The genetic information present in each normal somatic cell of a metazoan organism is practically identical. The genetically reproducible, hardwired exceptions are found in those few cells that have amplified or rearranged genes in order to perform specialized cellular functions. Of course, in various disease states chromosome integrity is altered (ie, cancer) sometimes even at the whole chromosome level (eg, trisomy 21, that causes Down syndrome). Expression of the genetic information must be regulated during ontogeny and differentiation of the organism and its cellular components. Furthermore, in order for the organism to adapt to its environment and to conserve energy and nutrients, the expression of genetic information must be cued to extrinsic signals and respond only when necessary. As organisms have evolved, more sophisticated regulatory mechanisms have appeared which provide the organism and its cells with the responsiveness necessary for survival in a complex environment. Mammalian cells possess about 1000 times more genetic information than does the bacterium Escherichia coli. Much of this additional genetic information is likely involved in regulation of gene expression during the differentiation of tissues and biologic processes in the multicellular organism and in ensuring that the organism can respond to complex environmental challenges.

In simple terms, there are only two types of gene regulation: positive regulation and negative regulation (Table 1). When the expression of genetic information is quantitatively increased by the presence of a specific regulatory element, regulation is said to be positive; when the expression of genetic information is diminished by the presence of a specific regulatory element, regulation is said to be negative. The element or molecule mediating negative regulation is said to be a negative regulator, a silencer or repressor; while the element mediating positive regulation is a positive regulator, an enhancer or activator. However, a double negative has the effect of acting as a positive. Thus, an effector that inhibits the function of a negative regulator will appear to bring about a positive regulation. Many regulated systems that appear to be induced are in fact derepressed at the molecular level.

Table1. Effects of Positive & Negative Regulation on Gene Expression

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مواضيع ذات صلة

Epigenetic Mechanisms Contribute Importantly to the Control of Gene Transcription

The Chromatin Template Contributes Importantly to Eukaryotic Gene Transcription Control

The Genetic Switch of Bacteriophage Lambda (λ) Provides Another Paradigm for Understanding the Role of Conditional Regulatory Protein-DNA Interactions in Transcriptional Control in Eukaryotic Cells

Analysis of Lactose Metabolism in E. coli Led to the Discovery of the Basic Principles of Gene Transcription

Biologic Systems Exhibit Three Types of Temporal Responses to a Regulatory Signal

Posttranslational Processing Affects the Activity of many Proteins

Like Transcription, Protein synthesis can be described in three phases: Initiation, Elongation, & Termination

Mutations result when changes occur in the Nucleotide Sequence

The Genetic Code is Degenerate, Unambiguous, Nonoverlapping, Without Punctuation, & Universal

RNAs can be Extensively Modified

RNA Molecules are Extensively Processed Before They Become Functional

Phosphorylation Activates Pol II