乳糖操縱子LactoseOperonforgraduatestudents.pptVIP

乳糖操纵子Lactose Operonfor graduate students 牛登科 生命科学学院 2010年10月 Lactose Operon Structural genes lac z, lac y, lac a Promoter Polycistronic mRNA Regulatory gene Repressor Operator Operon Inducer - lactose i Operon RegulatoryGene p o z y a DNA m-RNA β-Galactosidase Permease Transacetylase Protein Lactose Operon Inducer -- lactose Absence Active repressor No expression i p o z y a No lac mRNA Absence of lactose Active i p o z y a Β-Galactosidase Permease Transacetylase Presence of lactose Inactive Presence Inactivation of repressor Expression Negative control Is the story complete? These two pages are copied from the PPT of an American teacher. Has your textbook told you something more? If no, are you satisfied with this theory? No lactose, no genes transcribed, no proteins produced, and then nothing to transport lactose into the cell. A dead cycle Basal transcription The repressor could not entirely repress the genes. In absence of lactose, there is a low level of leaky gene expression of lactose operon genes. The negative control of lactose operon is not yes or no, but more or less. In fact, in eukaryotes, there are also many genes that are transcribed and translated, at low levels, in the cells that obviously do not need them. And the reason for this is not clear, awaiting for you to discover. Do you still have any question? Why operon? Operons contain genes belonging to the same functional pathway in order to guarantee them a similar expression level. About half of the protein-coding genes are organized in operons. However, prokaryotic genomes are rather unstable and only 5–25% of genes belong to strings shared by at least two distantly related species. The biological significance and mechanism(s) of operon formation are still under debate. At least seven different models have been proposed for operon formation. For details, see Res. Microbiol. 160: 502-512, 2009. More questions? Why negative control? That is, is there any trouble if the lactose operon gen