Definition: Gene is a segment of DNA that codes for a functional protein, rRNA, tRNA or ribozymes.
The definition of gene is continuously evolving with our expansion of our knowledge in the field of molecular biology. After discovering nucleus as the region responsible for inheritance, it took another century to put forward the definition of gene that I have given above.
I think without mentioning the efforts of great scientists behind this understanding, this definition is incomplete.
This is the time line that tells us how our understanding about genes evolved?
In 1880’s The first question: 1) What governs inheritance?
1900-It was Gregor Mendel, who provided the first insight into the matter. By his famous Pea experiment, he concluded that certain ‘factors’ are present in the cells of each parent and these factors are responsible for inheritance. Now we know that the Mendelian ‘factors’ are the genes.
The next question was 2) where these factors are located in the cell?
1860-Ernest Haeckel and Hertwig postulated that nucleus is responsible for heredity
1875-Strasburger discovered chromosomes and along with kolliker and Weismen formulated ‘Nuclear theory of heredity’
1900- Rediscovered Mendels laws by Devries, Correns and Tscermark
1902-Sutton proposed ‘Chromosome theory of heredity’ and postulated that Mendel's hereditary factors were physically located on chromosomes.
Definition of gene (1905)
‘Factors (genes) are located in the nucleus and is responsible for inheritance’
1909-Johannsen coined the term ‘gene’ and proposed genotype phenotype concept
1911- T.H Morgan postulated that “genes are located in a linear array on the chromosome’ in his book “The theory of gene”.
Definition of gene (1911)
Genes are located in a linear array on the chromosome and is responsible for a phenotype/trait or character.
Question number 3) What is the chemical nature of gene?
The more obvious question was is it DNA or protein?
At that time, majority of the scientific world voted for proteins. Proteins are everywhere inside a cell with maximum diversity in function and abundance. There are 20 amino acids compared to 4 variants in the case of DNA. Many group worked with the hypothesis that genes are proteins.
But surprisingly in 1944-Avery, Mc Leod and Mc Carty found out that genes were made of DNA not proteins by their experiments in Streptococcus pneumonia
1944- Beadle and Tatum proposed ‘one gene one enzyme hypothesis’
Each gene synthesises a protein and which in turn catalyses a chemical reaction
Definition of gene 1944: ‘A gene is capable of synthesising a protein which in turn catalyses a chemical reaction’
In 1953, Watson and Crick, proposed the double helical structure of DNA that could explain major processes of a genetic material like replication, protein synthesis etc
1965- Holley proposed a clover leaf model of tRNA molecule
This DNA model changed the perception about many molecular biology processes. Later Crick with many others proposed the existence of a RNA world before the takeover by DNA and the involvement the role of messenger RNA in protein synthesis.
Discovery of rRNA and tRNA as an adaptor molecules in protein synthesis (many workers involved)
A major discovery that supported the existence of RNA world was the discovery of RNA enzymes by
1983-Thomas Cech et al;
The ribozyme discovered was spliceosomes or self splicing intron in the ribosomal rRNA genes of the ciliated protozoan Tetrahymena thermophila
Later many ribozymes were discovered and confirmed the ability of RNA molecule to catalyze biological reactions. Ribozymes are involved in many critical reactions in the cell.
Our understanding of gene is evolving day by day…
I think the best definition that fits today is
“Gene is a segment of DNA that codes for a functional protein, rRNA, tRNA or ribozymes”