chapter 18. Regulation of gene expression

Q.what is promoter?

it is a base-pair sequence that sepecifies where transcription begins.


Q. what is significant that transcription and translation in prokaryotic cell?


prokaryotic had no nucleus to separate the processese of transcription and translation; so they are coupled.


Q. what is rRNA? ribosomal RNA with ribosomal proteins makes up the ribosomes. the prganelles translate the mRNA





facts


1.Gene transcription can be switched on and off by gene regulation proteins


2.As long as the operator remains free of the repressor, RNA polymerase that recognizes the promoter can transcribe the operon's structural genes into mRNA. The operon is ON.


3.In an E. Coli cell growing in the absence of lactose, a repressor protein binds to the operator, preventing RNA polymerase from transcribing the lac operon's genes. The operon is OFF.


4.Glucose and lactose levels control the initiation of transcription of the lac operon, i.e. whether the lac operon is switched "ON" or "OFF".


5..An operon is a cluster of bacterial genes along with an adjacent promoter that controls the transcription of those genes.




diagram





lactose absent, repressor active, peron off. the lac repressor is innately active, and in the absence of lactose it switches off the operon by binding to the operator. Lactose present, repressor inactive, operon on. allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. in this way, the enzymes for lactose utilization are induced.








video

http://www.youtube.com/watch?v=oBwtxdI1zvk

summary





The general term for the product of a regulatory gene is a regulatory protein. The Lac regulatory protein is called a repressor because it keeps RNA polymerase from transcribing the structural genes. Thus the Lac repressor inhibits transcription of the lac operon.The effect of the Lac repressor on the lac genes is referred to as negative regulation.Allolactose binds to an allosteric site on the repressor protein causing a conformational change. As a result of this change, the repressor can no longer bind to the operator region and falls off. RNA polymerase can then bind to the promoter and transcribe the lac genes.