68. Which of the following statements about IPTG and lactose is NOT correct?
1. IPTG is an analog of lactose
2. Both IPTG and lactose are inducers of the lac operon
3. IPTG is not metabolized while lactose is metabolized by the cells
4. Both IPTG and lactose need permease for entry into the cell.

Introduction

In bacterial systems, the lac operon plays a critical role in regulating the metabolism of lactose. The operon is induced by molecules that activate the lac repressor, allowing transcription of genes needed to break down lactose. Two well-known inducers of the lac operon are IPTG (Isopropyl β-D-1-thiogalactopyranoside) and lactose. While both act as inducers, they have different properties and roles. Let’s explore the correct and incorrect statements about IPTG and lactose.

Key Characteristics of IPTG and Lactose

1. IPTG as an Inducer:

   • IPTG is a synthetic molecule and a structural analog of lactose, designed to mimic the natural inducer of the lac operon. It binds to the lac repressor, causing it to dissociate from the operator, which in turn allows the transcription of the operon.

   • A significant feature of IPTG is that it is not metabolized by the cell. This allows researchers to use it to induce gene expression without the complication of degradation during the experiment.

2. Lactose as an Inducer:

   • Lactose is the natural inducer of the lac operon. It is a disaccharide sugar that can be metabolized by E. coli. When lactose enters the cell, it is metabolized to allolactose, which binds to the lac repressor, similarly inducing the lac operon.

3. Permease Requirement:
   Both IPTG and lactose require a permease (a transporter protein) for entry into the cell. The lacY gene encodes this permease, which facilitates the transport of lactose and IPTG into the bacterial cell.

Which Statement is NOT Correct?

1. IPTG is an analog of lactose:
   This statement is correct. IPTG mimics lactose in structure and function, but it is not metabolized by the cell.

2. Both IPTG and lactose are inducers of the lac operon:
   This statement is also correct. Both molecules can bind to the lac repressor and induce the lac operon.

3. IPTG is not metabolized while lactose is metabolized by the cells:
   This statement is correct. IPTG is not metabolized, whereas lactose is metabolized into glucose and galactose.

4. Both IPTG and lactose need permease for entry into the cell:
   This statement is correct. Both IPTG and lactose rely on the lacY permease to enter the cell.

Conclusion

All of the statements are correct regarding IPTG and lactose, except for the one in question. Therefore, there seems to be a misunderstanding in the question phrasing, as all the provided statements are accurate.

However, if the intent is to find a more specific difference or error, the given options remain valid as described.