The cell nucleus plays a key role in inheritance was recognized in the 1870s by the observation that the nuclei of male and female reproductive cells undergo fusion in the process of fertilization.
Soon thereafter, chromosomes were first observed inside the nucleus as thread-like objects that become visible in the light microscope when the cell is stained with certain dyes. Chromosomes were found to exhibit a characteristic “splitting” behavior in which each daughter cell formed by cell division receives an identical complement of chromosomes.
Further evidence for the importance of chromosomes was provided by the observation that, whereas the number of chromosomes in each cell may differ among biological species, the number of chromosomes is nearly always constant within the cells of any particular species. These features of chromosomes were well understood by about 1900, and they made it seem likely that chromosomes were the carriers of the genes.
By the 1920s, several lines of indirect evidence began to suggest a close relationship between chromosomes and DNA. Microscopic studies with special stains showed that DNA is present in chromosomes. Chromosomes also contain various types of proteins, but the amount and kinds of chromosomal proteins differ greatly from one cell type to another, whereas the amount of DNA per cell is constant. Furthermore, nearly all of the DNA present in cells of higher organisms is present in the chromosomes. These arguments for DNA as the genetic material were unconvincing, however, because crude chemical analyses had suggested that DNA lacks the chemical diversity needed in a genetic substance.
The favored candidate for the genetic material was protein, because proteins were known to be an exceedingly diverse collection of molecules. Proteins therefore became widely accepted as the genetic material, and DNA was assumed to function merely as the structural framework of the chromosomes. The experiments described below finally demonstrated that DNA is the genetic material.
Experimental Proof of the Genetic Function of DNA
The Griffth’s Experiment (1928):
An important first step was taken by Frederick Griffith in 1928 when he demonstrated that a physical trait can be passed from one cell to another.
He was working with two strains of the bacterium
Streptococcus pneumoniae identified as S (virulent= capable of killing the host) and R (non-virulent). When a bacterial cell is grown on solid medium, it undergoes repeated cell divisions to form a visible clump of cells called a colony.
The S type of S. pneumoniae synthesizes a gelatinous capsule composed of complex carbohydrate (polysaccharide). The enveloping capsule makes each colony large and gives it a glistening or smooth (S) appearance. This capsule also enables the bacterium to cause pneumonia by protecting it from the defense mechanisms of an infected animal. The R strains of S. pneumoniae are unable to synthesize the capsular polysaccharide, they form small colonies that have a rough (R) surface.
This strain of the bacterium does not cause pneumonia, because without the capsule the bacteria get inactivated by the immune system of the host.
Both types of bacteria “breed true” in the sense that the progeny formed by cell division have the capsular type of the parent, either S or R.
- Mice injected with living S cells get pneumonia.
- Mice injected either with living R cells remain healthy.
- Mice injected with heat-killed S cells remain healthy.
- The fourth observation was Griffith’s critical finding: mice injected with a mixture of living R cells and heat-killed S cells cause the disease—they often die of pneumonia. Bacteria isolated from blood samples of these dead mice contain virulent S cultures with a capsule typical of the injected S cells, even though the injected S cells had been killed by heat.
Evidently, the injected material from the dead S cells includes a substance that can be transferred to living R cells and confer the ability to resist the immunological system of the mouse and cause pneumonia. In other words, the R bacteria can be changed or undergo transformation into S bacteria. Furthermore, the new characteristics are inherited by descendants of the transformed bacteria.
The Avery–MacLeod–McCarty Experiment (Bacterial transformation) (1924):
Transformation in Streptococcus was originally discovered in 1928, but it was not until 1944 that the chemical substance responsible for changing the R cells into S cells was identified. In a milestone experiment, Oswald Avery, Colin MacLeod, and Maclyn McCarty showed that the substance causing the transformation of R cells into S cells was DNA.
In doing these experiments, they first had to develop chemical procedures for isolating almost pure DNA from cells, which had never been done before.
- They prepared cultures containing the heat-killed S strain and then removed lipids and carbohydrates from the solution.
- Next they treated the solutions with different digestive enzymes (DNase, RNase or protease) to destroy the targeted compound.
- Finally, they introduced living R strain cells to the culture to see which cultures would develop transformed S strain bacteria.
- Only in the culture treated with DNase did the S strain bacteria fail to grow (i.e. no DNA = no transformation)
- This indicated that DNA was the genetic component that was being transferred between cells.
Despite this finding, the scientific community was reluctant to accept the role of DNA as a genetic material. It was only 8 years later, when Hershey and Chase conducted their experiment, that the concept gained traction.