Control of Genes

In general speaking, all scientists believe that all body cells contain a complete set of genetic material. It is enough to build and operate the whole individual. Now, scientists found that in the complete set of genetic material of a human, only 2-3% is meaningful, that means that their functions are known and enough to build and operate the whole body. The rest is meaningless, and called the junk DNA. Scientists are still working for the role of junk DNA. Inside the functioning genetic material, scientist believe that no more than 0.1% of the functioning DNA inside a cell is working. Therefore, which gene should be activated at which time would be an essential problem. If genes are not activated at proper time and at proper place, undesired results would be achieved. For example, the gene for the production of hair is just activated on the epithelial cells on the top of the head, so, we can have hair. But if it is activated on the epithelial cells on the face, then, it would be impossible to see the face. The operon theory provides a hypothesis for explaining the activation of genes.

The operon theory states that a gene is composed of three parts. They are called the structural gene, the operator gene and the regulator gene.

(1)	The structural gene is the gene encoding the primary structure of a protein. It may be involved in the bodily structure or operation of the body. So, when it is activated, it would be transcribed into its mRNA and translates into the corresponding protein. Once this gene is activated, the corresponding protein will be produced in that location.
(2)	The operator gene is the part of the gene that is always activated and then, producing an activator. So, the activator can always activate the structural gene and enable the structural gene to be transcribed and translated.
(3)	The regulator gene is the part that would be continually transcribed and translated into a repressor. This repressor would act on the activator produced by the operator gene. Once the activator is sremoved from the structural gene, the transcription of the structural gene would be stopped. The production of the corresponding protein would be terminated. Then, it is said that the gene is inhibited.

If the above process is unchanged, no gene can be activated. But there is some alternative actions in the repressor material. So, genes can be activated at specific time and specific place. For example, E. coli can produce beta galactosidase. Beta galactosidase

can break down lactose into galactose and glucose. At normal situation, the enviornment does not contain lactose. So, E. coli would not produce beta galactosidase. Once lactose is added into the medium, lactose will activate the gene, then, E. coli can start to produce beta galactosidase to decompose lactose. The mechanism is called the Enzyme induction hypothesis

Enzyme induction hypothesis
When lactose occurs in environment, lactose will act on the repressor from regulator gene. Then, the repressor is unable to repress the structural gene. The structural gene is set free and starts transcription and translation. As a result, the beta galactosidase is produced and lactose in environment can be decomposed. When there is no more lactose, repressor molecules would be released and the inhibition of structural gene would resume.

The production of beta galactosidase in E. coli

Gene induced model
( Diagram adopted from : A-Level Biology, by Phillips and Chilton, Oxford University Press, 1989, pp. 59)

The process is summarized as follow :

Regulator gene transcribes to make its mRNA.
The mRNA is translated to make its repressor protein.
Normal (no lactose) In Environment	Lactose Present
Repressor represses the operator gene	Lactose combines with repressor. Repressor cannot repress the operator gene.
Operator gene cannot activate structural gene	Operator gene continues to activate the structural gene.
Structure gene cannot have transcription and translation	Structural gene is transcribed and translated.
No beta galactosidase is produced.	Beta galactosidase is produced.
..	Lactose is decomposed.
====================
..	When lactose is exhausted, no lactose is in environment.
..	Repressor is released to inhibit the operator gene.
..	Operator gene cannot activate the structural gene.
..	Structural gene stops transcription, translation. No beta galactosidase is in environment.

In another situation, the repressor may be incomplete. It needs something to make it complete and become possible to inhibit the operator gene. It is the enzyme inhibition hypothesis.

Tryptophan is an amino acid. E. coli requires this amino acid. If there is tryptophan in environment, E. coli can get it directly from the environment. If there is no tryptophan in environment, E. coli can activate the gene and start producing tryptophan synthetase to produce tryptophan.

Enzyme Inhibition Hypothesis.
In this hypothesis, the repressor from regulator gene is not a complete repressor. It must combine with tryptophan, then, become a functional repressor. So, if there is no tryptophan in environment, the repressor is not complete. The operator gene is not inhibited. So, the structural gene is activated for transcription and translation. Then, E. coli can produce tryptophan synthetase and produce tryptophan.

On the other hand, if tryptophan exists in environment, tryptophan combines with the repressor (from regulator gene) to become a functional repressor. This repressor represses the operator gene and the operator gene cannot activate the structural gene. Then, there is no transcription and translation. Also, no tryptophan synthetase produced and no tryptophan synthesized.

The production of tryptophan synthetase in E. coli.

Gene repressed model
( Diagram adopted from : A-Level Biology, by Phillips and Chilton, Oxford University Press, 1989, pp. 59)

It is summarized as follow.

Regulator gene transcribes to make its mRNA.
The mRNA is translated to make its repressor protein.
Tryptophan present	Tryptophan absent
Tryptophan combines with the repress to become a functional repressor.	The repressor is incomplete and not functional.
The tryptophan repressor complex successfully represses the operator gene.	The operator gene is not repressed.
The structural gene is not activated. No transcription and no translation occur.	The operator gene activates the structural gene. The structural gene is then transcribed and translated.
Tryptophan synthetase is not produced.	Tryptophan synthetase is produced.
No tryptophan is synthesized.	Tryptophan is synthesized.

(06.08.2008)