Regulation of Gene Expressions

Regulation of gene expression:

DNA, the chemical vehicle of heredity, is composed of functional units, namely genes. The term genome describes the total genetic information contained in a cell. The bacterium Escherichia coli contains about 4,400 genes present on a distinct chromosome. The genome of humans is more complex, with 23 pairs of (diploid) chromosomes containing 6 billion (6x 10⁹) base pairs of DNA, with an estimated 30,000-40,000 genes. At any given time, only a fraction of the genome is expressed. Regulation of gene expression constitutes of mechanisms cause by cells to increase or decrease the production of limited gene products (protein or DNA) and is known as gene regulation.

The living cells carry a remarkable property to adapt to changes in the environment by regulating the gene expression. Being insulin is synthesized by specialized cells of the pancreas and not by cells of other organs (like kidney, liver), although the nuclei of all the cells of the body consist the insulin genes. Molecular regulatory mechanisms facilitate the expression of the insulin gene in pancreas while preventing its expression in other cells.

What is gene

A gene is a sequence of DNA that codes a diffusible product. The product may be protein (cis-acting or trans-acting) or may be RNA (tRNA or rRNA). Trans-acting protein diffuse freely and function at a distant site after their synthesis. Cis-acting proteins are exclusively associated with DNA and regulate gene expression. It is the basic functional and physical unit of heredity. Gene varies in size from a few hundred DNA bases to more than two million bases in humans. Most of the genes are same in all human and a small number of the gene (less than one percent of the total) are slightly different between the human.

Types of genes:

Types of genes are given below:

1.Structural gene:

The structural gene encodes structural proteins, enzymes with catalytic activities and regulatory protein (that regulate the physiology of cell). They form templates for mRNA and responsible for the amino acid sequence of limited polypeptides.

2.Regulatory gene:

Gene that encodes a protein or RNA involved in regulating the expression of other genes. In this process, activator and repressor both are known to play important roles.

Gene Regulation:

The regulation of the expression of genes is entirely constitutional for the growth, development, differentiation and the very existence of an organism.

There are two types of gene regulation i.e. positive and negative.

1.positive regulation:

The gene regulation is called to be positive when its expression is enhanced by a regulatory element (positive regulator).

2.Negative regulation:

A decrease in the gene expression due to the existence of a regulatory element (negative regulator) is referred to as negative regulation.

Note: It may be cleared here that double negative effect on gene regulation concludes a positive phenomenon.

Constitutive and inducible genes:

1.Constitutive genes:

The products (proteins) of these genes are needed all the time in a cell. Therefore, the constitutive genes are expressed at more or less constant rate in essentially all the cells and, further, they are not subjected to regulate. Example: the enzymes of citric acid cycle.

2.Inducible genes:

The concentration of the proteins synthesized by inducible genes is systematized by various molecular signals. An inducer increases the expression of these genes while a repressor decreases, example: tryptophan pyrroles of the liver is induced by tryptophan.

The term pseudogenes are used to represent DNA sequences that have significant homology to a functional gene, but they cannot express due to mutations. Thus pseudogenes are non-functional. Though, they significantly increase the size of the eukaryotic genome without any contribution to the expression of genes.

One cistron-one subunit concept:

The compound product of a gene expression is a protein which can be an enzyme. It was originally believed that each gene codes for a definite enzyme, leading to the favored concept, one gene-one enzyme. This, however, is not significantly valid due to the fact that various enzymes (or proteins) are composed of two or more non-identical subunits (polypeptide chains).

The cistron is the smallest unit of phenotypic expression. It is the portion of DNA coding for the subunit of a protein molecule. The original concept of one gene-one enzyme is regained by cistron-one subunit.

Models to study gene expression:

Clarifying the regulation of gene expression in prokaryotes has largely supported to understand the principles of the flow of information from genes to mRNA to synthesize limited proteins. Features of prokaryotic gene expression are described which is followed by a brief account of eukaryotic gene expression.