Constitutive transcription involves the transcription of constitutive genes in a given cell. These are the genes containing the structural information of the constitutive proteins. The constitutive proteins are the ones that are continuously required by the cell to carry out its necessary function for survival and reproduction. Thus, the transcription of a constitutive gene containing the information of a constitutive protein need not be regulated as there are continuous requirements and utilization of the particular protein. Such a transcription is known as constitutive transcription.
Regulation of Gene Expression and Classification of Genes based upon its Regulation
Gene expression refers to the availability of the final product of the information contained within the gene - the protein. A cell requires different classes of proteins to carry out its routine functions for survivability and for maintaining its structural integrity. Some of these proteins may be continuously and permanently required by the cell such as the proteins responsible for maintaining the cytoskeleton of the cell. Some of these proteins may be required only to maintain the normal physiological processes of the cells. While certain proteins may be required for the adaptation of the cell to changing conditions in the surrounding environment. Thus, depending on the utility of the proteins as and when required the cells contain mechanisms that control the transcription of a particular gene of a particular protein. This is done by regulating either the transcriptional or translational processes within the cell. Thus, the regulation of gene expression is the control over the amount and timing of availability of the functional product of a gene.
The regulation of gene expression is vital for the cell to permit the cell to produce the gene product as and when required. This makes the cell capable and flexible to adapt to the variability of the surrounding environment, external stimulus, any damage to the cell, etc. This gives the cell control of the overall structure and function of the cell and is the basis for cellular differentiation and morphogenesis. Thus, in turn, it aids any living organism in adaptation and versatility.
Having learnt the importance of gene expression it is important to know about the categorization of genes depending on their need for regulation. These terminologies corresponding to their categorization is as follows -
It is a particular gene that is only expressed in the presence of an inducer. For example, bacteria generally utilizes glucose directly from the surrounding for its cellular activities. But in absence of glucose, it takes in lactose which is further turned into glucose inside the cell to carry out the activities. But in order to break down the lactose and produce glucose it needs a specialised set of proteins that are only expressed in the presence of lactose. Thus, lactose in this case is the inducer of the corresponding set of genes that provide the functional proteins.
These are special types of genes that are transcribed only when necessary. These are the genes expressed whenever the need arises which can be more frequent as compared to inducible genes but less frequent than other sets of genes required for continuing physiological processes.
A housekeeping gene is the one that is responsible for maintaining basic cellular functions and structure and hence is expressed in all types of cells of a living organism. Some of these genes are transcribed at a constant rate and because of this are used as references while conducting scientific experiments. Examples include actin, ubiquitin and GAPDH.
These are the genes that are transcribed continuously throughout the life cycle of a cell. Their functional products are continuously required by the cell and are essential for their activities. Examples include the enzymes required in the citric acid cycle.
Constitutive Transcription of Genes
Protein synthesis comprises two essential key steps: Transcription and translation. Transcription is the process of converting the information in the DNA to RNA and produce the messenger RNA or mRNA. The mRNA is then further processed in the ribosomes to produce proteins by translation of the information present in the nucleic acid form to a combination of amino acids, the basic building blocks of proteins. The process of the expression of information from gene to protein is thus highly regulated at both the levels of transcription and translation. To control the gene expression at the transcription step, there are numerous proteins known as transcription factors.
Having understood what are the different types of genes based upon their regulation, it is clear that the constitutive genes are constantly required by the cells. Hence, their transcription is a continuous process inside the cell. Thus, the constant transcription of the constitutive genes is known as constitutive transcription. An example of constitutive transcription includes the transcription of ribosomal genes which are regularly and constantly involved in protein synthesis. The ribosomal proteins are vital to the protein synthesis machinery and a failure in the mechanism will lead to catastrophic events inside the cell and eventual cell death. Thus, these proteins need to be constantly produced requiring the corresponding and respective genes to be constitutively transcribed in an unregulated manner.
Thus, in the end, it is very well understood that the constitutive transcription of constitutive genes is vital for a cell to maintain its functionality and viability.