A gene is the basic physical and functional unit of heredity. Genes are made up of DNA. Some genes act as instructions to make molecules called proteins. However, many genes do not code for proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases.
A gene is a nested sequence of nucleotides in DNA or RNA that encodes the synthesis of a gene product, either RNA or protein.
Lewin (2000) has defined gene to be a sequence of DNA that codes for a diffusible product.
A gene is:
(i) A unit of genetic material which is able to replicate,
(ii) It is a unit of recombination, i.e., capable of undergoing crossing over,
(iii) A unit of genetic material which can undergo mutation,
(iv) A unit of heredity connected with somatic structure or function that leads to a phenotypic expression.
The Human Genome Project estimated that humans have between 20,000 and 25,000 genes.
Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.
example, a gene on chromosome 7 that has been associated with cystic fibrosis is called the cystic fibrosis transmembrane conductance regulator; its symbol is CFTR.
Genes are made up of DNA. Each chromosome contains many genes.
Gene structure
Types of Genes:
- 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
- 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.
- 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.
- 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.
- Inducible 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.
- 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).
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Overlapping Genes:
- genes В E and К overlap other genes.
- 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.
- 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.
- Tissue Specific Genes:
- They are genes which are expressed only in certain specific tissues and not in others.
Functions of Genes
- Genes control the functions of DNA and RNA.
- Genes are components of genetic material and are thus units of inheritance
- They control the morphology or phenotype of individuals
- Replication of genes is essential for cell division
- Genes carry the hereditary information from one generation to the next
- They control the structure and metabolism of the body
- Reshuffling of genes at the time of sexual reproduction produces variations
- Different linkages are produced due to crossing over
- Genes undergo mutations and change their expression
- New genes and consequently new traits develop due to reshuffling of exons and introns
- Genes change their expression due to position effect and transposons
- Differentiation or formation of different types of cells, tissues and organs in various parts of the body is controlled by expression of certain genes and non-expression of others
- Development or production of different stages in the life history is controlled by genes
- Gene produces proteins through the process of gene expression( transcription, translation). Proteins are the most important materials in the human body which not only help by being the building blocks for muscles, connecting tissue and skin but also takes care of the enzymes production.
- These enzymes play an important role in conducting various chemical processes and reactions within the body. Therefore, protein synthesis is responsible for all activities carried on by the body and are mainly controlled by the genes.
- Genes consist of a particular set of instructions or specific functions. For example, globin gene was instructed to produce haemoglobin. Haemoglobin is a protein that helps to carry oxygen in the blood.
