A Model of How a Gene May be Regulated
The DNA within each cell contains thousands of genes. However, at any given time, only a small percentage of the genes are having their code being transcribed. Generally, only those genes which have to do with the particular cell type are transcribed on a regular basis within that cell.
As an example, for skin cells, only the genes having to do with making the cell a skin cell are transcribed. Those genes having to do with making nerve or blood cell proteins are "switched" off!
How then do the genes on a strand of DNA know when to be on or off?
Only genes that are directly needed are transcribed, others are ignored.
Lac Operon: Section of DNA with a promoter/operator in bacteria (one kind of prokaryitic cell) that acts as an on/off switch for several genes that control the breakdown of the sugar lactose into glucose + galactose.
Promoter: DNA section where RNA polymerase binds.
Operator: DNA section where repressor binds
Structural gene: Gene that makes an enzyme or other protein
Regulator gene: A gene not found next to the operon that makes a repressor protein.
Repressor: protein that shuts off operon. If bound to operator, it prevents RNA polymerase from initiating transcription.
*Repressor protein (always present) binds to operator.
*RNA polymerase is not allowed to move to structural gene as the repressor blocks its path.
*mRNA transcription is stopped, and thus enzyme production stops.
*This feedback system occurs when lactose concentration in cell is low.
Transcriptional activator: binds to DNA, holds RNA polymerase against promoter so transcription occurs.
*Activator protein binds to promoter. The activator is partly a molecule found only when energy is needed (because glucose levels are low).
*RNA polymerase now stays on promoter.
*RNA polymerase transcribes the structural genes
Positive Feedback & the Lac operon
*Activator (lactose) binds to repressor, changes its shape & it pops off operator.
*RNA polymerase binds to promoter and can now move past operator.
*RNA polymerase transcribes mRNA from structural gene.
*mRNA moves to ribosome where translation synthesizes the enzyme.
*The enzyme now breaks down the abundant lactose to glucose + galactose.