4. Molecular basis of Inheritance

The Discovery of DNA:

1.      Modern understanding of DNA has evolved from the discovery of nucleic acid to the development of the double-helix model.

2.      In 1869, Friedrich Miescher began working with white blood cells which are the major component of pus from infections. He collected a lot of pus from bandages at the local hospital. He used a salt solution to wash the pus off the bandages.

3.      When he added a weak alkaline solution to the cells, the cells lysed and nuclei precipitated out of the solution.

4.       From the cell nuclei, he isolated a unique chemical substance to which he called nuclein.

5.       Chemically, nuclein has high phosphorus content.

6.       It showed acidic properties. Hence it was named as nucleic acid.

7.                  By the early 1900s, we knew that Miescher's nuclein was a mix (mixture) of proteins and nucleic acids. There are two kinds of nucleic acids. - 1. DNA (deoxyribonucleic acid)

                                                      2.  RNA (ribonucleic acid).

The Genetic Material is a DNA:

1.      By the early 1900s, geneticist knew that genes control the inheritance of traits, that genes are located on chromosome and that chemically chromosomes are mainly composed of DNA and proteins.

2.      Initially, most geneticists thought that protein are large, complex molecules and store information needed to govern cell metabolism.

3.      Hence it was assumed that proteins caused the variations observed within species.

4.      On the other hand, DNA thought to be small, simple molecule whose composition varied little among species.

5.      Over the time, these ideas about DNA were shown to be wrong.

6.      In fact, DNA molecules are large and vary tremendously within and among species.

7.      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.

8.      Over a period of roughly 25 years (1928-1952), geneticists became convinced that DNA and not protein, was the genetic material.

Griffith’s experiments:

1.      In 1928, a British medical officer Frederick Griffith performed an experiment on bacterium Streptococcus pneumoniae that causes pneumonia in humans and other mammals.

2.      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.

3.      The two strains used were: - i. Virulent, smooth, pathogenic and encapsulated S type.

                                                   ii. Non-virulent, rough, non-pathogenic and non-capsulated R type.

4.      Griffith conducted four experiments on these bacteria.

5.       First, when he injected bacteria of strain R to mice, the mice survived because it did not develop pneumonia.

6.      Second, when he injected bacteria of strain S to mice, the mice developed pneumonia and died.

7.      In the third experiment, he injected heat-killed strain S bacteria to mice, once again the mice survived.

8.      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.

9.      In these four experiments, something had caused harmless strain R bacterium to change into deadly S strain bacterium.

10.       Griffith showed that the change was genetic.

11.      He suggested that genetic material from heat-killed strain S bacterium had somehow changed the living strain R bacterium into strain S bacterium.

12.      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.