Packaging of DNA Helix The length of DNA in humans can be calculated as : Distance between two consecutive base pairs is 0.34 nm = 0.34 × 10-9 m
Total number of base pairs in a DNA helix in a typical mammalian cell = 6.6 × 109
∴ Length of this DNA double helix = 6.6 × 109 × 0.34 × 10-9 m = 2.2 m (approx.)
Thus, we see that, the length of DNA is far greater than the dimension of a typical nucleus, which is approx 10-6m in diameter. Therefore, there must be some special arrangement to accommodate such a long polymer in a cell of both prokaryotes and eukaryotes.
Transforming Principle:
Frederick Griffith carried out a series of experiments with Streptococcus pneumoniae (a bacterium that cause pneumonia). He observed that when these bacteria (Streptococcus pneumoniae) where grown on a culture plate, some of them produced smooth, shiny colonies (S-type), whereas the others produced rough colonies (R-type).
This difference in appearance of colonies (smooth/rough) is due to the presence or absence of mucus (polysaccharide) coat on S-strains, but not on R-strains.
In his experiments, he first infected two separate groups of mice. The mice that were infected with the S-strain died from pneumonia as S-strains are the virulent strains causing pneumonia.
S-strain (virulent strain) → Injected into mice → Mice died. The mice that were infected with the R-strain did not develop pneumonia and they lived.
R-strain (non-virulent strain) → Injected into mice-Mice lived. In the next set of experiments. Griffith killed the bacteria by heating them. The mice that were injected with heat-killed Strain bacteria did not die and lived.
S-strain (heat killed) → Injected into mice → Mice lived. Whereas, on injecting a mixture of heat-killed S-strain and live R-strain bacteria, the mice died Moreover, living Sbacteria was recovered from the dead mice.
S-strain (heat killed) + R-strain (live) → Injected into mice → Mice died.
From all these observations Griffith concluded that the live R-strain bacteria, had been transformed by the heat-killed S-strain bacteria, i.e. some ‘transforming principle’ had transferred from the heat-killed S-strain, which helped the R-strain bacteria to synthesise a smooth polysaccharide coat and thus, become virulent. This must be due to the transfer of the genetic material. However, he was not able to define the biochemical nature of genetic material from his experiments.
Biochemical Characterisation of Transforming Principle:
Oswald Avery, Colin MacLeod and Maclyn McCarty worked to determine the biochemical nature of ‘transforming principle’ in Griffth’s experiment in an in vitro system. Prior to this experiment, the genetic material was thought to be protein.
During this experiment purified biochemicals (i.e. proteins, DNA, RNA, etc.) from the heat-killed S-cells were taken, to observe which biochemicals could transform live R-cells into S-cells. They discovered that DNA alone from heat-killed S-type bacteria caused the transformation of non-virulent Rotype bacteria into S-type virulent bacteria.
They also discovered that protein-digesting enzymes (proteases) and RNA digesting enzymes (RNase) did not inhibit this transformation. This proved that the transforming substance was neither protein nor RNA. DNA-digesting enzyme (DNase) caused inhibition of transformation, which suggests that the DNA caused the transformation. Thus, these scientists came to the conclusion that DNA is the hereditary material. But still, not all biologists were convinced.
