On

goosecord gene

BOX 18-5 FIGURE 1 Specification of the dorsal mesoderm in the Xenopus embryo, (a) The Xenopus egg contains a number of local-tied mRNAs including VegTand Vgl. VegT encodes a T-box DNA-binding protein while Vgl encodes an actwrVTGFji signaling molecule. They lead to the expression of Xnr in vegetal regions. Cortical rotation occurs after fertilization and leads to the stabilization of p catenin along the future dorsal surface. The point of intersection between the Xnr and (3-catenin domains defines the dorsal mesoderm and leads to the activation of a number of genes such as gaosecoid. (b) p-catenrn in dorsal regions leads to the activation of the siamois gene, which encodes a bomc-obox regulatory protein. The Xnr signaling molecule leads to the activation of another class of regulatory proteins, Smads. Both regulatory proteins. Smads and Siamois, are located only in the dorsal mesoderm. In this region they wort; synergpsfcaffy to actrvate the goosecwd gene (Source: (a) Adapted from Alberts B. et al. 2002 Afo-kajlar biobgyof the cell, 4th edition, p. 1211.121-66. Copyright © 2002 Reproduced by permission of Routfedge/laylor 8 Francis Books, Inc. (b) Adapted from Gilbert SF. 2000. Developmental btohgy, 6th edition, p. 322, fig, 1025 Copyright <P 2000 Sinauer Associates. Used with permission. And from Moon R and Kimelman D. ¡998 (Tom cortical rotation to organizer gene expression. fiicEssay? 20: 542, fig 3 Copyright C1 1998. Used by permission at John Wiley & Sons, Inc.)

Segmentation Is Initiated by Localized RNAs at the Anterior and Posterior Poles of the Unfertilized Egg

At the time of fertilization, the Drosopiula egg contains two localized mRNAs. One, the bicoid mRNA, is located at the anterior pole, while the other, the oskar mRNA, is located at the posterior pole (Figure 18-16a). The oskar mRNA encodes an RNA-binding protein that is responsible for the assembly of polar granules. These are large macromolecular complexes composed of a variety of different proteins and RNAs. The polar granules control the development of tissues thai arise from posterior regions of the early embryo, including the abdomen and the pole cells, which are tlie precursors of the germ cells (Figure 18-16b),

The oskar mRNA is synthesized within the ovary of the mother fly. It is first deposited al ihe anterior end of the immature egg, or oocyte, by "helper" cells called nurse cells. But, as the oocyte enlarges to form the mature egg, the oskar mRNA is Iransported from anterior lo posle-rior regions. This localization process depends on specific sequences within the 3' UTR of the oskar mRNA (Figure 18-17), We have already

a anterior pote pre-cellular embryo

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