The below mentioned article will highlight the seventeen important types of genes with regard to their role and activity. The seventeen important types of genes are:
(1) House Keeping Genes (2) Non-constitutive Genes (3) Inducible Genes (4) Repressible Genes (5) Multi-genes (6) Repeated Genes (7) Single Copy Genes (8) Pseudo-genes (9) Processed Genes (10) Split Genes (11) Transposons (12) Overlapping Genes (13) Structural Genes (14) Regulator Genes (15) Operator Genes (16) Promoter Genes and (17) Terminator Genes.
1. House Keeping Genes (Constitutive Genes):
They are those genes which are constantly expressing themselves in a cell because their products are required all the time for the normal cellular activities, e.g., genes for glycolysis, ATP-ase.
2. Non-constitutive Genes (Luxury Genes):
The genes are not always expressing themselves in a cell. They are switched on or off according to the requirement of cellular activities, e.g., gene for nitrate reductase in plants, lactose system in Escherichia coli. Non-constitutive genes are of further two types, inducible and repressible.
3. Inducible Genes:
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, e.g., nitrate for nitrate reductase.
4. Repressible Genes:
They are those genes which continue to express themselves till a chemical (often an end product) inhibits or represses their activity. Inhibition by an end product is known as feedback repression.
5. Multi-genes (Multiple Gene Family):
It is a group of similar or nearly similar genes for meeting requirement of time and tissue specific products, e.g, globin gene family (ε, δ, β, γ, on chromosome 11, α and δ on chromosome 16).
6. Repeated Genes:
The genes occur in multiple copies, e.g., histone genes, tRNA genes, rRNA genes, actin genes.
7. 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 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, e.g., several of snRNA genes.
9. Processed Genes:
They are eukaryotic genes which lack introns. Processed gene have been formed probably due to reverse transcription or retroviruses. Processed genes are generally nonfunctional as they lack promoters.
10. Split Genes:
Usually a gene has a continuous sequence of nucleotides. In other words, there is no interruption in the nucleotide sequence of a gene. Such nucleotide sequence codes for a particular single polypeptide chain. 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 interrupted sequence of nucleotides are referred to as split genes or interrupted genes. Thus, split genes have two types of sequences, viz., normal sequences and interrupted sequences (Fig. 8.3).
(i) Normal Sequence:
This represents the sequence of nucleotides which are included in the mRNA which is translated from DNA of split gene (Fig. 8.3). These sequences code for a particular polypeptide chain and are known as exons (or coding sequences).
(ii) Interrupted Sequence:
The intervening or interrupted non-coding sequences of split gene are known as introns (or Junk DNA). These sequences do not code for any peptide chain. Moreover, interrupted sequences are not included in mRNA which is transcribed from DNA of split genes.
The interrupted sequences are removed from the mRNA during processing of the same (Fig. 8.3). In other words, the intervening sequences are discarded in mRNA as they are non-coding sequences. The coding sequences or exons are joined by ligase enzyme.
Split genes are characteristic of eukaryotes. However, certain eukaryotic genes are completely exonic or non-split, e.g., histone genes, interferon genes. Split genes have also been recorded in some prokaryotes, such as thymidylate synthase gene and ribonucleotide reductase gene in T4. Split genes were discovered in 1977 by many workers but main credit was given to Philip Sharp and Richard J. Roberts (1997). They, for the first time, reported split genes in mammalian viruses (viz., adenoviruses) and were awarded Nobel prize.
11. Transposons (Jumping Genes; Hedges and Jacob, 1974):
They are segments of DNA that can jump or move from one place in the genome to another. Transposons were first discovered by Mc Clintock (1951) in case of Maize when she found that a segment of DNA moved into gene coding for pigmented kernels and produced light coloured kernels. Transposons possess repetitive DNA, either similar or inverted, at their ends, some 5, 7 or 9-nucleotide long. Enzyme transposes separates the segment from its original by cleaving the repetitive sequences at its ends.
12. Overlapping Genes:
A few genes in certain bacteria and animal viruses code for two different polypeptides. These are called overlapping genes, e.g., in φ x 174 genes B, E and K overlap other genes.
13. Structural Genes (Cistrons):
These genes code for chemical substances which contribute to morphological or functional trait of the cell. These are now-a-days called cistrons. They are continuous in prokaryotes and split into introns and exons in eukaryotes.
(i) Polypeptide-coding Genes:
These code for mRNAs which in turn code for polypeptides. Polypeptide may act as a component of an organelle, such as actin of muscle fibre; an enzyme such as DNA polymerase; a receptor or carrier protein of cell membrane; a transport protein such as hemoglobin; a hormone such as insulin, “an antibody” ; or antigen.
(ii) Polyprotein-coding Genes:
These genes code for more than one polypeptide per gene.
(iii) RNA-coding Genes:
These code for rRNAs and tRNAs. They are tandemly repeated in the chromosomes.
14. Regulator Genes:
These genes code for repressor proteins for regulating the transcription of cistrons.
15. Operator Genes:
An operator gene acts as a switch to turn on or off the transcription of a structural gene as and when the cell requires.
16. Promoter Genes:
These genes are DNA sequences (sites) where RNA polymerase binds for the transcription of RNAs by the structural genes.
17. Terminator Genes:
These genes are DNA regions where RNA polymerase activity stops to suspend transcription of structural genes.