Control of Gene Expression

By gene expression we mean the transcription of a gene into mRNA and its subsequent translation into protein. Gene expression is primarily controlled at the level of transcription, largely as a result of binding of proteins to specific sites on DNA. In 1965 Francois Jacob, Jacques Monod, and Andre Lwoff shared the Nobel prize in medicine for their work supporting the idea that control of enzyme levels in cells is regulated by transcription of DNA. occurs through regulation of transcription, which can be either induced or repressed. These researchers proposed that production of the enzyme is controlled by an "operon," which consists a series of related genes on the chromosome consisting of an operator, a promoter, a regulator gene, and structural genes.

The operator gene is the sequence of non-transcribable DNA that is the repressor binding site. There is also a regulator gene, which codes for the synthesis of a repressor molecule hat binds to the operator

The repressor is normall bound to the operator, effectively blocking transcription. If lactose binds to the repressor, the repressor is released, and RNA polymerase can then proceed with transcription.


Binding of tryptophan to the repressor activates the repressor and prevents RNA polymerase from transcribing more mRNA.


Control of Gene Expression in Eukaryotes

Eukaryotic cells have similar mechanisms for control of gene expression, but they are more complex. Consider, for example, that prokaryotic cells of a given species are all the same, but most eukaryotes are multicellular organisms with many cell types, so control of gene expression is much more complicated. Not surprisingly, gene expression in eukaryotic cells is controlled by a number of complex processes which are summarized by the following list.

Showing how segments of DNA are wrapped around histones


The initial mRNA transcript has introns, i.e., segments of RNA that are then removed. The remaining exons are then spliced together to create the final transcript which has the correct coding sequence.




Post-translational processing of insulin involves folding, cleavage of the bend, and insertion of disulfide cross-links between the two resulting strands.


Gene expression being influenced by other cells. The signalling cell elaborates a signal molecule that binds to a receptor on a target cell, setting in motion a sequency of events that initiate synthesis of a particular protein.



Some RNA virus will invade cells and introduce double-stranded RNA which will use the cells machinery to make new copies of viral RNA and viral proteins. The cell's RNA interference system (RNAi) can prevent the viral RNA from replicating. First, an enzyme nicknamed "Dicer" chops any double-stranded RNA it finds into pieces that are about 22 nucleotides long. Next, protein complexes called RISC (RNA-induced Silencing Complex) bind to the fragments of double-stranded RNA, winds it, and then releases one of the strands, while retaining the other. The RISC-RNA complex will then bind to any other viral RNA with nucleotide sequences matching those on the RNA attached to the complex. This binding blocks translation of viral proteins at least partially, if not completely. The RNAi system could potentially be used to develop treatments for defective genes that cause disease. The treatment would involve making a double-stranded RNA from the diseased gene and introducing it into cells to silence the expression of that gene. For an illustrated explanation of RNAi, see the short, interactive Flash module at

The RNA interference system is also explained more completely in the video below from Nature Video.

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