Mendel the "Father of Genetics" performed an experiement in 1857 that led
to increased interest in the study of genetics. Mendel who became
a monk of the Roman Catholic church in 1843, studied at the University
of Vienna where he mastered mathematics, and then later performed many
scientific experiments. The greatest experiment that Mendel performed
involved growing thousands of pea plants for 8 years. He was forced
to give up his experiment when he became abbot of the monastery because
of the political problems of the time. He died in 1884, but has been
remembered for the great contribution to science that he made. To
learn about his experiment and what it led to read: Genetics.
In 1928 a scientist named
Frederick Griffith was working on a project that enabled others to point
out that DNA was the molecule of inheritance. Griffith's experiment
involved mice and two types of pneumonia, a virulent
and a non-virulent kind. He injected the virulent pneumonia into
a mouse and the mouse died. Next he injected the non-virulent pneumonia
into a mouse and the mouse continued to live. After this, he
heated up the virulent disease to kill it and then injected it into a mouse.
The mouse lived on. Last he injected non-virulent pneumonia and virulent
pneumonia, that had been heated and killed, into a mouse. This mouse
Why? Griffith thought
that the killed virulent bacteria had passed on a characteristic to the
non-virulent one to make it virulent. He thought that this characteristic
was in the inheritance molecule. This passing on of the inheritance
molecule was what he called transformation.
Mouse with virulent pneumonia
Mouse with heated virulent pneumonia and non-virulent
pneumonia mixed together
Mouse with non-virulent pneumonia
Mouse with heated, killed virulent pneumonia
Fourteen years later a scientist
named Oswald Avery continued with Griffith’s experiment to see what the
inheritance molecule was. In this experiment he destroyed the lipids,
ribonucleic acids, carbohydrates, and proteins of the virulent pneumonia.
Transformation still occurred after this. Next he destroyed the deoxyribonucleic
acid. Transformation did not occur. Avery had found the inheritance
To understand the DNA molecule
better scientists were trying to make a model to understand how it works
and what it does. In the 1940’s another scientist named Erwin Chargaff
noticed a pattern in the amounts of the four bases: adenine, guanine, cytosine,
and thymine. He took samples of DNA of different cells and found
that the amount of adenine was almost equal to the amount of thymine, and
that the amount of guanine was almost equal to the amount of cytosine.
Thus you could say: A=T, and G=C. This discovery later became Chargaff’s
Rosalind Franklin and Maurice
Two scientists named, Rosalind
Franklin and Maurice Wilkins, decided to try to make a crystal of the DNA
molecule. If they could get DNA to crystallize, then they could make
an x-ray pattern, thus resulting in understanding how DNA works.
These two scientists were successful and obtained an x-ray pattern.
The pattern appeared to contain rungs, like those on a ladder between to
strands that are side by side. It also showed by an “X” shape that
DNA had a helix shape.
James Watson and Francis
In 1953 two scientists, James
Watson and Francis Crick, were trying to put together a model of DNA.
When they saw Franklin and Wilkin's picture of the X-ray they had enough
information to make an accurate model. They created a model that
has not been changed much since then. Their model showed a double
helix with little rungs connecting the two strands. These rungs were the
bases of a nucleotide (see
nucleic acids). At first Watson and Crick were set back with
a problem, how to bond the bases together, and how to solve the problem
of the sizes of the bases. Adenine and Guanine were purines having
two carbon-nitrogen rings in their structures. Thymine and Cytosine
were pyrimidines having one carbon-nitrogen ring in its structure. If DNA
were to have its bases pair up so that the purines and the pyrimidines
were together, then it would look wobly and crooked. Watson and Crick
then found that if they paired Thymine with Adenine and Guanine with Cytosine
DNA would look uniform. This pairing was also in accordance with
Cargaff's rule. They also found that a hydrogen bond could be formed
between the two pairs of bases. In all DNA strands if one side has
a Thymine base then the other has the opposite: Adenine and so on with
Guanine and Cytosine. Each side is a complete compliment of the other.
A Person to Praise
By using the picture of the crystallized
DNA, Watson and Crick were able to put together the model of DNA.
Some have speculated that they did not give Rosalind Franklin enough credit
for her work; she had certainly made history. Watson and Crick did
use the new information very quickly as it is shown by the fact that their
paper showing the model of DNA was published in the same issue of Nature
as Franklin's picture. Watson and Crick, did, though, use this new
information and information from Avery, Chargaff, Griffith, and others.
They simply pieced together the puzzle. The Nobel Prize was awarded
a few years after the presentation of the model to Watson, Crick, and Maurice
Wilkins. Rosalind Franklin did not receive the prize because she
had died of cancer by this time. Maurice Wilkins was able to share
the prize with Watson and Crick, though, because of his work with Franklin.
Her accomplishment should never be forgotten.