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Index >> Regulation of Protein Synthesis - Operon >> Regulation of Gene Activity in Eukaryotes

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Regulation of Protein Synthesis - Operon Structural Genes Operator Genes Promoter Gene Regulator Gene Control Mechanism - Induction Control Mechanism - Repression Inducible System Repressible System Negative Control of Protein Synthesis Positive Control of Protein Synthesis Mutation in the Operon

Regulation of Gene Activity in Eukaryotes

	Regulation of Gene Activity in Eukaryotes - Genes are active only when their products are required by the cell. The rest of the time they are 'switched off'. Thus in mouse liver cells only about 3% of the genes are active (i.e. transcribe RNA) and in brain cells about 9% are turned on. In prokaryotes control of gene activity takes place by means of special repressor proteins transcribed by regulator genes;
Binding of the repressor protein to the operator gene blocks the movement of the transcribing enzyme RNA polymerase, and thus prevents RNA transcription by the structural genes. In eukaryotes gene regulation takes place by a different mechanism. The possible candidates for regulation of gene activity are the proteins associated with DNA in the chromosomes. These include histones and non histone chromosomal proteins (NHC proteins). It is unlikely that histones selectively repress gene activity since they have almost identical amino acid compositions in different organisms.
It has been suggested that histones probably mask DNA in a nonspecific manner (non-specific repression). Because histones are rich in lysine and arginine they have a net positive charge and are basic proteins. The positively charged groups, which appear to be arranged in clusters, could interact with the negative charge of the phosphate backbone of DNA. This would greatly reduce the charge repulsion of the phosphate groups and would facilitate supercoiling of DNA.
The RNA polymerase molecule would not be able to move along the supercoiled DNA, and thus no transcription would be possible. It is thus possible that histones prevent transcription by bringing about supercoiling of DNA. This is shown by the fact that incorporation of I RNA precursors in eukaryote cells takes place only in regions of the chromosome where the chromatin is diffuse and not in the highly compact regions. The highly compact heterochromatic regions of the chromosome are therefore inert and cannot carry out transcription.
	Among the models proposed to explain gene regulation in higher cells are: (i) Frenster's (1965) model of gene specific derepressor RNA, (ii) models in which non histone proteins act as derepressors e.g. those proposed by Paul and co workers (1971) and Stein et aZ (1975) and (iii) Britten and Davidson's operon-aperator model (1969).

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Regulation of Gene Activity in Eukaryotes Frenster's Model of Gene Regulation Nonhistone Protein as Derepressors Britten Davidson Operon Operator Model Sensor Genes Producer Genes Integrator Genes Receptor Genes

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