Index
Introduction
The control of gene expression
The lac operon
Adapting the environment
Four situations are possible
The control of lac operon
The regulation of lac operon
Bibliography
Lac operon
Introduction
what exactly is the lac operon? The lac operon is an
operon, or group of genes with a single promoter (transcribed as
a single mRNA). The genes in the operon encode proteins that
allow the bacteria to use lactose as an energy source.
What makes the lac operon turn on?
E. coli bacteria can break down lactose, but it’s not their
favourite fuel. If glucose is around, they would much rather use
that. Glucose requires fewer steps and less energy to break down
than lactose. However, if lactose is the only sugar available, the
E. coli will go right ahead and use it as an energy source.
To use lactose, the bacteria must express the lac operon genes,
which encode key enzymes for lactose uptake and metabolism.
To be as efficient as possible, E. coli should express the lac
operon only when two conditions are met:
Lactose is available, and
Glucose is not available
The control of gene expression
Each cell in the human contains all the genetic Material
for the growth and development of a human
Some of these genes will be need to be expressed all the
time
These are the genes that are involved in of vital
biochemical processes such as respiration
Other genes are not expressed all the time .
operon
An operon is a group of genes that are transcribed at the
same time.
They usually control an important biochemical process
They are not found in prokaryotes
The lac operon
The lac operon consists of one regulatory gene( i-gene) and
three structural gene(z,y,a)
The i gene codes for repressor of lac operon
The z gene codes for beta - galactosidase which are
responsible for hydrolysis of disaccharide , lactose , galactose and
glucose
The y gene codes for permease which increase the permeability
of the cell to beta - galactose
The a gene codes for transacetylase
Hence all the three gene product in lac operon are required for
metabolism of lactose
Adapting to environment
E. coli can use either glucose, which is a monosaccharide, or
lactose, which is a disaccharide
However, lactose needs to be hydrolysed (digested) first
So the bacterium prefers to use glucose when it can.
Four situations are possible
When glucose is present and lactose is absent the E. coli does
not produce β‐galactosidase
When glucose is present and lactose is present the E. coli does
not produce β‐galactosidase
When glucose is absent and lactose is absent the E. coli does not
produce β‐galactosidase
When glucose is absent and lactose is present the E. coli does
produce β‐galactosidase.
The control of the lac operon
1. When lactose is absent
A repressor protein is continuously synthesised. It sits on
a sequence of DNA just in front of the lac operon, the operator site
The repressor protein blocks the Promoter site where the
RNA polymerase settles before it starts transcribing.
2.When lactose is present
A small amount of a sugar all lactose is formed within the
bacterial cell. This fits onto the repressor protein at another active site
(allosteric site)
This causes the repressor protein to change its shape (a
conformational change). It can no longer sit on the operator site.
RNA polymerase can now reach its promoter site
3.When both glucose and lactose are present
This explains how the lac operon is transcribed only
when lactose is present.
BUT….. this does not explain why the operon is not
transcribed when both glucose and lactose are present.
When glucose and lactose is present RNA polymerase
can sit on promotor site but it is unstable and it keeps falling off
4.When glucose is absent and lactose is present
Another protein is needed, an activator protein. This
stabilises RNA polymerase.
The activator protein only works when glucose is absent
In this way E. coli only makes enzymes to metabolise other
sugars in the absence of glucose
The regulation of lac operon
The repressor of operon is synthesized from i gene .
The repressor protein binds to the operator region of the operon
region of the operon and prevent RNA polymerase from transcribing the
operon .
In the presence of inducer , such as lactose , the repressor is
inactivated by interaction with the inducer . This allows RNA
polymerase access to promoter and transcription proceeds .
Essentially ,the regulation of lac operon can also be visualised
as regulation of enzyme synthesis by its substrate.
Regulation of lac operon by repressor is referred to as negative
regulation .Lac operon is under control of positive regulation.
Bibliography
https://gmch.gov.in
https://en.wikipedia.org
