DNA: The Genetic Material

Althought DNA was first discovered in the late ninetheenth century it was not until the middle of the twentieth centruy that the role of DNA as the genetic material was established. The following are brief summaries of two of the now classic experiments that helped demonstrate the role of DNA as the carrier of genetic information. A more complete description of their experiments can be found in your text book.

Avery, MacLeod and McCarty:

This research group expanded the earlier work of Griffith, who discovered the process of transformation in 1928. As you may remember, Griiffith was studying two different genetic strains of Streptococcus pneumoniae, a bacterium that casues pneumonia in mammals. One of the strains, termed the S strain, was pathogenic and mice injected with this strain soon died. You should also note that living S bacteria could be isolated from dead mice. The second strain, the R strain, was not pathogenic and an injection of living R strain did not kill the mice. The researcher then injected heat-killed bacteria (both S and R strains) and neither of the 'dead' strains had any effect on the mice. Thus it was established that only living S strain bacteria were pathogenic. Griffith then incubated living R and heat-killed S prior to injecting the mixture into the mice. He subsequently found that this mixture was pathogenic and killed the mice. When he isolated bacteria from these dead mice, he found living S strain bacteria! (Remember, he did not inject any living S type bacteria. He concluded that some factor from the dead S was assimilated into the R bacteria and transformed them into the S type. Since this transformation was inherited by subsequent generations of bacteria, it was assumed that the factor that was the genetic material.

Avery and colleagues investigated the chemical nature of this transforming factor - more specifically, whether the transforming factor was a protein or a nucleic acid. They, like Griffith, attempted to transform the R strain into the S strain by incubating living R and heat-killed S. However, they pretreated the heat-killed S with either a protease (an enzyme that degrades proteins) or with DNAase, and enzyme that degrades DNA. They reasoned that if the transforming factor was a protein, treatment of the heat-killed S with a protease would destroy the protein and inhibit transformation and treatment with DNAase should have no effect on the transformation process. On the other hand, if DNA were the genetic material, the opposite would be true. In their experiments, Avery et al. found that protease did not affect the ability of 'dead' S to transform R but DNAase did, therefore they concluded that the genetic material in transformation is most likely DNA.

Hershey and Chase

In 1952, the research team of Hershey and Chase published a report that concluded that DNA is the genetic material of the bacteriophage (a virus that infects bacteria) T2 (strange name!). They knew that when a bacteria was infected with this phage, the bacteria soon became a machine that produced new phages. Phages have a very simple structure - they are composed of a strand of DNA surrounded by a protein coat. Hershey and Chase set out to determine which component (the DNA or the protein) was responsible for the ability of the phage to 'take over' and control the metabolic aciivity of the bacteria to produce new phages. In their experiments, they used two radioactive markers to label the proteins and the DNA of the phages. The proteins were labeled with 35S (a radioactive form of sulfur) and the DNA was labeled with 32P, a radioactive form of phosphorus. This allowed the researchers to easily differentiate between a sample that contained protein (thus would have 35S present) and a sample that only contained DNA (thus would have 32P present).

During a phage infection, it was hypothesized that some part of the phage was injected into the bacterium and it was this injected material that conveyed the genetic material necessary to produce new phages. Hershey and Chase determined that the phage injected only the DNA into the bacterium and concluded that DNA must be the genetic material in phages. This was done by a series of two experiments in which different sets of non-radioactive bacteria were incubated with phages that had either their protein or their DNA labeled (as described above). They allowed the phages to infect the bacteria for a short time, they agitated the incubations to dislodge any loose parts of the phages. The bacteria cells were then pelleted in a centrifuge and the location of the radioacivity (in the pellet with the bacteria or in the supernatant). They found that the radioactive DNA was always found with the bacteria cells and that the radioactive protein was always in the suupernatant. This suggested that the DNA was injected into the bacteria but the protein coat was not. Thus all of the information needed to produce new viruses was contained in the DNA and not the protein.

There will be no quiz associated with this page, however, it is expected that you know this material for the exam.

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