Question

Match the components of $lac$ operon of Escherichia $coli$ and find out the correct combination.

	Column I		Column II
a.	Structural gene	p	Binding site for repressor protein
b.	Operator	q	Codes for repressor protein
c.	Promoter	r	Induces lactose transport from the medium
d.	Regulator gene	s	Codes for enzyme proteins
		t	Binding site for RNA-polymerase

• A.
  $\begin{array}{cccc}\text{a}& \text{b}& \text{c}& \text{d}\\ \text{t}& \text{s}& \text{q}& \text{p}\end{array}$
  
• B.
  $\begin{array}{cccc}\text{a}& \text{b}& \text{c}& \text{d}\\ \text{s}& \text{p}& \text{t}& \text{q}\end{array}$
  
• C.
  $\begin{array}{cccc}\text{a}& \text{b}& \text{c}& \text{d}\\ \text{q}& \text{t}& \text{p}& \text{r}\end{array}$
  
• D.
  $\begin{array}{cccc}\text{a}& \text{b}& \text{c}& \text{d}\\ \text{r}& \text{s}& \text{t}& \text{p}\end{array}$
  

Answer 1

The correct option is B

$\begin{array}{cccc}\text{a}& \text{b}& \text{c}& \text{d}\\ \text{s}& \text{p}& \text{t}& \text{q}\end{array}$

Structural gene - Codes for enzyme proteins
The structural genes present in lac operon are $lacz$, $lacy$ and $laca$. The $lacz$ gene codes for β-galactosidase enzyme, $lacy$ gene codes for permease enzyme, $laca$ gene codes for transacetylase enzyme.

Operator - Binding site for repressor protein
Operator is a genetic sequence that lies adjacent downstream to the promoter region of DNA and regulates the synthesis of RNA from the downstream structural genes. The repressor protein binds to the operator region and helps to switch off the operon. This is because the RNA polymerase cannot bind properly to the promoter which has overlaps with the operator region.

Promoter - Binding site for RNA polymerase
Promoter is the sequence of DNA that serves as the site for initial binding of RNA polymerase to proceed with transcription.

Regulator gene - Codes for repressor protein
Regulator gene ($laci$) codes for the repressor protein which binds to the operator region and switches off the operon.