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1、 GMAT考试:2022年GMAT考试阅读材料14(附答案)Nearly a century ago， biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of its life， it would survive and develop as two normal embryos. This led them to believe that the cells in the early embryo are undetermined in
2、the sense that each cell has the potential to develop in a variety of different ways. Later biologists found that the situation was not so simple. It matters in which plane the embryo is cut. If it is cut in a plane different from the one used by the early investigators， it will not form two whole e
3、mbryos.A debate arose over what exactly was happening. Which embryo cells are determined， just when do they become irreversibly committed to their fates， and what are the “morphogenetic determinants” that tell a cell what to become？ But the debate could not be resolved because no one was able to ask
4、 the crucial questions in a form in which they could be pursued productively. Recent discoveries in molecular biology， however， have opened up prospects for a resolution of the debate. Now investigators think they know at least some of the molecules that act as morphogenetic determinants in early de
5、velopment. They have been able to show that， in a sense， cell determination begins even before an egg is fertilized.Studying sea urchins （any of numerous echinoderms （class Echinoidea） that are usually enclosed in thin brittle globular tests covered with movable spines海胆）， biologist Paul Gross found
6、 that an unfertilized egg contains substances that function as morphogenetic determinants. They are located in the cytoplasm of the egg cell; i.e.， in that part of the cells protoplasm that lies outside of the nucleus. In the unfertilized egg， the substances are inactive and are not distributed homo
7、geneously. When the egg is fertilized， the substances become active and， presumably， govern the behavior of the genes they interact with. Since the substances are unevenly distributed in the egg， when the fertilized egg divides， the resulting cells are different from the start and so can be qualitat
8、ively different in their own gene activity.The substances that Gross studied are maternal messenger RNAsproducts of certain of the maternal genes. He and other biologists studying a wide variety of organisms have found that these particular RNAs direct， in large part， the synthesis of histones， a cl
9、ass of proteins that bind to DNA. Once synthesized， the histones move into the cell nucleus， where section of DNA wrap around them to form a structure that resembles beads， or knots， on a string. The beads are DNA segments wrapped around the histones; the string is the intervening DNA. And it is the structure of these beaded DNA strings that guide the fate of the cells in which they are located.
