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.
