MOLECULAR BASIS OF INHERITANCE - PART-I

In this session Bio Strips Media takes through the important topics of Molecular Basis of Inheritance. It is one of the most important and scoring topics in Plus Two Biology Exam and NEET Exam. 

This lecture covers:

·         History of genetic material

·         Transforming principle by Frederick Griffith

·         Biochemical characterization of Transforming Principle by Avery, McLeod and McCarty

·         Transduction Experiment by Hershey and Chase

The content being discussed in this video will be helpful for those appearing for the Plus Two Biology Exam and NEET Biology Exam.

The information in this video is very useful to them and it helps those NEET aspirants to score maximum marks in Biology.

The Biology content in this video will be helpful for those candidates appearing for Kerala SET Zoology Exam, Kerala SET Botany Exam, KTET Natural Science Exam and various other Entrance Exams. 

Evidences of DNA as the genetic material

The following experiments conducted by the molecular biologists provide direct evidences of DNA being the genetic material.

Bacterial transformation or Griffith's Experiments : Griffith  (1928) injected into mice with virulent and smooth (S-type, smooth colony with mucilage) form of Streptococus pneumoniae. The mice died due to pneumonia. No death occurred when mice were injected with nonvirulent or rough (R-type, irregular colony without mucilage) form or heat- killed virulent form. However, in a combination of heat killed S-type and live R-type bacteria, death occurred in some mice. Autopsy of dead mice showed that they possessed S-type living bacteria, which could have been produced only by transformation of R-type bacteria.

Biochemical analysis

The transforming chemical was found out by O.T. Avery, C.M. Mc. leod and M. Mc. Carty. They fractionated heat-killed S-type bacteria into DNA, carbohydrate and protein fractions. DNA was divided into two parts, one with DNAase and the other without it. Each component was added to different cultures of R-type bacteria. Transformation was found only in that culture which was provided with intact DNA of S-type. Therefore, the trait of virulence is present in DNA. Transformation involves transfer of a part of DNA from surrounding medium or dead bacteria (donor) to living bacteria (recipient) to form a recombinant.

Bacteriophage experiment

Hershey and Chase (1952) conducted their experiment on T2 bacteriophage, which attacks on E.coli bacterium. The phage particles were prepared by using radioisotopes of S35 and P32 in the following steps :

(i) Few bacteriophages were grown in bacteria containing 35S. Which was incorporated into the cystein and methionine amino acids of proteins and thus these amino acids with 35S formed the proteins of phage.

(ii) Some other bacteriophages were grown in bacteria having ³²P. Which was restricted to DNA of phage particles. These two radioactive phage preparations (one with radioactive proteins and another with radioactive DNA) were allowed to infect the culture of E.coli. The protein coats were separated from the bacterial cell walls by shaking and centrifugation.

The heavier infected bacterial cells during centrifugation pelleted to bottom. The supernatant had the lighter phage particles and other components that failed to infect bacteria. It was observed that bacteriophages with radioactive DNA gave rise to radioactive pellets with 32P in DNA. However in the phage particles with radioactive protein (with35S) the bacterial pellets have almost nil radioactivity indicating that proteins have failed to migrate into bacterial cell. So, it can be safely concluded that during infection by bacteriophage T₂, it was DNA, which entered the bacteria. The above experiment clearly suggests that it is phage DNA and not protein, which contains the genetic information for the production of new bacteriophages. However, in some plant viruses (like TMV), RNA acts as hereditary material (being DNA absent).