Search For Genetic Material – Molecular Basis Of Inheritance

CBSE Class12- Molecular Basis Of Inheritance- The Search For Genetic Material: The molecules which determine the inheritance are known as genes and it consists of two parts known as Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA). DNA acts as genetic material in all organisms except in viruses.

The search for Genetic Material started during the nineteenth century. The genetic material which is present in our cells is the DNA (deoxyribonucleic acid) which is responsible for the inheritance of traits and generally passed from the parents to the offspring. Many scientists performed numerous experiments to have evidence that determines that DNA is the genetic material.

The Search for Genetic Material

There are many scientists who performed various experiments to have evidence that determines that DNA is the genetic material. Previous discoveries made by Gregor Mendel, Walter Sutton, Thomas Hunt Morgan, and other scientists had narrowed the search to the chromosomes which are located in the nucleus of most cells but still the question of what molecule was actually the genetic material arises. Therefore the following experiments below determine the DNA as genetic material.

Transforming Principle

A couple of experiments were performed with Streptococcus pneumoniae by Frederick Griffith in 1928. When on a culture plate Streptococcus pneumoniae are grown some of them produce smooth shiny colonies (S) while others are producing rough colonies (R). The mice die from pneumonia which is infected with the S strain (virulent) while the other does not develop pneumonia which is infected with the R strain.

• S Strain – Inject into mice – Mice die
• R Strain – Inject into mice – Mice live

Griffith was able to kill the bacteria by heating them. When the heat-killed S strain bacteria were injected into the mice, it doesn’t kill the mice while when the heat-killed S strain and live R strain bacteria the mice died. Therefore, he recovered the living S bacteria from the dead mice.

• S strain – Inject into mice – Mice live
• S strain + R strain – Inject into mice – Mice die

Lastly, Griffith concludes that R-strain bacteria were transformed by the heat-killed S-strain bacteria which happens due to the transfer of the genetic material But he cannot conclude about the biochemical nature of genetic material.

Biochemical Characterization of Transforming Principle

Oswald Avery, Colin Macleod, and Maclyn McCarty worked together to determine the biochemical nature of the transforming principle in Griffith’s experiment. They purified the biochemicals such as proteins, DNA, RNA, and so on from the heat-killed S cells to analyze which one could transform live R cells into S cells.

They discovered that DNA alone from the S bacteria causes R bacteria to become transformed. They also discovered that the transformation doesn’t affect the protein-digesting enzymes and RNA-digesting enzymes. Therefore it was concluded that transforming substances are not RNA or protein and DNA was the cause of transformation. Thus, DNA is the hereditary material.

Hershey and Chase Experiment

Viruses that infect the bacteria are known as bacteriophages. Alfred Hershey and Martha Chase worked on bacteriophages. They grew some viruses on a medium that consists of radioactive phosphorus and also grew some other viruses on a medium that consists of radioactive sulfur. Phosphorus is present in DNA but it is absent in protein. Therefore, viruses that are grown in the presence of phosphorus consist of radioactive DNA. Sulfur is present in proteins but it is absent in DNA. Therefore, viruses consist of radioactive proteins doesn’t have radioactive DNA in them.

Radioactive phages can attach to E. coli bacteria. Virus particles are separated from the bacteria by spinning them in a centrifuge. The bacteria which were infected by viruses that had radioactive DNA were radioactive which shows that DNA was the material that passes from the virus to the bacteria. The bacteria which were infected by the viruses had radioactive proteins were not affected. 

Conclusion

Thus, it indicates that protein did not enter the bacteria from the viruses which proved that DNA passes from virus to bacteria and DNA is a genetic material.

Properties of Genetic Material

• It should be stable both structurally and chemically.
• It should express itself in the form of Mendelian Characters.
• It should follow mutation for evolution.
• It should obey replication and therefore can able to replicate.

Replication of DNA

Replication of DNA is the process in which two strands separate and act like a template for the synthesis of new complementary strands. When replication is completed each DNA molecule consists of one parental and one new synthesis strand. The schema of replication was known as semiconservative DNA replication.

FAQs on the Search for Genetic Material

Q1: What do you mean by the Transforming Principle?

Answer:

It is a principle in which a couple of experiments were performed with Streptococcus pneumoniae by Frederick Griffith in 1928 to conclude the transfer of genetic material.

Q2: What has been concluded from the Hershey and Chase Experiment?

Answer:

It has been concluded that DNA was the material that passes from virus to the bacteria. The bacteria which were infected by the viruses had radioactive proteins and were not affected. Thus, it indicates that protein did not enter the bacteria from the viruses which proved that DNA passes from virus to bacteria and DNA is a genetic material.

Q3: What are the two properties of genetic material?

Answer:

The two properties of Genetic material are as follows:-

• It should follow mutation for evolution.
• It can able to replicate.

Q4: What do you mean by Replication of DNA?

Answer:

It is the process in which two strands separate and acts like a template for the synthesis of new complementary strands.