Based on the function and activity, the genes are classified as follows.
Housekeeping Genes or Constitutive Genes:
Housekeeping genes are involved in basic cell maintenance and, therefore, are expected to maintain constant expression levels in all cells and conditions. They are functional in all types of body cells of a multicellular organism and all the time. They are required for basic cellular activity. They are not regulated.
Example: Genes associated with glycolysis are active in all types of cell and all the time throughout the life.
housekeeping genes are instrumental for calibration in many biotechnological applications and genomic studies. Advances in our ability to measure RNA expression have resulted in a gradual increase in the number of identified housekeeping genes.
Luxury Genes or Noncontitutive Genes:
These genes are not always expressing themselves in a cell. They remain inactive for most of the time in the lifespan of an individual and is expressed in certain cells or at a certain time only when their products are needed. These are called luxury genes or specialist genes.
Humans comprise approximately 200 different types of cells, such as skin cells, liver cells, and nerve cells. Each cell varies in both the structure and the function because different sets of genes are expressed in each of them. For example, the serum albumin gene is expressed only in hepatocytes (liver cells), while the insulin gene is expressed only in pancreatic beta cells.
They are switched on or off according to the requirement of cellular activities.
Example: the gene for nitrate reductase in plants, lactose system in Escherichia coli. There are some genes in the human body which are present in all the body cells but some are functional in kidney cells, some in liver cells and some in intestine or stomach. They are associated with adaptive enzymes synthesis.
Luxury genes are of further classified as inducible and repressible. The genes are switched on in response to the presence of a chemical substance or inducer which is required for the functioning of the product of gene activity are called inducible genes, e.g., nitrate for nitrate reductase. The genes which continue to express themselves till a chemical (often an end product) inhibits or represses their activity are called repressible genes.
Structural Genes (Cistrons):
These genes code for chemical substances which contribute to the morphological or functional trait of the cell. These are called cistrons. They are continuous in prokaryotes and split into introns and exons in eukaryotes. They are further classified as
Polypeptide-coding Genes: These genes code for mRNAs which in turn code for polypeptides. The polypeptide produced may act as a component of an organelle (as actin of muscle fibre); an enzyme (as DNA polymerase); a transport protein (as haemoglobin); a hormone (as insulin); a receptor or carrier protein of cell membrane; an antibody, an antigen.
Polyprotein-coding Genes: These genes code for more than one polypeptide per gene.
RNA-coding Genes: These genes code for rRNAs and tRNAs.
These genes code for repressor proteins for regulating the transcription of cistrons.
An operator gene acts as a switch to turn on or off the transcription of a structural gene as and when required by the cell.
These genes are DNA sequences (sites) for binding of RNA polymerase for the transcription of RNAs by the structural genes.
These genes are DNA regions (lying t end of message) where RNA polymerase activity stops to suspend transcription of structural genes.
Uninterrupted Genes or Continuous Genes Or Collinear Genes:
In prokaryotes, the sequence of nucleotides in the gene corresponds exactly with the sequence of amino acids in the protein. Such nucleotide sequence codes for a particular single polypeptide chain. Each gene is a continuous stretch of DNA whose length is related to the size of protein to be synthesized. Thus these genes and proteins are collinear.
Interrupted Genes or Discontinuous Genes or Split Genes:
Generally, a gene has a continuous sequence of nucleotides. However, it was observed that the sequence of nucleotides was not continuous in case of some genes, the sequences of nucleotides were interrupted by intervening sequences. Such genes with the interrupted sequence of nucleotides are called split genes or interrupted genes. Thus, split genes have two types of sequences, viz., normal sequences and interrupted sequences
The Concept of Exons and Introns:
The coding units containing biological information are called exons. and intervening non-coding DNA segment are called introns. Introns are present in the genes of eukaryotes, viruses and archaebacteria. Interrupted genes produce the primary transcript RNA. It acts as a precursor as it is a faithful copy of the interrupted gene.
The functional RNA is formed by the removal of introns and rejoining exons. This process is known as RNA splicing.
Overlapping Genes or Alternate Genes:
A few genes in certain bacteria and animal viruses code for two different polypeptides (more than one protein). These are called overlapping genes. In this case, the specific sequence is shared between two non-homologous proteins. In these genes, the first and second half of the gene codes fora specific protein that represents the first or second half of the protein, specified by the full gene.
The concept of alternative genes was given by Gilbert and is known as Gilbert hypothesis. They are formed when exons from different discontinuous genes get connected forming several new combinations. These genes produce proteins in which one part is common while another part is different.
Jumping Genes or Transposons:
They are segments of DNA that can jump or move from one place in the genome to another. Transposons were first discovered by Nobel prize winner Mc Clintock (1951) in case of Maize when she found that a segment of DNA can move from one position to another in the genome of the cell. Recently they have been described in snapdragon, Drosophila, mice and bacteria.
Transposons possess repetitive DNA, either similar or inverted, at their ends. The two major event took place during transposition. There is a duplication of the target sequence in the recipient DNA molecule and insertion of transposons between the repeated target sequences.
Gene Families and Pseudogenes:
They are genes which have homology to functional genes but are unable to produce functional products due to intervening nonsense codons, insertions, deletions and inactivation of promoter regions, Pseudogenes are genomic DNA sequences similar to normal genes but non-functional; they are regarded as defunct relatives of functional genes.
Most of the prokaryotic genes are represented only once in the genome. But many eukaryotic genes are presented in multiple copies. These multiple copies of genes are called gene families or pseudogenes.
They may be clustered in the same region of DNA or dispersed to different chromosomes.
e.g., several of snRNA genes.
Single Copy Genes:
The genes are present in single copies (occasionally 2-3 times). They form 60-70% of the functional genes. Duplications, mutations and exon reshuffling between two genes form new genes.
They are eukaryotic genes which lack introns. Processed genes are generally nonfunctional as they lack promoters.
Multi-genes (Multiple Gene Family):
It is a group of similar or nearly similar genes for meeting requirement of time and tissue-specific products.