Q.In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?

Concept and Intuition

The lac operon (Jacob and Monod's model of inducible gene regulation) has a single promoter-operator controlling three structural genes transcribed as one polycistronic mRNA: lacZ ($\beta$-galactosidase, hydrolyses lactose into glucose + galactose and also converts some lactose to allolactose, the real inducer), lacY (permease, transports lactose across the cell membrane into the cytoplasm), and lacA (transacetylase, whose exact metabolic role is less central but is transcribed along with z and y). The repressor protein, coded by the separate lacI gene (with its own independent promoter), binds the operator to block transcription in the absence of lactose.

Step-by-Step Solution

1. Identify the gene naming convention: lacZ, lacY, lacA correspond to $\beta$-galactosidase, permease, and transacetylase respectively.
2. Gene 'y' = lacY $\rightarrow$ codes for permease. …

Concept and Intuition

The lac operon is a classic example of negative inducible regulation. The lacI

gene constitutively produces a repressor protein. In the absence of lactose (the

inducer), this repressor binds the operator — a DNA sequence that overlaps/lies

next to the promoter — and this bound repressor sterically blocks RNA polymerase

from moving from the promoter into the structural genes, so transcription is

switched off. When lactose (via allolactose) is present, it binds the repressor,

changes its shape, and the repressor can no longer bind the operator, so RNA

polymerase transcribes lacZ, lacY, lacA freely.

Step-by-Step Solution

1. Identify the four choices as operon elements: Promoter (where RNA polymerase binds), Operator (the repressor's binding site), Inducer (lactose/allolactose, which binds the repressor, not DNA), Terminator (where transcription ends).
2. Recall the definition of the operator: a DNA sequence whose occupation by the repressor blocks transcription initiation.
3. The repressor protein's job, by definition, is to bind the operator — this is the textbook mechanism of lac operon repression. …

Concept and Intuition

The lac operon is the classic model of inducible gene regulation in bacteria. In the absence of an inducer, the repressor protein (encoded by the i gene) binds tightly to the operator region, physically blocking RNA polymerase from transcribing the structural genes (z, y, a), so lac mRNA is not made. When an inducer (allolactose, a derivative of lactose, or synthetic analogs like IPTG) is present, it binds to the repressor, causing a conformational change that reduces the repressor's affinity for the operator. The repressor then falls off the operator, RNA polymerase gains access, and transcription of the lac structural genes (hence lac mRNA synthesis) proceeds.

Step-by-Step Solution

1. Assertion: lac mRNA is synthesized in the presence of inducer - true, this is the defining feature of an inducible operon.
2. Reason: inducer suppresses repressor activity - true, this is the actual molecular mechanism (inducer binds repressor, changing its conformation). …

Concept and Intuition

The lac operon of E. coli is organised as a cluster of structural genes under the control of a single promoter/operator, plus a separate regulatory gene. The z gene encodes beta-galactosidase, the enzyme that cleaves lactose into glucose and galactose (and also catalyses the conversion of lactose into the actual inducer, allolactose). The y gene encodes permease, a membrane transport protein that actively pumps lactose into the bacterial cell. The a gene encodes transacetylase, whose exact physiological role is less central but is thought to detoxify non-metabolisable thiogalactosides. The i gene, positioned upstream, is the regulatory gene that codes for the lac repressor protein, which binds the operator to control transcription of z, y, and a.

Step-by-Step Solution

1. Identify z gene product: beta-galactosidase (lactose-hydrolysing enzyme).
2. Identify y gene product: permease (lactose transporter).
3. Identify a gene product: transacetylase.
4. Identify i gene product: repressor protein (regulatory, controls the operon). …

Concept and Intuition

The operon concept — a cluster of structural genes under the control of a common promoter/operator, switched on or off as a unit — was proposed in 1961 by François Jacob and Jacques Monod, based on their work on the lac operon of E. coli. Monod was primarily a biochemist who had studied bacterial growth kinetics and enzyme induction (diauxic growth, allostery), while Jacob was a geneticist who worked on bacterial conjugation and gene transfer with Élie Wollman. Together they explained how lac genes (lacZ, lacY, lacA) are coordinately regulated by a repressor protein and an operator sequence.

Step-by-Step Solution

1. Identify the two scientists credited with the lac operon concept: Jacob and Monod.
2. Assign roles: Monod = biochemist (enzyme/metabolic studies); Jacob = geneticist (gene transfer/genetic mapping studies). …

The key to this question is knowing the standard lac operon map and the function of each gene product. The operon is an inducible system: the repressor protein (made from the i gene) normally blocks transcription at the operator (o). When the inducer (lactose) binds the repressor, the repressor releases from the operator, allowing transcription of the z, y, and a genes. The diagram labels the repressor as D and the inducer as E, and the three enzymes from the polycistronic mRNA as A, B, and C in the order they are transcribed.

1. Identify the repressor and inducer (D and E).

   The diagram shows a red oval (D) with a small yellow/orange oval (E) attached. This represents the repressor protein (D) bound to the inducer molecule (E). In the lac operon, the repressor is encoded by the i gene, and the inducer is lactose (or allolactose). So D = repressor protein, E = inducer (lactose).

2. Identify the three enzymes (A, B, C) from the lac mRNA.

   The lac mRNA is transcribed from the z, y, and a genes in that order (left to right on the gene map). The three enzymes produced are:

   • z gene → β‑galactosidase (cleaves lactose into glucose and galactose)
   • y gene → permease (transports lactose into the cell)
   • a gene → transacetylase (a less critical enzyme that acetylates lactose derivatives) …