How is gene expression regulated Describe pretranscriptional

How is gene expression regulated? Describe pre-transcriptional control, particularly with respect to prokaryotice operon model systems. What is one example of a post-transcriptional control?

Solution

Gene regulation is a label for the cellular processes that control the rate and manner of gene expression.A complex set of interactions between genes,RNA molecules,proteins (including transcription factors) and other components of the expression system determine when and where specific genes are activated and the amount of protein or RNA product produced.Some genes are expressed continuously,as they produce proteins involved in basic metabolic functions;some genes are expressed as part of the process of cell differentiation;and some genes are expressed as a result of cell differentiation.Transcription factors are proteins that play a role in regulating the transcription of genes by binding to specific regulatory nucleotide sequences.
Mechanisms of gene regulation include:

Bacterial gene expression can be regulated by operons,which can either be inducible or repressible.Usually these genes, which are effectively turned on all the time,code for enzymes that the cell needs in fairly large amounts for its major life processes.Glycolysis enzymes are examples.
Pre-transcriptional Control-Two genetic control mechanisms known as repression and induction regulate the transcription of mRNA,and consequently,the synthesis of enzymes from them.These mechanisms control the formation and amounts of enzymes in the cell, not the activities of the enzymes.
Repression- The regulatory mechanism that inhibits gene expression and decreases the synthesis of enzymes is called repression.Repression is usually a response to the overabundance of an end product of a metabolic pathway;it causes a decrease in the rate of synthesis of the enzymes leading to the formation of that product Repression is mediated by regulatory proteins called repressors,which block the ability of RNA polymerase to initiate transcription from the repressed genes.
Induction-The process that turns on the transcription of a gene or genes is induction.A substance that initiates transcription of a gene is called an inducer,and enzymes that are synthesized in the presence of inducers are inducible enzymes.The genes required for lactose metabolism in E. coli are a well-known example of an inducible system.

Post-transcriptional regulation mechanisms comprise various processes such as mRNA processing (polyadenylation,capping and splicing),mRNA export and localization,mRNA decay,and mRNA translation.