Biology · Molecular Basis of Inheritance · NEET
The lac operon has four kinds of DNA regions in a row: a promoter (where RNA polymerase binds), an operator (where the repressor sits), and three structural genes z, y and a. Just upstream there is the regulatory gene i that makes the repressor. NEET often asks you to pick which part is NOT in the operon (answer: an enhancer, because enhancers are eukaryotic).
The three structural genes are z, y and a. Remember them as "Zoya". The z gene makes beta-galactosidase, the y gene makes permease, and the a gene makes transacetylase. These three share a single common promoter and are transcribed together as one long mRNA (polycistronic).
The i gene codes for the repressor protein. "i" stands for inhibitor, not "inducer" (this trips many students). The i gene is a regulatory gene and has its own promoter, so strictly it is not a structural gene of the operon. It is expressed all the time (constitutively) to keep making repressor.
beta-galactosidase (z) breaks lactose into glucose and galactose. Permease (y) makes the cell membrane more permeable so lactose can enter the cell. Transacetylase (a) transfers an acetyl group. NEET frequently gives a matching question: i-repressor, z-beta-galactosidase, y-permease, a-transacetylase.
Francois Jacob (a geneticist) and Jacques Monod (a biochemist) worked out the lac operon. It was the first example of transcriptional gene regulation to be explained. NEET has repeatedly asked this in match-the-scientist questions, so memorise the pair Jacob and Monod for lac operon.
The lac operon is an inducible operon. It stays OFF by default and is switched ON when lactose (the inducer) is present. This is the opposite of a repressible operon (like the trp operon), which is normally ON and switched off when the end product builds up.
Match the following genes of the Lac operon with their respective products: (a) i gene (b) z gene (c) a gene (d) y gene | (i) beta-galactosidase (ii) Permease (iii) Repressor (iv) Transacetylase
All of the following are part of an operon except
Which of the following statements about the lac-operon is correct?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An operon is a group of genes controlled together as one unit. It has a promoter, an operator and structural genes that are switched on or off as a set. The lac operon of E. coli is the classic example.
"Lac" comes from lactose. The operon controls the enzymes needed to take in and break down lactose sugar, so it is the lactose (lac) operon.
Three: z, y and a. The z gene makes beta-galactosidase, y makes permease, and a makes transacetylase. They share one promoter and are transcribed as a single polycistronic mRNA.
The i gene is a regulatory gene that lies close to the operon but has its own separate promoter and is expressed all the time. It codes for the repressor, so it is not counted as a structural gene of the operon.
The lac operon is inducible (normally OFF, turned ON by lactose). The trp operon is repressible (normally ON, turned OFF when tryptophan builds up). NEET can ask you to compare these two types.
