罗帕同
第8题
治疗室性快速性心律失常首选药物是
A.奎尼丁
B.维拉帕米
C.利多卡因
D.普萘洛尔
E.胺碘酮
第9题
治疗室性快速性心律失常首选药物是
A.奎尼丁
B.维拉帕米
C.利多卡因
D.普萘洛尔
E.胺碘酮
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