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Lac operon is the operon or the gaggle of genes with a single promoter, the purpose of which is to help in the transport or metabolism of lactose in E, coli and a number of other bacteria.
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Keyterms: Lac operon, operon, metabolism, lactose, E. coli, bacteria cell, glucose, energy, beta-galactosidase
Read more: Central dogma
Although the breakdown of glucose holds the preferred place to fuel up the bacteria (because glucose takes less energy as well as steps to complete the whole process) but when the glucose is not available in the bacteria cell, E. coli looks for the availability of the lactose and then it starts the breakdown process through the activity of beta-galactosidase for fulfilling the same purpose.
The Lac Operon
The video below explains this:
Lac Operon Detailed Video Explanation:
Before the breakdown process happens, the bacteria needs to stimulate the lac operon genes which again sparks up the enzymes for lactose uptake as well as for metabolism. Two regulatory proteins work for checking the levels of lactose and glucose. The first one is a lac repressor which works as a lactose sensor and the other one is catabolite activator protein (CAP) which acts as a glucose sensor. These proteins regulate the DNA transcription on the basis of lactose and glucose levels.
The lac operon consists of one regulatory i gene (‘i’ is for ‘inhibitor’) and three structural genes (lacZ, lacY, lacA). These all genes are transcribed as a single mRNA and all of these three genes code for the three different yet three integral parts which take place in-order-to serve the one common purpose i.e., fueling up the bacteria.
- i gene helps in ciphering for the repressor of the lac operon.
- Z gene codes for beta-galactosidase.
- Y gene codes for permease.
The genes here These all three gene products perform an integral role for the metabolism of lactose. Additionally, the lac operon consists of a few other DNA sequences which regulate the process of DNA transcription.
- The promoter signifies the binding place for RNA polymerase to perform transcription.
- The operator is a negative site bound by the lac repressor. This operator overlays with the promoter.
- CAP is a positive site which stays bounded by CAP. CAP helps in the advancement of the transcription process by helping RNA polymerase get into the promoter.
Read More: Bioinformatics
The lac repressor
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- This protein inhibits the transcription of the lac operon.
- It works by binding with the operator and it also partly overlaps with the promoter.
In the absence of lactose, the binding of lac repressor and operator gets tight to prevent the transcription by RNA polymerase but when lactose is present the lac repressor can’t get binded to DNA.
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Catabolite Activator Protein
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- CAP binds to a place of DNA just before the lac operon binds with the promoter and helps RNA polymerase to get binded to the promoter to achieve the level of transcription.
- CAP only gets activated when the glucose level is low and obviously when the cAMP levels are high enough to help the lac operon to be transcripted.
Read More: DNA Packaging
Sample Questions
Ques. Why is the regulation by the repressor in lac operon called negative regulation? (1 mark)
Ans. In the lac operon control system, the expression remains blocked by an active repressor that switches off the transcription process.
Ques. Is the lac operon present in the human body? (1 mark)
Ans. Lac operon is common in bacteria but it is very rare in human bodies. Actually, lac operon consists of those kinds of genes that encode proteins which are raised through the uptake and metabolism of lactose.
Ques. What are the two types of lac operon? (1 mark)
Ans. The two types of lac operon are inducible and repressible. Inducibility gets switched on in which a special chemical is presented whereas repressibility is normally active and the enzymes produced by its natural genes are already present in the cell.
Ques. How long does the lac operon process work? (1 mark)
Ans. As long as the lactose is present, the lac operon process continues, but the process gets stopped as soon as all the lactose is converted into galactose and glucose.
Ques. What will be an example of lac operon? (1 mark)
Ans. An inducible operon will be an example of lac operon. As long as the lactose is present in the cell, it will be converted into allolactose which works as an inducer and binding to the repressor and prevents the repressor from binding to the operator.
Ques. Describe Meselson and Stahl's experiment that was carried in 1958 on E.Coli. Write the conclusion they arrived at after the experiment.(3 marks)
Ans. The experiment led out by Meselson and Stahl was to denote the DNA semi-conservative nature of replication. For the experiment they made use of two sort E. coli cultures. In one culture it was growing in medium of N14 isotope and in case of the second it was growing in N15 isotopes. They were left to develop for several generations and put through density gradient centrifugation after which it was observed that the cells growing on N14 medium had the two strands of N14 whereas the cells of N15 medium had the two strands of N15, which did settle down during the process of centrifugation because of the increased density. While in case of the second experiment the N15 culture was moved to the N14 medium. The cells were put through density gradient centrifugation after a single generation after which it was found that the daughter cells gave rise to the bands of DNA which was placed in between the N14 and N15. It was concluded that the final daughter DNA had two strands of N14 and N15 each that provides a proof of semiconservative nature of DNA replication.
Ques. Write the different components of a lac-operon in E.coli. Explain its expression while in an ‘open’ state.(3 marks)
Ans. The different components of a lac-operon in E. coli are:
- Regulatory gene
- Promoter gene
- Operator gene
- Three structural genes
- Inducer
In case when the lactose is absent the gene gives rise to repressor proteins and the repressor protein helps in binding the operator. It helps in preventing the RNA polymerase to bind the operon, hence the operon is switched off.
When the lactose is present, it helps in binding the inducer to the repressor and form an inactive repressor. Afterwards the RNA polymerase is bound to the operator and reproduces lac mRNA. The lac operon is switched on and leads to the production of three types of enzymes: β - galactosidase, transacetylase and permease.
Ques. How are the structural genes activated in the lac operon in E.coli?(2 marks)
Ans. Lactose is the base inducer in the lac operon model which helps in activating the structural genes. When the lactose is present the repressor gets diffused and the RNA polymerase helps in binding to the operator to upgrade the translation of m-RNA strands into the relevant proteins by the structural genes. The structural genes that inscribes the proteins include: β - galactosidase being inscribed by lacZ (LacZ), lactose permease inscribed by lacY (LacY), galactoside O-acetyltransferase inscribed by LacA (LacA).
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