04 Molecular Basis of Inheritance - part 01 - Genetic Material is a DNA

04 Molecular Basis of Inheritance - part 01 - Genetic Material is a DNA

The Discovery of DNA:

• Modern understanding of DNA has evolved from the discovery of nucleic acid to the development of the double-helix model. 
• In 1869, Friedrich Miescher began working with white blood cells which are the major component of pus from infections. 
• He collected a lot of pus from bandages at the local hospital. He used a salt solution to wash the pus off the bandages. 
• When he added a weak alkaline solution to the cells, the cells lysed and nuclei precipitated out of the solution. From the cell nuclei, he isolated a unique chemical substance to which he called nuclein. 
• Chemically, nuclein has high phosphorus content. Moreover it showed acidic properties. Hence it was named as nucleic acid. 
• By the early 1900s, we knew that Miescher's nuclein was a mix (mixture) of proteins and nucleic acids. 
• There are two kinds of nucleic acids- 

1. DNA (deoxyribonucleic acid) and 
2. RNA (ribonucleic acid).

The Genetic Material is a DNA:

•  DNA molecules are large and vary tremendously within and among species. 
• Variations in the DNA molecules are different than the variation in shape, electrical charge and function shown by proteins so it is not surprising that most researchers initially favored proteins as the genetic material. 
• Over a period of roughly 25 years (1928-1952), geneticists became convinced that DNA and not protein, was the genetic material. 

Griffith's experiments :

• In 1928, a British medical officer Frederick Griffith performed an experiment on bacterium Streptococcus pneumoniae that causes pneumonia in humans and other mammals. 
• Griffith used two strains or two genetic varieties of Streptococcus to find a cure for pneumonia,which was a common cause of death at that time. 
• The two strains used were : 
  

1. Virulent, smooth, pathogenic and encapsulatedS type. 
2. Non-virulent, rough, non-pathogenic and non-capsulated R type. 

• Griffith conducted four experiments on these bacteria. 
• First, when he injected bacteria of strain R to mice, the mice survived because it did not develop pneumonia. 
• Second, when he injected bacteria of strain S to mice, the mice developed pneumonia and died. 
• In the third experiment, he injected heat-killed strain S bacteria to mice, once again the mice survived. 
• In fourth experiment, he mixed heat-killed S bacteria with live bacteria of strain R and injected to mice. The mice died and Griffith recovered large numbers of live strain S bacteria from the blood of the dead mice. 
• In these four experiments, something had caused harmless strain R bacterium to change into deadly S strain bacterium. 
• Griffith showed that the change was genetic. 
• He suggested that genetic material from heat-killed strain S bacterium had somehow changed the living strain R bacterium into strain S bacterium. 
• Griffith concluded that the R-strain bacterium must have taken up, to what he called a "transforming principle" from the heat killed S bacterium, which allowed R strain to get transformed into smooth-coated bacterium and become virulent.

Avery, McCarty and MacLeod's experiment: 

• In 1944, after some 10 years of research and experimentation, U. S. microbiologists Oswald T. Avery, Colin M. MacLeod and Maclyn McCarty (all at Rockefeller University in New York) first evidenced to prove the DNA is a genetic material (transforming principle), through the experiments. 
• They purified DNA, RNA, Proteins (enzymes) and other materials from cell free extract of S cells/ strain and mixed with heat killed S strain and R cells seperately to confirm which one could transform living R cells into S cells. 
• Only DNA was able to transform harmless strain R into deadly strain S.
• They also discovered that protein ­digesting enzymes (proteases), RNA-digesting enzyme (RNAases) did not affect transformation, so the transforming substance was neither a protein nor RNA. 
• DNA digested with DNAse did inhibit the transformation, suggesting that DNA caused the transformation. 
• These experiments proved that the transforming principle is DNA but all biologists were not convinced.
• Finally, Alfred Hershey and Martha Chase(1952) proved that DNA is the genetic material and not proteins, by using bacteriophages.

Hershey - Chase Experiment:

• Hershey and Chase worked with viruses that infect bacteria i.e. bacteriophages, which are composed of DNA and protein. 
  
• They used radioactive phosphorous ³²P in the medium for some viruses and radioactive sulphur³⁵S for some others. 
• They grew some viruses on a medium that contained radioactive phosphorus and some others on medium that containedradioactive sulphur. 
• Viruses grown in the presence of radioactive phosphorus contained radioactive DNA(labelled DNA), but not radioactive proteins because DNA contains phosphorus (labelled DNA) but proteins do not. 
• Similarly, viruses grown on radioactive sulphur contained radioactive protein but not radioactive DNA because DNA does not contain sulphur. 
• Radioactive phages were allowed to infect E.colibacteria grown on the medium containing normal `P' and `S'. 
• Then, as the infection proceeded, the viral coats were removed with the help of centrifuge. 
• Bacteria which were infected by viruses with radioactive DNA, were radioactive, indicating that DNA was the material that passed from the viruses to the bacteria. 
• Bacteria which were infected by viruses having radioactive sulphur (protein) were not radioactive. 
• This indicates that proteins from the viruses, did not enter the bacteria. 
• DNA is, therefore, the genetic material that is passed from virus to bacteria (fig. 4.3). 
• In other words, sometime after infection, radioactivity for `P' and `S' was tested. 
• Only radioactive `P' was found inside the bacterial cell, indicating that DNA is the genetic material.