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  3. Cholesterol plays a key role in the development of embryo cells, biologists have discovered. They say that this much maligned fa

Cholesterol plays a key role in the development of embryo cells, biologists have discovered. They say that this much maligned fa

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问题     Cholesterol plays a key role in the development of embryo cells, biologists have discovered. They say that this much maligned fatty molecule, best known for its links to heart disease, helps to position a family of "controller" proteins that tell embryo cells how they should form.
    In laboratory studies on fruit flies and mice, Phil Beachy and his colleagues at Johns Hopkins University in Baltimore looked at members of a family of controller proteins known as Hedgehog. Fruit fly embryos that lack the genes for these proteins develop a spiky appearance — hence the name. Biologists already know that Hedgehog proteins help to direct the development of embryo cells by delivering signals at the cell surface. The proteins only deliver their signals when they split in two, and one half binds to a lipid, a fatty molecule. Beachy’ s team discovered that this lipid is cholesterol.
    In previous test-tube experiments, scientists have found that the concentration and distribution of Hedgehog proteins on the surface of embryo cells can be critical in determining their fate. For example, cells taken from the embryonic nervous system become motor neurons when exposed to low concentrations of Hedgehog proteins, while at high concentrations they turn into structural cells known as floor plates. Until now, however, no one knew what determined the concentration of the proteins on the surface of cells in the embryo.
    Beachy’ s findings suggest that cholesterol’ s role is to allow the signaling proteins to enter cells. Since the lipid molecule is water-hating or hydrophobic, the researchers argue, it may act as a key, allowing the Hedgehog proteins to move through the hydrophobic membranes of target cells, or to stick to them. The results appear in the current issue of Science.
    Researchers already know that cholesterol does some important jobs: for example, it is involved in the production of hormones and is vital for fertility and for the healthy membrane structure of cells. Beachy’ s latest results show that cholesterol is also needed for embryonic development. They also help to explain a long-known but mysterious finding that pregnant rats given drugs that block the manufacture of cholesterol produce deformed fetuses.
    Gail Martin, a developmental biologist at the University of California, San Francisco, describes the findings as "exciting". They may also have wider implications, she says: it is likely that cholesterol’ s action is needed more generally for signaling between cells.
    Beachy too believes that the findings may have "strong connections for human biology". For example, they throw light on a birth disorder that has been linked to the mother’ s inability to make cholesterol. This disorder, Smith-Lemli-Opitz syndrome, affects about 1 in 9000 newborn babies, causing defects such as small head size.
When Hedgehog proteins split in two, they______.
选项 A、can be found in a fatty molecule
B、develop a spiky appearance
C、deliver signals at a lipid
D、receive the signals from cholesterol
答案A
解析 由题干找到其所在原文第二段最后几句话：“Biologists already know…by deliveringsignals at the cell surface．The proteins only deliver their signals when they split in two，and one．half binds to a lipid, a fatty molecule．”。根据这些内容，可以看出该题的正确答案应该为A。
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