Heredity and Development Second Edition (1972) / Chapter Skim
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13 Developmental Control of Genetic Systems
13 Developmental Control of Genetic Systems
Pages 279-288
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From page 279... ...
We are sure of this because the genetic systems of ova and sperm can be tested by uniting them and studying the phenotype of the new individual. Only in exceptional circumstances, can we test as rigorously the genetic systems of differentiated somatic cells.
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From page 280... ...
The differentiation of these two cell types begins in the early embryo: red blood cells arise from the mesoderm and islet cells from the endoderm. As the embryo develops, the two types of cells become increasingly different morphologically and finally differ in the specific proteins that they synthesize.
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From page 281... ...
Spemann had been able to show that a single nucleus from a salamander embryo in the 16-cell stage, plus some cytoplasm, was able to develop into an entire embryo. Technical problems prevented his testing the developmental potentialities of older nuclei.
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From page 282... ...
Subsequently, John Gurdon of Oxford University was able to obtain normal embryos when the injected nucleus came from a larva, in which the cells were fully differentiated, or even from nuclei of adult cells. These discoveries gave added support to the hypothesis that all of an individual's nuclei are genetically identical.
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From page 283... ...
Its contents flow back into one of the cells, known as D at the end of the second cleavage. Third cleavage divides the embryo into four slightly smaller animal cells, known as 1a, 1b, 1c, and 1d, and four slightly larger vegetal cells known as 1A, 1B, 1C, and 1D.
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From page 284... ...
Similarly, the materials necessary for the apical tuft are first in the vegetal hemisphere and then in the first polar lobe, CD cell, D cell, and then in the 1d cell. The polar lobes do not contain a nucleus so the substances responsible for the apical tuft and the posttrochal region must be cytoplasmic.
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From page 285... ...
They conceive, instead, that external conditions or cytoplasmic substances interact with a uniform cellular genetic system to provide for differentiation. Though the genetic system specifies what a cell may do, nongenetic phenomena influence what it actually does.
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From page 286... ...
Thus an initial stimulus, which has nothing to do with the nucleus, controls the beginning of a series of events that is of profound importance in cellular differentiation. The protuberance begins when the embryo contains a single nucleus.
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From page 287... ...
Once this has been brought about, one can suggest models for the manner in which cellular differentiation can proceed, even with nuclei that are at first functionally identical. Cellular differentiation may be thought of as an interaction between nuclei, which are genetically identical, and different cytoplasmic regions.
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From page 288... ...
1968. ‘The cytoplasmic control of nuclear activ ity in animal development.' Biological Reviews 43:233–67.
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