DNA contains genetic information in the form of genes. Gene expression is regulated through positive and negative regulation, and this regulation is essential for organism growth and development. The lac operon in E. coli regulates the expression of genes involved in lactose metabolism. It consists of structural and regulatory genes. In the absence of lactose, a repressor protein binds to the operator gene and prevents transcription. In the presence of lactose, it binds to the repressor and induces a conformational change, allowing transcription and expression of the structural genes.

2.  DNA is chemical basis of heredity &
regarded as reserve bank of genetic
information, composed of functional
units, genes.
 Genome refers to the total genetic
information contained in a cell.

3.  Regulation of the expression of genes is
essential for the growth, development,
differentiation & existence of an organism.
 Two types of gene regulation-positive &
negative.
 Positive regulation:
 In this expression is increased by a
regulatory element (positive regulator).

4.  Negative regulation:
 A decrease in the gene expression due to the
presence of a regulatory element (negative
regulator) is referred to as negative
regulation.
 Constitutive genes:
 Products of these genes are required all the
time in a cell (housekeeping genes).
 Not subjected to regulation.

5.  Inducible genes:
 The concentration of the proteins synthesized
by inducible genes is regulated by various
molecular signals.
 An inducer increases the expression of these
genes while a repressor decreases.

6.  The operon is the coordinated unit of
genetic expression in bacteria.
 Introduced by Jacob & Monod, based on
their observations on the regulation of
lactose metabolism in E.coli.
 This is known as lac operon.

7.  Structure of lac operon:
 The lac operon consists of
 A regulatory gene (l; I for inhibition)
 Operator gene (O)
 Three structural genes (Z, Y, A).
 Besides these genes, there is a promoter site
(P) next to the operator gene, where the
enzyme RNA polymerase binds.

8.  Structural genes Z, Y & A respectively, code
for the enzymes β-galactosidase, galactoside
permease & galactoside acetylase.
 β – Galactosidase hydrolyses lactose (β -
galactoside) to galactose & glucose while
permease is responsible for transport of
lactose into the cell.
 The function of acetylase not known.

9.  Structural genes Z, Y & A transcribe into a
single large mRNA with 3 independent
translation units for the synthesis of 3
distinct enzymes.
 An mRNA coding for more than one protein
is known as polycistronic mRNA.
 Prokaryotic organisms contain a large
number of polycistronic mRNAs.

11.  The regulatory gene (I) is constitutive.
 It is expressed at a constant rate leading to
the synthesis of lac repressor.
 Lac repressor is a tetrameric (4 subunits)
regulatory protein (total mol. wt. 150,000)
which specifically binds to the operator
gene (O).

12.  This prevents the binding of the enzyme RNA
polymerase to the promoter site (P), thereby
blocking the transcription of structural genes
(Z, Y & A).
 This is what happens in the absence of
lactose in E. coli.
 The repressor molecule acts as a negative
regulator of gene expression.

13.  In the presence of lactose (inducer) in the
medium, a small amount of it can enter the E.
coli cells.
 Repressor molecules have a high affinity for
lactose.
 Lactose molecules bind & induce a
conformational change in the repressor.
 The result is that the repressor gets inactivated &
cannot bind to operator gene (O).

14.  The RNA polymerase attaches to the DNA at
promoter site & transcription proceeds, leading to the
formation of polycistronic mRNA (for genes Z, Y & A)
& finally, the 3 enzymes.
 Thus, lactose induces the synthesis of the three
enzymes β – galactosidase, galactoside permease &
galactoside acetylase.
 Lactose acts by inactivating the repressor molecules,
hence this process is known as derepression of lac
operon.

15.  There are certain structural analogs of lactose
which can induce the lac operon but are not
the substrates for the enzyme β-galactosidase.
 Such substances are known as gratuitous
inducers.
 Isopropylthiogalactoside (IPTC) is a gratuitous
inducer.

16.  The cells of E. coli utilize glucose in
preference to lactose; when both of them are
present in medium.
 After the depletion of glucose in the medium,
utilization of lactose starts.
 This indicates that glucose somehow
interferes with the induction of lac operon.

17.  Gene amplification:
 The expression of a gene is increased several fold.
 Commonly observed during the developmental
stages of eukaryotic organisms.
 Gene amplification also been reported in humans.
 Methotrexate is an anticancer drug which inhibits the
enzyme dihydrofolate reductase.
 The malignant cells develop drug resistance to long
term administration of methotrexate by amplifying
the genes coding for dihydrofolate reductase

18.  Textbook of Biochemistry – U Satyanarayana