GMAT 考满分题库

Nearly a century ago, biologists found that if theyseparated an invertebrate animal embryo into two partsat an early stage of its life, it would survive and developas two normal embryos. This led them to believe that thecells in the early embryo are undetermined in the sensethat each cell has the potential to develop in a variety ofdifferent ways. Later biologists found that the situationwas not so simple. It matters in which plane the embryois cut. If it is cut in a plane different from the one usedby the early investigators, it will not form two wholeembryos.

A debate arose over what exactly was happening.Which embryo cells are determined, just when do theybecome irreversibly committed to their fates, and whatare the "morphogenetic determinants" that tell a cellwhat to become? But the debate could not be resolvedbecause no one was able to ask the crucial questionsin a form in which they could be pursued productively.Recent discoveries in molecular biology, however, haveopened up prospects for a resolution of the debate.Now investigators think they know at least some of themolecules that act as morphogenetic determinants inearly development. They have been able o show that,in a sense, cell determination begins even before an eggis fertilized.

Studying sea urchins, biologist Paul Gross foundthat an unfertilized egg contains substances that function as morphogenetic determinants. They are locatedin the cytoplasm of the egg cell; i.e., in that part of thecell's protoplasm that lies outside of the nucleus. In theunfertilized egg, the substances are inactive and are notdistributed homogeneously. When the egg is fertilized,the substances become active and, presumably, governthe behavior of the genes they interact with. Since thesubstances are unevenly distributed in the egg, when thefertilized egg divides, the resulting cells are differentfrom the start and so can be qualitatively different intheir own gene activity.

The substances that Gross studied are maternalmessenger RNA's --products of certain of the maternalgenes. He and other biologists studying a wide varietyof organisms have found that these particular RNA'sdirect, in large part, the synthesis of histones, a classof proteins that bind to DNA. Once synthesized, thehistones move into the cell nucleus, where section ofDNA wrap around them to form a structure that resembles beads, or knots, on a string. The beads are DNAsegments wrapped around the histones; the string is theintervening DNA. And it is the structure of these beadedDNA strings that guides the fate of the cells in whichthey are located.