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

Griffith Experiment:

Background:

  • 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.

Experiment:

  • 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.



Conclusion:

  • 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.

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