Unit – I A
Gene and Its Nature
- A gene may be defined as a segment of DNA which is responsible for inheritance and expression of a particular character.A gene is a segment of DNA that provides instructions for synthesis of a specific protein or a particular type of RNA.
- A gene provides instructions for synthesis of a specific protein or a particular type of RNA.
Characteristics of Genes:
- Genes are the functional unit of heredity, variation, mutation and evolution. They are situated in chromosomes.
- Every gene occupies fixed position in a chromosome. They are arranged in a single linear order in a chromosome.
- They express them by synthesis of proteins, which control cell metabolism. Thus they determine the physical and metabolic characteristics of the cell.
- In a single a gene they may occur in several different forms called alleles.
- They may show a sudden change from one form to another. This sudden change is called mutation and the new allele is called mutant.
Modern Concept of Gene:
- Seymour Benzer in 1955 introduced the terms cistron, muton and recon
Cistron (Unit of function):
- It is a segment of DNA having information of synthesis of particular protein or RNA.
- It is responsible for expression of a trait.
- It can be several bp (base pairs) long.
Muton (Unit of mutation):
- It is segment of DNA that can undergo mutation.
- It consists of few nucleotides (one to few bp long).
Recon (Unit of recombination)
- It is a segment of DNA that participates in recombination through crossing over during meiosis.
- It consists of few to many base pairs.
Significance of DNA in Heredity:
- For continued existence of a biological species, it requires a genetic material which should have properties like a) It should be chemically stable b) It should express accurately c) It should replicate with minimum errors d) It should provide a scope for changes (mutation) which is required for evolution.
- DNA shows all these features
- By 1926 the mechanism for genetic inheritance had reached the molecular level.
- The work by Gregor Mendel, Walter Sutton, Thomas Hunt Morgan, and numerous other scientists had narrowed the search to the chromosomes located in the nucleus of most cells.
- But exactly which molecule is responsible for heredity was not confirmed.
- In 1928 Frederick Griffith, in a series of experiments with Diplococcus pneumoniae (bacterium responsible for pneumonia), witnessed a miraculous transformation in the bacteria. During the course of his experiment, the bacteria (living organism) had changed in physical form.
- The pneumococcus bacterium occurs naturally in two forms with distinctively different characteristics. The virulent or pathogenic (S-strain) form has a smooth polysaccharide capsule that is essential for infection. The nonvirulent or nonpathogenic (R-strain) lacks the polysaccharide capsule, giving it a rough appearance.
Step – 1
S-type of the pneumococcus bacteria were injected to healthy mice. The mice were infected and died from pneumonic infection within a few days,
Step – 2
R-type of the pneumococcus bacteria were injected to healthy mice. The mice were not infected and continue to live.
Step – 3:
Heat Killed S-type of the pneumococcus bacteria were injected to healthy mice. The mice were not infected and continue to live.
Step – 4:
A mixture of heat-killed S-type and live R-type pneumococcus bacteria were injected to healthy mice. It produced lethal results. The mice died. On observation, Griffith discovered a mixture of R-Type and living forms of the S-type bacteria in the infected dead mice.
- Griffith hypothesized that something has transformed the non-lethal R-type bacteria into lethal s – Type bacteria.The heat-killed S-strain bacteria should be responsible for it.
- Some “transforming principle”, enabled the R-strain to synthesize a smooth polysaccharide coat and become virulent.
- He further observed that “transforming principle” was transferred to the next generation. Thus “transforming principle” should be a genetic material. Further, it was proved that the “transforming principle” referred by Griffith is DNA.