The origin of the word gene is from the Greek word genes meaning born. The smallest piece of DNA that controls an organism’s characteristics and causes variation, mutation and evolution is called a gene. It is called the blue print of life. The location of a gene on a chromosome is called a locus. A locus on a chromosome contains two identical or different genes. Two genes are called alleles or allelomorphs of each other.
Discovery and naming of genes
Gregor Johann Mendel called the carriers of heredity as particles or factors. In 1909 Johansen named this particle or factor as gene. In 1912, scientist Thomas Hunt Morgan proved that genes are located on chromosomes. Indian scientist Har Gobinda Khorana won the Nobel Prize in 1969 for discovering artificial genes.
In 1908, scientist Garrol first introduced the one gene, one enzyme theory. This was later called the one gene one polypeptide chain theory. In 1941, scientist Bidom and Tatum studied the fungus Neurospora crassa and said that specific genes produce specific enzymes.
Number of genes
The number of genes can range from a few thousand to several million depending on the species. Mammalian cells can contain over 50,000 genes. Human diploid cells have 30,000-40,000 genes. Human chromosome 1 has the highest number of 2968 genes and Y-chromosome 23 has the lowest number of 231 genes.
Nature of genes
Genes create new variations through mutation. A gene controls a trait. Sometimes more than one gene controls a trait. Gene expression can also be regulated by the environment. Mammalian cells may contain over 50,000 genes. Each gene contains a certain number of nucleotides. The smallest gene has 75 and the largest gene has 40,000 nucleotides recorded.
The coding part of a gene is called an exon and the non-coding part is called an intron. Exons synthesize proteins. Sometimes exons are non-coding like introns. Such a gene is called a speit gene.
Genome
All the genes present in a set of chromosomes are collectively called the genome. German botanist Hans Winkler first used the term genome in 1920. The human genome contains about 3000 million base pairs and 20,000-25,000 genes. In 2007, 2900 million nucleotides and about 30,000 genes were recorded in the human genome. 99.9 percent of human genomes (genes) are identical. Due to the difference in the structure of only 0.1% of the genes, there are different types of people in the world. Only 2% of human genes express the trait. The remaining 98% of genes are inactive. These inactive genes (98%) are called junk DNA. Mycoplasma genitalium has 517 genes, E. coli has 5416 genes and Arabidopsis thaliana has 25,000 genes.
Characteristics of genes
1. A gene is the smallest piece of DNA.
2. Genes are hereditary components. It carries hereditary characteristics.
3. It is self-reproducing. That is, genes have the ability to reproduce.
4. It is located in the chromosomal locus.
5. It causes variation, mutation and evolution of organisms.
6. It is made up of nucleic acids.
7. Multiple genes are responsible for a trait.
8. New genes are formed by rearranging the elements of the gene.
9. Genes are arranged in linear order on chromosomes.
8. 3 constitutive genes of lactose operon and 5 constitutive genes of tryptophan.
9. Multiple genes combine to express a trait. Drosophila eye color is controlled by 20 genes.
10. A gene controls multiple traits. The skin and hair color of albinos (humans) is caused by a single gene mutation.
11. The genes of some viruses (plant viruses) are made up of RNA.
Modern information of genes
1. A gene is a segment of deoxyribonucleic acid.
2. Gene sequences are unique throughout the genome.
3. All the elements needed to produce protein, tRNA or rRNA are encoded in genes.
4. Crossovers and mutations occur between genes for rearrangement.
5. Genes can influence other traits and this happens often.
6. Some genes may be partially responsible for the expression of a specific trait. In this case another gene acts to fully express the trait.
Units of genes
1. Cistron: The smallest piece of DNA that makes a protein is called a cistron. It carries almost the same meaning as jinn. A cistron contains multiple recons and mutons. Hence the length of cistron is much greater than the length of ricin and muton. A cistron of E. coli bacteria consists of about 1500 pairs of nucleotides.
2. Muton: The smallest piece of DNA that causes mutation is called muton. It is made up of one or two pairs of nucleotides.
3. Recon: The smallest piece of DNA that causes recombination is called Recon. It is made up of one or two pairs of nucleotides.
4. Replicon: The smallest piece of DNA that replicates is called a replicon.
Type of gene
1. Holandric gene: The gene present in Y-chromosome is called Holandric gene. This is the human ear hair expressing gene.
2. Sex-chromosomal gene: The gene present in X-chromosome is called sex-chromosomal gene. It is responsible for hemophilia, color blindness etc.
3. Autosomal gene: The gene present in autosome is called autosomal gene. It is responsible for baldness, albinism etc.
4. Split gene: A gene that consists of introns and exons is called a split gene.
5. Trans gene: A gene that takes characteristics from one cell and replaces it in another cell is called trans gene.
6. Lethal gene: The expression of the gene causes the death of the organism is called lethal gene.
7. Pseudo gene: A gene that is inactive or does not form a polypeptide is called a pseudo gene.
8. Oncogene: The gene that causes cancer is called oncogene gene.
9. House keeping gene: These are called constitutive genes. These genes are constantly active to carry out the normal functions of metabolic cells.
10. Luxury gene: These are called non-constitutive genes. These genes are not constantly active. It is activated only when needed.
11. Luxury gene: The gene used in genetically modified crops is the terminator gene. It prevents the plant from producing fertile seeds.
Functions of gene
1. Self-replication: Each gene can reproduce itself through replication.
2. Mutation: Genes can change the structure of DNA through mutation.
3. Heterocatalysis: Genes can control any property of an enzyme. This is called heterocatalysis.
4. Transmission of Hereditary Traits: Traits of parents are passed on to offspring through genes. Heredity is the process of passing on characteristics of parents to offspring. It is also called genetic transmission.
5. Self-reproductive power: Genes have self-reproducing power.
6. Synthesis: It synthesizes proteins, hormones and enzymes in the body.
7. Control of traits: A trait of an organism is controlled by multiple genes. Again, in some cases more than one trait is controlled by a gene.
8. Body Structure and Metabolism: Genes shape the organism’s body and control metabolism.
9. Creation of new variation: The exchange of genes during sexual reproduction creates new variation.