lecturenotes1 - Lecture 1: Introduction: The genetic...

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1 Lecture 1: Introduction: The genetic control of development 1. What is development? Development is the process by which a single cell (the fertilized egg or zygote) 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 of cells (10 12 in humans for example) of many different types that are 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, early philosophers and scientists had a difficult time imagining how a simple, uniform appearing structure like a chicken egg could undergo such a remarkable transformation over the course of a number of days to give rise to the newly hatched chick, a complex organism of many different parts. Some thought that a tiny, preformed organism was present in the egg or sperm and simply grew during embryogenesis (preformationism). 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 leads to 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 isolated blastomere), or to direct the complete development of an entire organism (for a transplanted nucleus). If an isolated blastomere or a transplanted nucleus is totipotent , then it cannot 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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This note was uploaded on 07/07/2010 for the course MCDB 138 taught by Professor Lyon during the Summer '07 term at UCLA.

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lecturenotes1 - Lecture 1: Introduction: The genetic...

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