Discovery of DNA and DNA Sequencing

The beginning of DNA exlpoitation

It is a fundamental part of human behaviour to need to know more about how humans work and the discovery of DNA is one of the greatest aids in this research.

Interest in genetics can be traced as far back as the 1700’s when Maupertuis wrote an early attempt at a materialistic explanation of the origin of species. Nearly a century later in 1859 Charles Darwin published his book “On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life” or better known as “The origin of the species” which was produced as a result of his  research whilst serving as naturalist aboard the H.M.S. Beagle on a British science expedition around the world. Darwin's theory of natural selection issued a profound challenge to orthodox thought and belief at that time. link

Further interest was initiated by Gregor Mendel the "Father of Genetics" who performed an experiment in 1857 which  involved growing thousands of pea plants for 8 years. He died in 1884, but is remembered for his great contribution to science. Details of experiment

In the late nineteenth century, a German biochemist, Meischer found that nucleic acids, long-chain polymers of nucleotides, were made up of sugar, phosphoric acid, and several nitrogen-containing bases. Later it was found that the sugar in nucleic acid can be ribose or deoxyribose, giving two forms: RNA and DNA. In 1943, American Oswald Avery proved that DNA carries genetic information. He even suggested DNA might actually be the gene. Most people at the time thought the gene would be protein, not nucleic acid, but by the late 1940s, DNA was largely accepted as the genetic molecule. Scientists still needed to discover this molecule's structure and to understand how it worked.

Johann Friedrich Miescher

In 1948, Linus Pauling discovered that many proteins take the shape of an alpha helix, spiralled like a spring coil. In 1950, biochemist Erwin Chargaff found that the arrangement of nitrogen bases in DNA varied widely, but the amount of certain bases always occurred in a one-to-one ratio. These discoveries were an important foundation for the later description of DNA.

Linus Pauling

The Structure of DNA

The biggest breakthrough however was in the early 1950s. At Cambridge University, graduate student Francis Crick and research fellow James Watson  had become interested in this research having been  impressed by Pauling's work. Meanwhile at King's College in London, Maurice Wilkins and Rosalind Franklin were also studying DNA. The Cambridge team's approach was to make physical models to narrow down the possibilities and eventually create an accurate picture of the molecule. The King's team took an experimental approach, looking particularly at x-ray diffraction images of DNA.

In 1951, Watson attended a lecture by Franklin on her work to date. She had found that DNA could exist in two forms, depending on the relative humidity in the surrounding air. This had helped her deduce that the phosphate part of the molecule was on the outside. Watson returned to Cambridge with a confused recollection of the facts Franklin had presented and based on this information, Watson and Crick made a failed DNA model.

Rosalind Franklins x-ray photographs proved that DNA is a double helix

Franklin, working mostly alone, found that her x-ray diffractions showed that the "wet" form of DNA (in the higher humidity) had all the characteristics of a helix. She suspected that all DNA was helical but did not want to announce this finding until she had sufficient evidence on the other form as well. Wilkins was frustrated and in January, 1953, he showed Franklin's results to Watson, apparently without her knowledge or consent. Watson and Crick then took a crucial conceptual step, suggesting the molecule was made of two chains of nucleotides, each in a helix as Franklin had found, but one going up and the other going down. Crick had just learned of Chargaff's findings about base pairs in the summer of 1952. He added that to the model, so that matching base pairs interlocked in the middle of the double helix to keep the distance between the chains constant. Watson and Crick showed that each strand of the DNA molecule was a template for the other. During cell division the two strands separate and on each strand a new "other half" is built, just like the one before. This way DNA can reproduce itself without changing its structure - except for occasional errors, or mutations. The structure so perfectly fit the experimental data that it was almost immediately accepted. By 1962, when Watson, Crick, and Wilkins won the Nobel Prize for physiology/medicine, Rosalind Franklin had died. The Nobel Prize only goes to living recipients, and can only be shared among three winners.

About DNA Sequencing

In 1951 the world's first commercially available computer, the Ferranti Mark 1, was delivered to the University of Manchester, which would ultimately combine with molecular biological research to allow sequencing of DNA.

Again in the 1950s Fred Sanger and his colleagues developed many of the sequencing techniques still used in genomic biology. In the 1940s and 1950s, new methods were developed in separation and purification and it seemed that it might at last be possible to determine the chemical structure of protein molecules. Over several years, Sanger developed methods to determine the order (sequence) of the building blocks of the protein insulin. In 1958 he was awarded the Nobel Prize for his work in sequencing proteins. Sanger  was then struck by the challenge of determining the order of bases in DNA - DNA sequencing. It was clear by now that DNA was a linear code and although the code was being unravelled, no methods existed to read the code in even the simplest genome. Over the next 15 years, Sanger and his team developed several and ever-improving methods to sequence nucleic acids (DNA and RNA). The work for his second Nobel Prize (awarded in 1980) was developing the technique still used today - 'dideoxy' or 'Sanger' sequencing. In this method, stretches of 500-800 bases at a time can be read.

http://osulibrary.orst.edu/specialcollections/coll/pauling/dna/people/

http://www.sanger.ac.uk/Info/Intro/sanger.shtml

http://cogweb.ucla.edu/ep/DNA_history.html

http://www.accessexcellence.org/RC/AB/WYW/wkbooks/SFTS/part1.htm