Soal dan Pembahasan TBI (Reading Comprehension) Bagian 28

        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 embryo. This led them to believe that the cells in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways. Later biologist found that the situation was not so simple. It matters in which place the embryo is cut. If it is cut in a plane different from the one used by the early investigators, it will not from two whole embryos.

        A debate arose over what exactly was happening. Which embryo cells are determined, just when do they become irreversibly committed to their fades, and what are the “morphogenetic determinants” that tell a cell what tp become? But the debate could not be resolved because no one was able to ask the crucial question a form in which they could be pursued productively.

        Recent discoveries in molecular biology, homever, have opened up prospects for a resolution of the debate. Now investigators think they know at least some of the molecules that act as mophogenetic determinants in early development. They have been able to show that, in a sense, cell determinant begins even before a egg is fertilized.

        Studying sea urchins, biologist Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants. They are located in the cytopiasm of the egg cell; e,. In that part of the cell’s protoplasm that lies outside of the nucleus. In the unfertilized egg; the substances are inactive and are not distributed  homogeneously. When the egg is fertilized, the substances become active and, presumaably, govern the behaviour of the genes they interact with. Since the substances are unevenly distributed in the egg; when the fertilized egg devides, the resulting cells are different from the start and so can qualitatively diffrent in their own gene activity.

(Adapted from various sources – PTT BPPK)