6. MOLECULAR BASIS OF INHERITANCE
REGULATION OF GENE EXPRESSION
In eukaryotes,
gene expression occurs by following levels:
1.
Transcriptional level (formation of primary transcript).
2.
Processing level (splicing, capping etc.).
3.
Transport of mRNA from nucleus to the cytoplasm.
4. Translational level (formation of a polypeptide).
The
metabolic, physiological and environmental conditions regulate gene expression.
E.g.
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In
E. coli, the beta-galactosidase enzyme hydrolyses lactose
into galactose & glucose. In the absence of lactose, the synthesis of beta-galactosidase
stops.
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The
development and differentiation of embryo into adult are a result of the expression
of several set of genes.
If a substrate is added
to growth medium of bacteria, a set of genes is switched on to metabolize it.
It is called induction.
When a metabolite
(product) is added, the genes to produce it are turned off. This is called repression.
OPERON CONCEPT
§
“Each metabolic reaction
is controlled by a set of genes”
§
All
the genes regulating a metabolic reaction constitute an Operon. E.g. lac operon, trp operon, ara operon,
his operon, val operon etc.
Lac Operon in E. coli
- The operon controlling lactose metabolism.
- It is proposed by Francois Jacob & Jacque Monod.
It
consists of
a)
A regulatory or inhibitor
(i) gene: Codes for
repressor protein.
b)
3 structural genes:
i. z
gene: Codes for b galactosidase. It hydrolyses lactose to galactose and glucose.
ii. y
gene: Codes for permease. It increases permeability
of the cell to b-galactosides (lactose).
iii. a
gene: Codes for a transacetylase.
- Genes in the operon function together in the same or
related metabolic pathway.
- If there is no lactose (inducer), lac operon remains switched off. The regulator gene
synthesizes mRNA to produce repressor
protein. This protein
binds to the operator
region and
blocks RNA polymerase movement. So the structural genes are not
expressed.
- If lactose or allolactose is provided in the growth
medium, it is transported into E. coli cells by the action of permease. Lactose (inducer) binds with repressor protein. So
repressor protein cannot bind to operator region. The operator region becomes
free and induces the RNA polymerase to bind with promoter. Then transcription starts.
- Regulation of lac operon by repressor is called negative regulation.
 |
| In the absence of Inducer |
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