DNA+carries+Hereditary+Information

Today, scientists understand that DNA is the carrier of hereditary information. However, if you were to go back in time a little less than 50 years ago, you would see the ambiguities that existed around the role of DNA and proteins in the acquisition of certain traits. Various experiments were performed by many different scientists to develop fundamental concepts about the properties of DNA.

Alfred Hershey and Martha Chase were able to demonstrate that DNA is involved in the replication of bacteriophages, which are viruses that infect bacterial cells. Two identical populations of phages were grown; one was grown in a radioactive isotope of sulfur to label the protein viral coat while the other one was grown in a radioactive isotope of phosphorous to label the DNA. Both groups were exposed to bacteria cells for them to infect the bacteria with the phages. After the viruses infected the bacteria, the contents were separated by centrifugation into two distinct sections. One section of the test tubes would contain the empty viral coats while the denser section of the test tube would contain the bacteria that were infected with the viral DNA. Viral reproduction occurred in the denser section of the test tubes, known as the pellet. However, the protein coats labeled with radioactive sulfur were found in the supernatant, which indicated that the protein did not enter the cells. After new viruses were produced, the DNA labeled with radioactive phosphate was found in the viruses. In contrast, the protein coats labeled with radioactive sulfur were not found anywhere in the viruses. This indicated that DNA is necessary for the synthesis of new phages and DNA was discovered as the genetic material in phages. This led to future demonstrations that DNA, which genes are composed of, was the molecular basis of inheritance for all living organisms.
 * //__A. Hershey and Chase - bacteriophage replication__//**

Fredrick Griffith and Oswald T. Avery conducted experiments that provided evidence that a genetic trait may be transmitted from one bacterial strain to another, otherwise known as bacterial transformation. In the first experiment, Griffith injected mice with a rough strain of bacteria that could not cause disease and the mouse was able to survive. In the second experiment, he injected mice with a smooth strain of the same bacteria that was considered virulent; as a result, the mice developed the disease and died. In the third test, the cells of the virulent strain were killed by exposing them to a high degree of heat and the mice were able to live. In the final trial, the avirulent, rough strain was mixed with the heat-killed virulent strain and injected into the mice. Even though these two strains did not kill the mice individually, the combination of the two did. After an autopsy of the mice was done, it was shown that the living cells of the virulent strain were present. This indicated that some substance, referred to as a “transforming principle”, was transferred from the dead bacteria to living cells, causing transformation. Avery and his team were able to chemically identify the transforming principle as DNA by treating living avirulent cells with nucleic acids taken from virulent cells. The cells that exhibited avirulence were transformed to into the smooth, virulent cells. This indicated that DNA could be transmitted between various bacterial strains and cause transformation.
 * //__B. Griffith and Avery - bacterial transformation__//**

__//**C. Meselson and Stahl - DNA replication in bacteria**//__ Meselson and Stahl carried out experiments that described the method by which DNA replication occurs in bacteria. They grew the bacteria E. coli in a heavy nitrogen growth medium for many generations to allow the bacteria to incorporate the heavy nitrogen into their DNA. The bacteria were later transferred to a medium containing light nitrogen and allowed to replicate and divide for only one generation. The resulting offspring cells were spun in a centrifuge and found to be midway between the bacteria grown in heavy and light nitrogen. That indicated that the DNA of the new bacteria consisted of one new strand and one old strand, proving that DNA replicates in a semi-conservative fashion.

Sources:


 * Solomon Biology Textbook, 8th Edition
 * Biology Online (See link in navigation)
 * Barron's AP Biology Review Book (2008 Edition)