Types of Genes
The human body is made of around 200 types of cells, such as bone cells, blood cells, skin cells, nerve cells, liver cells, etc. These cells make different organs and vary in terms of their structure and function as a different set of genes are expressed in these cells as per the requirement of respective organs. Such as the serum albumin gene which becomes active only in liver cells (hepatocytes). Similarly, the insulin gene expresses itself only in the beta cells of the pancreas. Besides this, some genes are present in all cells of the human body but some of them express themselves in kidney cells and some are expressed in liver cells, and more.
Genes can be of different types. Some of gene types that are present in eukaryotes are described below:
1. House Keeping Genes
They are also known as constitutive genes. They constantly express themselves as they are needed to carry out basic cellular functions that occur continuously. In other words, they code for proteins that are continuously needed by the cell to perform basic cellular activities necessary for the maintenance of a cell. So, they are always present and expressed in the cells under normal conditions. Thus, they are necessary for the existence of a cell irrespective of the role of a cell. Some examples of housekeeping genes are genes for glycolysis and genes of ATPase enzyme.
2. Non-constitutive Genes
These genes do not express themselves continuously in a cell. They are also called luxury genes or specialist genes. They can be switched on or off as per the requirement of cellular reactions or activities. Some examples include the gene required for nitrate reductase in plants and the gene for lactose system in E. coli.
They remain inactive or switched off for most of the time during the lifespan of an organism. They become active and express themselves in certain cells only when their products are needed.
They are further of two types: Inducible and repressible.
- Inducible genes: These type of non-constitutive genes are switched on or becomes active in response to the presence of a chemical or inducer that is needed for the functioning of the product of these gene’s activity. For example, nitrate for nitrate reductase.
- Repressible genes: They keep expressing themselves until their activity is inhibited or repressed by a chemical or an end product. The inhibition of this gene’s activity by an end product is called feedback repression. For example, trp genes are repressed by tryptophan.
3. Structural Genes (Cistrons)
They are also called cistrons as they are protein-coding genes which means they code only for proteins not for any type of RNA or any other product. They are a continuous stretch of DNA that contains the protein coding sequences exons and non-coding sequences that are called introns. These sequences are present repeatedly and alternatively inside the cistron.
They code for the synthesis of chemical substances or protein for a specific morphological and functional trait of the cell. The structural gene remains under the control of regulator gene, operator gene and promoter gene and these three genes are present upstream to the cistron.
4. Pseudogenes
They are similar or homologous to functional protein-coding genes, but they cannot produce a functional protein due to disordered open reading frame (ORF). Similarly, they may be highly similar to RNA encoding genes but cannot produce an RNA transcript.
We can say that they are the DNA sequence that may be similar to functional genes but are not able to code for proteins. So, they are non-functional genomic sequences that resemble functional genes but are not able to produce functional products due to nonsense codons, deletions, insertions and deactivation of promoter regions such as many snRNA genes. They are also called defunct relatives of functional genes.
5. Transposons (Jumping Genes)
As the name suggests, they are those genes or segments of DNA that can jump or move from one place in the genome to another place; from one chromosome to another chromosome within the genome of an individual. Jumping genes were first discovered by Nobel Prize winner Mc Clintock (1951) in Maize when she observed that a DNA segment can jump from one location to another within the genome of a cell. They are mostly present towards the telomeric region of the chromosome.
Besides this, they do not code for a protein. They can only jump or discontinue themselves from a particular region of the chromosome generally from the end region of the chromosome and can ligate themselves to another chromosome.
Furthermore, Transposons contain repetitive DNA (similar or inverted) sequences at their ends. During transposition, the target sequence in the recipient DNA gets duplicated and a transposon is inserted between the repetitive target sequences. The enzyme transposase separates the repetitive segment from its original’s end by cleaving or slicing.
6. Single Copy genes
These genes exist as single copies. It has one physical location in a genome and can have orthologs or orthologous in other species.
7. Processed genes
They are found in eukaryotes and they lack introns. They are formed due to reverse transcription or retroviruses. Processed genes are mostly non-functional as they lack promoters.
8. Overlapping genes
They are DNA fragments that can code for more than one product or polypeptide by using different reading frames or initiations codons. They code for more than product and do not code for a particular product, so they are called overlapping. They are not found in higher organisms or vertebrates. They are mostly found in organelles, bacteria, animal viruses, nuclear eukaryotic genomes, DNA virus, RNA virus, etc. for example; ø x 174, SV-40 virus.
9. Split genes
They were discovered by Sharp and Roberts. They were awarded the Nobel Prize in 1993. It is a gene that contains a non-functional part along with the functional part. They possess extra or non-essential part interspersed with the essential or coding part. So, they have intervening sequences that do not code for a protein. The non-functional part is called introns or spacer DNA or intervening sequences (IVS) and functional parts are called exons. Introns are removed before RNA passes out into the cytoplasm. They are also known as interrupted genes. They are mostly found in eukaryotes, however, some are also found in prokaryotes.
10. Multi genes
These are the groups of similar genes. They work or express continuously to fulfil the needs of a cell depending on a particular protein product. These genes are homologous genes and they are work for a particular pathway and they synthesize the product as per the demand of the cellular environment.
11. Regulator gene
It controls another gene. They turn on or off the transcription of structural genes. It controls the expression of structural genes of an operon through the synthesis of a repressor protein. It is called regulator as it regulates whether the cistron will be expressed or not.
They code for repressor protein and are present upstream to the coding sequence of cistron or structural gene. They produce a repressor mRNA which translates a particular repressor protein that binds to the operator region which is present next to the regulator gene in a cistron. So, if the repressor gene binds with the operator gene, automatically the cistron will remain switched off and will not express.
12. Operator gene
This gene acts as a switch, for example, if the repressor protein produced by the regulator gene binds in the operator region, it (operator gene) remains switched off and it does not allow the mRNA polymerase enzyme to move forward and transcribe the structural gene of cistron so it acts as a switch to turn on or off the transcription of the cistron.
13. Promoter gene
Promotor gene is present upstream (towards the starting point) to the cistron’s structural gene. It must be present and active only then the cistron will work or express itself. It provides the site for the binding of RNA polymerase enzyme which binds to the promoter region of the cistron so that it can scan the nucleotides of the structural genes and can transcribe the mRNA. So, promoter gene is the site where the RNA polymerase bind.
14. Terminator gene
This gene (a segment or part of DNA) is present towards the downstream of the cistron where transcription terminates and where RNA polymerase enzyme dissociates from DNA. So, it is a DNA site where RNA polymerase activity stops and transcription terminates.
15. Silencer gene
They silence the activities of other genes. If the protein product of a gene is not needed by the cell, that particular gene will be silenced by the silencer gene.
16. Enhancer gene
Theses genes enhance the activities of other genes related to the production of a particular protein product when demand for this product is high in a cell.
17. Integrated gene
They can be present in different chromosomes or different parts of a single chromosome. All these genes are activated simultaneously whey they are supposed to involve in a particular biochemical pathway which involves 10 or more enzymes. So, there must be 10 or more proteins for 10 or more enzymes and similarly, there must be 10 or more number of cistrons to produce the required number of proteins. So all these cistrons are protein-coding genes that are switched on simultaneously when the metabolic pathway is going on.