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STUDY GUIDE—CHAPTER 19 BIOLOGY II Differential Gene Expression in Development - Development -The process by which multicellular organisms undergo a sequence of changes that allow them to acquire the forms that characterize their life cycle. Major events in development: o Determination -setting of the developmental fate of cell—even before it has taken on the feature of that cell type. o Differentiation - when determined cells become a particular type of cell. o Morphogenesis -shaping of differentiated cells into the tissues and organs of the multicellular organism. o Growth -the increase in size of the organism via mitosis and cell expansion. o Cell Movement -limited to animals since the rigid cell wall of plants prevents movement. o Apoptosis -programmed cell death [Fig 19.1] CELL FATE: WHAT A CELL BECOMES (AND DOES) IS A RESULT OF: -Differential Gene Expression Which is determined by internal cell events which, in turn, may be triggered by external signals. o Which signal(s) a cell receives (influencing its developmental path via determination and differentiation) often depends on its location within the developing organism. [Fig 19.2] -Cell fate in early embryo are not determined, but can change depending on the environment (localization embryo)* In general, the potential fates of successively produced cells narrow over developmental time (especially during embryotic development) CAN DIFFERENTIATION BE REVERSED? - By definition, a zygote it totipotent: has the ability to produce cell descendants via mitosis that can differentiate into every type of cell Descendant cells of animals usually lose totipotency (result of determination and differentiation) –But, some early cells may retain totipotency (embryotic stem cells). However, in plants, many cells of the adult retain totipotency.
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[Fig 19.3] -In animals: differentiated descendants cells usually retain a complete copy of the genome (they are genomically equivalent to the zygote) and, therefore have the potential to reacquire totipotency. Nuclear transpiantation experiments have demonstrated that for many animals early embryonic cells retain both genomic equivalency and the ability to have all of their genes expressed in the cytoplasmic environment of an enucleated egg. -The cloning of Dolly (the sheep) demonstrated that even nuclei of adult animal cells may be stimulated to restore totipotency. [Fig 19.4] - Pluripotent cells (stem cells with a narrower range of fates than embryonic stem cells) and embryonic stem cells (totipotent) may be used therapeutically. [Fig 19.6-19.8] DEVELOPMENTAL GENES PLAY A MAJOR ROLE IN DEVELOPMENT BY ACTING AS ON-OFF SWITCHES -The turning on of a developmental gene usually leads to a cascade of other genes being turned on (or off) and results in differentiating in particular ways—i.e. the control of gene expression is crucial to development.
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