By Dr Ananya Mandal, MD
The history of genetic research began with Gregor Mendel the "Father of Genetics". He had performed an experiment with plants 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 from where he studied mathematics, and then later performed many scientific experiments.
His experiment involved growing thousands of pea plants for 8 years. He was forced to give up his experiment when he became abbot of the monastery. He died in 1884 but his experiments still form the basis of genetics and gave a fair idea of inheritance.
Friedrich Miescher and Richard Altmann
Friedrich Miescher (1844-1895) discovered a substance he called "nuclein" in 1869. Later he isolated a pure sample of the material now known as DNA from the sperm of salmon, and in 1889 his pupil, Richard Altmann, named it "nucleic acid". This substance was found to exist only in the chromosomes.
Frederick Griffith, a scientist, was working on a project in 1928 that formed the basis that DNA was the molecule of inheritance. Griffith's experiment involved mice and two types of pneumonia – one was virulent and the other non-virulent. 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 survived.
After this, he heated up the virulent disease to kill it and then injected it into a mouse. This time the animal survived as predicted. Last he injected non-virulent pneumonia and virulent pneumonia that had been heated and killed, into a mouse. This time the mouse died.
Griffith speculated that the killed virulent bacteria had passed on a characteristic to the non-virulent one to make it virulent. He believed this characteristic was in the inheritance molecule. This passing on of the inheritance molecule was what he called transformation.
Oswald Avery continued with Griffith’s experiment around a decade later 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. He had found the basis of the inheritance.
In 1929 Phoebus Levene at the Rockefeller Institute identified the components that make up a DNA Molecule. Those components are:
The four bases
He showed that the components of DNA were linked in the order phosphate-sugar-base. He said that each of these units is a nucleotide and suggested the DNA molecule consisted of a string of nucleotide units linked together through the phosphate groups. He suggested that these form a 'backbone' of the molecule.
However, Levene thought the chain was short and that the bases repeated in the same fixed order. It was Torbjorn Caspersson and Einar Hammersten who showed that DNA was a polymer.
Erwin Chargaff and Chargaff’s rule
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 found the 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 Rule.
Rosalind Franklin and Maurice Wilkins
Thereafter two researchers Rosalind Franklin and Maurice Wilkins tried to make a crystal of the DNA molecule. They wanted to take X ray pictures of the DNA to understand 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. They found that DNA had a helix shape.
Watson and Crick
In 1953, two scientists, James Watson and Francis Crick, were trying to put together a model of DNA. They took a look at Franklin and Wilkin's picture of the X-ray and made their 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.
They also found that if they paired Thymine with Adenine and Guanine with Cytosine DNA would look uniform. This pairing was also in accordance with Chargaff's rule.
They also found that a hydrogen bond could be formed between the two pairs of bases. In addition, each side is a complete complement of the other.
DNA profiling was developed a few years later in 1984 by English geneticist Alec Jeffreys of the University of Leicester, and was first used to convict Colin Pitchfork in 1988 in the Enderby murders case in Leicestershire, England. Thus began the journey of DNA research.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)