The regulation of the genes for metabolism of lactose is one of the best understood systems of regulation of gene expression (i.e. it's a "model system"). Control of gene expresssion can be exerted at several different points such as at transcription of DNA into mRNA and at translation.
In prokaryotes such as bacteria, no nucleus separates DNA from ribosomes in the cytoplasm. When nutrient supply is high, transcription proceeds rapidly. Translation occurs even before mRNA transcripts are finished. Seventy-five different operons controlling 250 structural genes have been identified for E. coli. Can you spot the location of the lac operon on this bacterial chromosone gene map?
The lactose operon is composed of three genes coding for different proteins, plus a promoter gene and an operator gene. A regulator gene nearby codes for a repressor protein that binds to the operator when lactose concentrations are low and effectively blocks RNA polymerase's access to the promoter. Transcription is blocked
This animation (Audio - Important) describes the lac operon.
When milk is consumed and lactose levels are high , the lactose binds to the repressor changing its shape and effectively removing its blockage of the promoter. Then RNA polymerase can initiate transcription of the genes.
Here is a lac operon animation from the University of Virginia.
Take a look at the lac operon tutorial.
These two animations (Audio - Important) review negative control of the lac operon:
negative control of the lac operon 1,
negative control of the lac operon 2.
So, in the case of low glucose , an activator protein called CAP becomes active and forms a complex with cAMP and turns on the lactose metabolism genes.. CAP will adhere to promoter only when in complex with cAMP.
When glucose levels are high and there is little cAMP, CAP cannot be activated. The promoter is not good at binding RNA polymerase. The lactose-metabolizing genes are not transcribed very much.
When glucose levels are low, cAMP accumulates. The CAP-cAMP complex forms and binds to the promoter. RNA polymerase can now bind and the lactose-metabolizing genes are transcribed rapidly.
This animation (Audio - Important) reviews positive control of the lac operon.
REVIEW: The lactose operon includes
REVIEW: The obvious advantage of the lactose operon system is that
REVIEW: The positive control of the lactose operon in bacteria is
REVIEW: A base sequence signalilng the start of a gene is a(n) _____.
REVIEW: In prokaryotic cells but not eukaryotic cells, a(n) _____ precedes the genes of an operon .
REVIEW: An operator most typically governs _____ .
REVIEW: Eukaryotic genes guide _____ .
a. fast short-term activities
b. overall growth
c. development
d. all of the above
REVIEW: Which of the following is the region that is the binding site for RNA polymerase?
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