4. Molecular Basis Of Inheritance

-CREATED BY-

PROF. DESHMUKH A. B.

ASST. TEACHER

AGASTI ARTS, COMMERCE AND DADASAHEB RUPWATE SCIENCE JUNIOR COLLEGE, AKOLE

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4.2 The Genetic Material is A DNA

Prof. Deshmukh A. B’s. Biology Class

4.2 The Genetic Material is a DNA:

• By the early 1900s, geneticist knew that
• genes control the inheritance of traits,
• genes are located on chromosome and
• chemically chromosomes are mainly composed of DNA and proteins.
• Initially, most geneticists thought that
• protein are large, complex molecules and
• store information needed to govern cell metabolism.
• Hence it was assumed that proteins caused the variations observed within species.

Prof. Deshmukh A. B’s. Biology Class

• On the other hand DNA thought to be small, simple molecule whose composition varied little among species.
• Over the time, these ideas about DNA were shown to be wrong.
• Infact 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.
• Let us study three important contributions that helped cause this shift of opinion.

Prof. Deshmukh A. B’s. Biology Class

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 :
• i. Virulent, smooth, pathogenic and encapsulated S type.
• ii. Non-virulent, rough, non-pathogenic and non-capsulated R type.

Prof. Deshmukh A. B’s. Biology Class

S- Strain

R- Strain

• 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.

Prof. Deshmukh A. B’s. Biology Class

• 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.

Prof. Deshmukh A. B’s. Biology Class

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.

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• 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 separately 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.

Prof. Deshmukh A. B’s. Biology Class

• 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.

Prof. Deshmukh A. B’s. Biology Class

Prof. Deshmukh A. B’s. Biology Class

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 sulphur ³⁵S for some others.
• They grew some viruses on a medium that contained radioactive phosphorus and some others on medium that contained radioactive sulphur.

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Prof. Deshmukh A. B’s. Biology Class

• 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.coli bacteria grown on the medium containing normal ‘P’ and ‘S’.
• Then, as the infection proceeded, the viral coats were removed with the help of centrifuge.

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Prof. Deshmukh A. B’s. Biology Class

• 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.

Prof. Deshmukh A. B’s. Biology Class