4. MOLECULAR BASIS OF INHERITANCE

Transformation:-

1) Griffith started experiment for identification of genetic material.

2) He selected the bacterium Diplococcus pneumoniae which exist in two different forms such as smooth-strain (s-strain) and rough-strain (R-strain)

3) The s-strain is capsulated in nature and is responsible for pneumonia in mammals.  Hence it is pathogenic or virulent.

4) R-strain is without capsule and is non-pathogenic or avirulent.

5) Griffith’s experiment can be represented as follows:

6) Griffith stated that presence of heat killed s-strain is responsible for change in cells of R-strain.  He described this that bacterial transformation.

7) Transformation occur through the transfer of some active substance or factor responsible for virulence.

8) Due to this reason non-virulent or Avirulent strain changed into virulent strain i.e. R-strain change to S-strain.

9) However Griffith could not found the nature of transforming substance.  He called it as transforming principle.

 

Avery, Macleod and Mc Carty experiment:-

1) Avery, Mc dead and Mc Carty perform same experiment as that of Griffith and identified nature of genetic material.

2) They separate carbohydrates, proteins and DNA fractions of s-strain.  This fractions were added separately into the nutrient medium containing R-strain.

3) They observed following facts:-

    A)    When carbohydrates were added to medium there was no change or transformation.

    B)    When proteins were added, no transformation was observed.

    C)    With the addition of DNA extract, there was change/transformation.

    D)    When DNA extract was added a long with DNase ,DNA destroying enzymes there was no transformation.

4) In the third case, when DNA extract was added, R-strain changed into s-strain.

5) Such type of change was not observed when proteins and carbohydrates we are added to the medium.

6) Similar types of results were obtain when DNA and the enzyme DNase were added together

7) Thus it was confirmed that the DNA is genetic material.

Transduction:-

1) The transfer of genetic information from one bacterial cell to another through the agency of viruses or bacteriophages is called as transduction.

2) Lederberg and Zinder discovered the phenomenon of transduction.

3) It was observed that new phages are produced in bacterial cells, fragments of bacterial DNA get incorporated into the DNA of newly formed phage.

4) This newly formed phage when affects or attacks new bacterial cell incorporate its DNA top the DNA of bacterial cell easily.

5) As a result of which new characters are introduced into the bacterial due to genetic recombination.

6) The transduced or transuded fragment induces new character indicating that DNA is the genetic material.

Hershey and chase experiment:-

Ans.: 1) Hershey and Chase proved that DNA is the genetic material while working with viruses that infect bacteria called as bacteriophages.

2) Phage attached to the bacteria and its genetic material enters the bacterial cell.

3) The bacterial cell treats the viral genetic material as if it was its own and subsequently manufactures/produces more viral particles.

4) Hershey and chase worked out to discover whether it was protein or DNA from the virus that entered the bacteria.

5) They cultured some viruses on a medium containing radioactive isotopes of phosphorus of some other on the medium that contained radioactive sulphur.

6) Viruses grown in the presence of radio-active phosphors contained radio-active DNA but not the ratio-active protein because DNA contains phosphorus but protein does not.

7) Similarly viruses grown on radio-active sulphur contained radio-active protein but not radio-active DNA because DNA does not contain sulphur of this labelled phages or radio-active phages allow to attach to E. coli bacteria.

9) Then as infection proceeded the viral coats were remove from the bacteria by agitating them in a blender.

10) The viral particles were separated from the bacteria by spinning them in centrifuge. 

11) Bacteria which was infected with viruses that had radio-active DNA were radio-active indicating that DNA is the material that passed viral to bacteria.

12) Bacteria that was infected with viruses that radio- active protein not radioactive. This indicates that protein did not enter the bacteria from the viruses.

Thus from above experiment it can be concluded that DNA is the genetic material.