Genes are made up of DNA. Each chromosome contains many genes.

Gene structure

Types of Genes:

1. House Keeping Genes (Constitutive Genes):
   • They are those genes which are constantly expressing themselves in a cell because their products are required 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.
     1. 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.
     2. 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.
3. Multigenes (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 (e, 5, (3, у on chromosome 11, oc and 8 on chromosome 16).
4. Repeated Genes:
   • The genes occur in multiple copies because their products are required in larger quantity, e.g., histone genes, tRNA genes, rRNA genes, actin genes.
5. Single Copy Genes:
   • The genes are present in single copies (occasionally 2—3 times), e.g., protein coding genes. They form 60—70% of the functional genes. Duplica­tions, mutations and exon reshuffling can form new genes.
6. 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, e.g., several of snRNA genes.
7. Processed Genes:
   • They are eukaryotic genes which lack introns. Processed genes have been formed probably due to reverse transcription or retroviruses. Processed genes are generally non-functional as they lack promoters.
8. Split Genes:
   • Split genes are those genes which possess extra or nonessential regions interspersed with essential or coding parts. The nonessential parts are called introns, spacer DNA or intervening sequences (IVS). Essential or coding parts are called exons. Transcribed intronic regions are removed before RNA passes out into cytoplasm. 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 prokaryotes.
9. 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 possess repetitive DNA, either similar or inverted, at their ends, some 5, 7 or 9-nucleotide long. Enzyme transposase separates the segment from its original by cleaving the repetitive sequences at its ends.
   • There are many types of transposons. e.g., proto-oncogenes → oncogenes. New genes may develop by exon shuffling. Other changes caused by transposons are mutations, through insertions, deletions and translocations.
10. Overlapping Genes:
    • genes В E and К overlap other genes.
11. Structural Genes:
    • Structural genes are those genes which have encoded informa­tion for the synthesis of chemical substances required for cellular machinery.
    • The chemical substances may be:
      • (a) Polypeptides for the formation of structural proteins (e.g., colloidal complex of protoplasm, cell membranes, elastin of ligaments, collagen of tendons or carti­lage, actin of muscles, tubulin of microtubules, etc.).
      • (b) Polypeptides for the synthesis of enzymes,
      • (c) Transport proteins like haemoglobin of erythrocytes, lipid transporting pro­teins, carrier proteins of cell membranes, etc.
      • (d) Proteinaceous hormones, e.g., insulin, growth hormone, parathyroid hormone,
      • (e) Antibodies, antigens, certain toxins, blood coagu­lation factors, etc.
      • (f) Non-translated RNAs like tRNAs, rRNA. Broadly speaking, structural genes either produce mRNAs for synthesis of polypeptides/proteins/enzymes or noncoding RNAs.
12. Regulatory Genes (Regulatory Sequences):
    • Regulatory genes do not transcribe RNAs for controlling structure and functioning of the cells. Instead, they control the func­tions of structural genes. The important regulatory genes are promoters, terminators, operators and repressor producing or regulator genes. Repressor does not take part in cellular activity. Instead, it regulates the activity of other genes. Therefore, repressor producing gene is of intermediate nature.
13. Tissue Specific Genes:
    • They are genes which are expressed only in certain specific tissues and not in others.