In this article we will discuss about the role of different organisms in the advancement of genetics.
Bacteria are unicellular free living organisms. They have a simple chromosome which is not enclosed in a nucleus. Bacteria have many features which make them suitable objects for genetical investigations. For example, they can be grown easily and rapidly and are simple in their needs as compared to multicellular organisms. The bacterium Escherichia coli has been most widely studied for genetical investigations.
It is a harmless bacterium found in the human colon. It has a single chromosome which is circular in nature. Besides single circular DNA molecule, E. coli possesses one or more smaller chromosomes called as plasmids. It has been named after its discoverer, Theodore Escherich, the species name has been derived from the colon where it resides.
The concept of genetic recombination was discovered by Beadle and Tatum on E. coli and operori concept was developed on E. coli by Jacob and Monod (1961) for which they were awarded Nobel Prize for Physiology or Medicine in 1965. They studied lactose metabolism in lac region of E. coli. Later on, operon model was studied in several other organisms.
Bacteriophages are viruses which grow only inside bacteria and kill them. Work on phage genetics was initiated in early thirties independently by Delbruck (a physicist), Schlesinger and Burnet. Most significant contribution in phage genetics was made by Delbruck. He along with Luria and Hershey discovered genetic recombination in phages in 1940s.
Later on phages have been extensively used as tools for the study of gene structure and function. The principle of transduction was invented by the study of bacteriophages. Structurally, viruses are the smallest organisms consisting of a nucleic acid (either DNA or RNA) and a protein coat of capsid.
Inside the host cell, they exist in the form of replicating nucleic acid molecules devoid of protein coats. The nucleic acid is double stranded DNA or single stranded RNA in most viruses. Benzer (1955) divided gene into cistron, recon and muton working on r-II locus of T4 phage of E. coli.
The fruitfly (Drosophila melanogaster) has been widely used for genetic studies.
Drosophila has several features, viz:
1. Short life cycle,
2. Produces large number of offspring,
3. Easy for handling,
4. Can be raised under controlled conditions,
5. Inexpensive to maintain and
6. Variation and recombination.
These features make Drosophila a very convenient tool for genetic investigations. Drosophila was first used for the study of linkage by Morgan (1910). Concepts of sex linked inheritance, sex determination; chromosomal aberrations, multiple alleles and mutation were developed by the study of Drosophila.
Muller used Drosophila for the study of induced mutations. The low chromosome number (2n = 8) of fruitfly has greatly facilitated the construction of linkage maps.
Neurospora crassa is a fungus, which belongs to ascomycetes and is also known as red bread mould or pink bread mould. Its diploid chromosome number is 14. Biochemical mutations have been studied on large scale in Neurospora by Beadle and Tatum. The important concept of one gene one enzyme was developed on Neurospora for which Beadle and Tatum won Nobel Prize for physiology or medicine in 1958.
According to this hypothesis each gene controls the reproduction, function and specificity of a particular enzyme. The study of Neurospora also contributed to the understanding of linkage and crossing (Tver. Concept of negative interference was also developed from studies on Neurospora.
5. Corn (2n = 20):
The maize plant has been extensively used for genetic and cytogenetic studies. Linkage maps have been constructed for all the ten chromosomes. Genetical studies on maize have significantly contributed to the advancement of concepts related to linkage, chromosomal aberrations, mutation, sex determination, cytoplasmic inheritance and heterosis. Mc Clintock discovered jumping genes working on maize.
6. Garden Pea:
The basic principles of heredity were discovered on garden pea by Mendel. Later on the coupling and repulsion concepts of linkage were also discovered by Bateson and Punnett (1905) on garden pea. The interaction of non-allelic genes was observed in garden pea.