MCDB 138 Lecture 1


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LECTURE 1: INTRODUCTION: THE GENETIC CONTROL OF DEVELOPMENT 1. WHAT IS DEVELOPMENT? Development is the process by which a single cell (the fertilized egg) containing a single nucleus undergoes cleavage and growth, giving rise to a multicellular organism consisting of many different cell types (carrying out different functions). Consider the problem that confronts the fertilized egg: one cell (~100 μ m in diameter) gives rise to an organism composed of trillions (10 12 ) (in humans, for example) of cells (of many different cell types) organized into tissues, which are organized into organs, which are organized into organ systems. The most important concept that we begin with in this course is that development is controlled by genes . Although this seems obvious to everyone now, this was not always the case. Lacking the understanding of genetics that we have today, earlier philosophers and scientists had a difficult time imagining how a simple, uniform appearing structure like an egg (think of a chicken egg) could undergo such a remarkable transformation over the course of a number of days to give rise to a complex organism of many different parts (think of the chicken that hatches out of the egg). Some thought that a tiny, preformed organism was present in the egg or sperm and simply grew during embryogenesis. We now know that embryogenesis is a process of generating increasing complexity (this process is called epigenesis ) by the action of different genes in different cells. As we discuss below, this process depends on the fact that, although all cells in the organism have the same genetic information, different cells express different genes at different times. The goal of the present day developmental biologist is to understand how this differential gene activity is brought about, and how the expression of different genes in different cells causes them to have different developmental fates and functions. 2. GENOMIC EQUIVALENCE Blastomere isolation and nuclear transplantation experiments demonstrate the principle of genomic equivalence . This is the idea that each genome (the sum total of genetic information, or DNA) in each cell of the body has equivalent potency , or ability to direct development. Totipotency means the ability to develop into a whole organism (for a fertilized egg or an isolated blastomere), or to direct the complete development of an organism (for a transplanted nucleus). If an isolated blastomere or a transplanted nucleus is totipotent , then it cannot
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have undergone irreversible changes prior to the time it is isolated or transplanted. If a nucleus can be transplanted from a differentiated cell and shown to be totipotent or pluripotent (able to direct development of multiple cell types, in addition to its own prospective cell fate), then irreversible changes (loss of genetic information) cannot have taken place during differentiation. Genomic equivalence
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