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050304GeneControlLactose

050304GeneControlLactose - 1 GENE CONTROL LACTOSE...

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1 GENE CONTROL GENE CONTROL LACTOSE LACTOSE
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2 INTRODUCTION TO MACROMOLECULAR SYNTHESIS Although there are exceptions, control of metabolic processes usually occurs at the initiation stage. A. Replication Replication of the chromosome is tightly regulated such that replication is begun once and only once per cell cycle and at the right time in the cell cycle. This regulation is exerted by a regulatory protein or proteins acting at the replication origin to control the initiation stage. Once DNA synthesis has begun, it normally proceeds to complete duplication of the entire chromosome. B. Gene Expression Gene expression generally means all the processes necessary to produce a functional product. That is, both transcription and translation (except for genes which do not produce protein products, such as tRNA and rRNA genes, in which case gene expression would involve transcription and processing of the RNA products). Gene expression also is tightly regulated, usually by regulating transcription. Transcription usually is regulated by controlling the initiation of transcription. Genes are subject to negative, positive, or negative and positive controllers. (Some genes are not regulated; they are transcribed at a constant, usually low, level and are said to be expressed constitutively.) Negative control is exerted by repressors which bind operators and inhibit initiation of transcription. Repressors can be either active or inactive, depending on the presence or absence of low molecular weight compounds in the environment (or other environmental factors). Positive control is exerted by activator proteins which bind activator binding sites near promoters and stimulate initiation of transcription. Activator proteins can be either active or inactive, depending on the presence or absence of low molecular weight compounds in the environment (or other environmental factors).
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