遗传分析-第二十三章Developmental Genetics.doc

Chapter 23 Developmental GeneticsKey Concepts A programmed set of instructions in the genome of a higher organism establishes the developmental fates of cells with respect to the major features of the basic body plan. Developmental pathways are formed by the sequential implementation of various regulatory steps. The zygote is totipotent, giving rise to every adult cell type; as development proceeds, successive decisions restrict each cell lineage to its particular fate. Gradients of maternally derived regulatory proteins establish polarity along the major body axes of the egg; these proteins control the local transcriptional activation of genes encoding master regulatory proteins in the zygote. Many proteins that act as master regulators of early development are transcription factors; others are components of pathways that mediate signaling between cells. Some fate decisions are made autonomously by individual cells; many fate decisions require communication and collaboration between cells. The same basic set of genes identified in Drosophila and the regulatory proteins that they encode are conserved in mammals and appear to govern major developmental events in many—perhaps all—higher animals. Introduction In all higher organisms, life begins with a single cell, the newly fertilized egg. It reaches maturity with thousands, millions, or even trillions of cells combined into a complex organism with many integrated organ systems. The goal of developmental biology is to unravel the fascinating and mysterious processes that achieve the transfiguration of egg into adult. Because we know more about development in these organisms, we will restrict most of our discussion to animal systems. The different cell types of the body are distinguished by the variety and amounts of the proteins that they express—the protein profile of each cell (that is, the quantitative and qualitative array of proteins that it contains). The protein profile of a cell in a multicellular organism i