The History of Nucleic Acids

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•1909, Levene discovered D-ribose

•1910, Kossel was awarded the Nobel Prize in Physiology or Medicine for research in nucleic acids.

• 1912, Levene formulated his “ tetranucleotide hypothesis”

Two famous experiments:

1. 1944, Avery completed the transformation of Streptococcus pneumoniae

2. 1952, Bacteria , T2 phages ,

DNA component with ³²P

Protein component with ³⁵S

Transformation of Pneumococcus:

•Frederick Griffith, 1928

•Transforming principle

•Two strains of the Streptococcus pneumoniae bacterium: R-strain & S-strain.

S strain
- Smooth mucous polysaccharide coat
- Resistant to immune system
- Virulent
R strain
- Lacks the coat
- Destroyed by immune system of the host
- Non-virulent

Class_12_Molecular_Basis_Of_Inheritance_R-Strain_S-Strain

The experiment was performed in multiple steps:

S-strain (virulent) was injected into mouse. It was found that the mouse died of pneumonia

R-strain (non-virulent) was injected into mouse. It was found that the mouse remained alive

Class_12_Molecular_Basis_Of_Inheritance_R-Strain

Heat killed S-strain (S-strain bacteria were killed by heating) was injected into mouse. It was found that the mouse remained alive

Class_12_Molecular_Basis_Of_Inheritance_S-Strain1

Heat killed S-strain & live R-strain were injected into mouse. It was found that the mouse died of pneumonia

Griffith found that Live S-strain bacteria could be recovered from the dead mouse (Step 4).

Griffith thus arrived at the following conclusion:

Something caused bacteria to change from one type (R) to another type (S)
Some ‘Transforming principle’, transferred from heat-killed S strain to R strain and transformed it virulent.

Rockefeller University in New York, Avery

Separated a crude extract, including protein, nucleic acids, polysaccharide and lipids, from heat-killed S. pneumococcus and injected into non-virulent R strain

He found out only the nucleic acids of the dead S strain can transform the non-virulent R stain into a virulent form.

conclusion:

• The smooth strain (S strain) had a polysaccharide capsule and was virulent when injected, caused pneumonia and killed mice in a day or two. • The rough strain (R strain) did not cause pneumonia when injected into mice (it was avirulent) since it lacked a capsule.

However, the biochemical nature of the ‘Transforming principle’ was still unknown.

Hershey-Chase Experiment or Waring Blender experiment

•hershey and chase, 1952

•To confirm: DNA was the genetic material

•Experimented with bacteriophage, Bacteriophages are the viruses that infect & replicate within bacteria.

Bacteriophages were grown in two different mediums.

Some bacteriophages were grown in radioactive phosphorus medium. It was found that these Bacteriophages came up with radioactive DNA
Some bacteriophages were grown in radioactive sulfur medium. It was found that these Bacteriophages with radioactive protein.

Class_12_Molecular_Basis_Of_Inheritance_RadioactiveDNA_&_RadioactiveProtein

Bacteriophages with Radioactive DNA were brought in contact with bacteria
Bacteria got infected
Agitated in a blender to separate phage particles from bacterial cells
Centrifugation leaves Phage particles as supernatant
Bacterial cells were found to be radioactive
No radioactivity was detected in the phage particles

Bacteriophages with Radioactive protein were brought in contact with bacteria
Bacteria got infected
Agitated in a blender to separate phage particles from bacterial cells
Centrifugation leaves Phage particles as supernatant
Phage particles were found to be radioactive
No radioactivity was detected in the bacterial cells

It was therefore concluded that it was not the proteins, rather DNA which entered into the bacteria. Therefore, DNA causes the replication of viruses inside the bacteria.

DNA was thus proved to be the genetic material.

Have a look on: Classification, Composition, and Distribution of Nucleic acids