Note on Gene Regulation

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Gene regulation

Following terms are used in gene regulation:

1) Split genes :The non-essential or non-coding parts intermixed with essential or coding parts are called split genes. The coding parts or genes are called exons and non-coding or non-essential parts or genes are called introns.

2 ) Splicing :Splicing is the process in which introns are removed and exons are joined together.

Splicing
Splicing

3) Constitutive genes (Housekeeping genes) :The genes which are constantly expressing themselves in a cell are called as constitutive genes. Expressions of these genes are not controlled by any factors. These genes always express the character for normal cellular activities. Eg- genes for glycolysis, They are required all the time in a cell.

4) Non-constitutive genes (Luxury genes) :These are the genes which are not always expressing themselves in a cell. They are switched on or off according to the requirement of cellular activities. Eg- Lactose system inE.coli.Non-constitutive genes are of two types: Inducible and repressible.

A) Inducible genes and Inducer -All the genes present on the chromosome are not expressed simultaneously. The genes that get activated when a certain substrate (i:e an inducer) is present in the medium are called inducible genes. The phenomenon of the action of these genes is called enzyme induction and the substrate is called inducer.

For example,E.coligrown in a medium without lactose doesn't produce enzymes required for lactose metabolism. But when the same bacteria is placed in a lactose supplemented medium, it starts producing enzymes like β-galactosidase required for converting lactose to glucose and galactose. Therefore, since lactose is used to induce this enzyme, it is called inducer and this phenomenon is called as enzyme induction.

B) Repressible genes and repression-The genes which get inactivated due to the presence of some chemicals are called repressible genes. When E. coliids supplied with certain metabolite more than required, the action of some genes, responsible for the formation of some specific enzymes, can be inhibited or repressed. Repression may take place in the case even if the metabolite is being provided from an outer source. As a result, certain genes are repressed and do not produce enzymes. Such inactivated genes are known as repressible genes and the phenomenon is called enzyme repression.

5) Co-repressor :Molecules that binds with the repressor protein to form a functional repressor complex is called co-repressor. In a tryptophan operon, tryptophan acts as a co-repressor by binding with the repressor protein to form a complex which on binding to the promoter switches it off and hence no transcription takes place.

6) Pseudogenes :These are the genes which have homology to functional genes but are unable to produce functional products.

7) Structural genes :These genes code for substances that contribute to the specific morphological or functional trait of the cell. These genes are also known as cistrons. The genes that contain the information to determine the sequence of amino acids are called as structural genes. Such genes have structural functions coding for the proteins needed by the cells.

8) Regulatory gene :The gene codes for the product that regulates the level of expression of the structural gene. Although it is located at the site away from the structural genes, it is the key element of an operon. These genes code for special proteins known as repressor proteins that regulate transcription.

9) Promoter gene :Gene that forms the binding site of RNA polymerase for transcription is called promoter gene. Each gene may be regulated by a specific promoter.

10) Operator gene :The gene, that operates the activity of structural genes, is called operator gene. It lies adjacent to the promoter site. Structural genes are expressed or not expressed depending upon whether the operator gene is 'on' or 'off'.

11) Transpogenesor transposons (Jumping genes) :They are the segment of DNA that can move or jump from one place to another place. These types of genes cause mutations through insertions, deletion, and translocation. Barbara McClintok discovered jumping genes. She is called lady Mendel because she was the first lady geneticist.

12) Apo- repressor :It is a protein produced by the regulatory gene for blocking the working of operator gene in the presence of co-repressor.

13) Co-repressor :It is non-protein or component of repressor and product of reactions catalysed by enzymes. In tryptophan operon, the tryptophan functions as co-repressor.

14) Operon :An operon is a coordinated genetic unit consists of an operator, a promoter, and one or more structural genes whose activity is influenced by a regulator.

Operon model
Operon model

Operon concept:

An operon is a coordinated genetic unit consists of an operator, a promoter, and one or more structural genes whose activity is influenced by a regulator. Jacob and Monad proposed Operon concept in 1961 in the prokaryote. There are many operons but the Lactose or Lac Operon and Tryptophan Operon are important ones.

A) Lactose (Lac) operon concept:

Jacob and Monad studied the lactose metabolism in E.coli.The bacterium when grown in a medium containing lactose, produces three enzymes namelyβ-galactosidase, galactoside permease, and thiogalactoside transacetylase. These enzymes help to break lactose into glucose and galactose.

Lactose (+ the three enzymes)→ Glucose + galactose.

The structure of lac operon:

The structure of lactose operon consists of :

1) Structural genes - The lac operon consists three structural genes named Z,Y, and,A codes for three enzymes β-galactosidase, galactoside permease, and thiogalactoside transacetylase respectively. These genes are located in a row adjacent to each other and are known as polycistrons,

2) Operator gene - A single operator gene regulates all the three structural genes. The operator gene acts as a switch.

3) Promoter gene - A single promoter gene directs proper initiation of transcription in structural genes.

4) Regulatory gene - It is located away from the structural genes and is known as inhibitor gene. The regulatory gene constantly transcribes mRNA to produce repressor protein. It is the key element of operon because the function of the operon is dependent on it.

A)When E.coli is grown in a medium in absence of lactose:

When E.coli is grown in a medium in the absence of lactose, the regulator gene produces a repressor protein that binds the operator gene and blocks its activity. RNA polymerase can bot move from promoter to structural genes. It stops the transcription of mRNA from structural genes and thus protein synthesis is switched off. Hence, no enzymes are produced.

When E.coli is grown in a medium in absence of lactose
When E.coli is grown in a medium in absence of lactose

B)When E.coli is grown in a medium in the presence of lactose:

When the lactose is introduced in the medium, lactose binds to the repressor protein. In this way, repressor protein fails to bind to the operator gene. Then the operator gene remains active and the switch is turned on. Then, RNA polymerase moves from promoter gene to the structural genes through operator gene. These structural genes Z,Y, and A genes show transcription in the presence of RNA polymerase and show transcription process. Then three enzymes are formed.

When E.coli is grown in a medium in the presence of lactose
When E.coli is grown in a medium in the presence of lactose

Synthesis of enzymes is continued unless and until all lactose molecules are consumed.

Tryptophan Operon:

Tryptophan operon is a segment of DNA that regulates the synthesis of protein.The formation of protein, in this case, is controlled by five structural genes: A,B,C,D, and E.

The presence or absence of tryptophan switches off or on the transcription mRNA and protein synthesis,

1)When E.coli is grown in a medium having tryptophan:

The regulatory gene produces repressor protein known as apo-repressor binds with tryptophan to form a repressor-co-repressor complex. This functional repressor protein binds with operator gene. The operator gene is switched off. RNA polymerase cannot be transferred to structural genes. So, there is no formation of any enzymes for expression of character.

When E.coli is grown in a medium having tryptophan
When E.coli is grown in a medium having tryptophan

2) When E.coli is grown in a medium without tryptophan:

The regulatory gene produces repressor protein known as apo-repressor. The protein cannot bind to the operator gene. Hence, the operator gene is switched on. The RNA polymerase moves forward and structural genes produce five enzymes which help in the formation of tryptophan amino acid.

When E.coli is grown in a medium without tryptophan
When E.coli is grown in a medium without tryptophan

Thus operon model can be completed.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.



  • Various terms used in gene expression.
  •  Barbara McClintok discovered jumping genes. She is called lady Mendel because she was the first lady geneticist.
  • Operon concept: Lactose and tryptophan operon model.
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