Good Shepherd Academy Int'l

The Discovery of the Structure of DNA (Deoxyribonucleic Acid)

The molecular structure and shape of Deoxyribonucleic Acid (DNA), the unique biological molecule that carries the genetic information (traits) from one generation to another for every living organism (with the exception of certain viruses that have RNA as the repository of their genetic information) was discovered by Francis Crick and James Watson in 1953. The Structure of DNA is designed to carry, store, modify and express genetic information.

The nucleic acid; DNA for short, is required by every living organism except some viruses as stated earlier for storage, transmission and expression of genetic information. The genetic information found in DNA is first copied and transmitted to daughter cells through a highly complicated process called DNA replication. For example, the DNA found in a fertilized egg (or zygote) contains the encoded information that directs the development of a new organism.

The flow of information from DNA through RNA to Protein (where such information as encoded in the DNA are expressed) is termed The “Central Dogma” of Molecular Biology.

The British biochemist Francis Crick, and his American colleague, James Watson, created the first accurate and acceptable model of the molecular structure of deoxyribonucleic acid, (DNA), the unique master code responsible for the storage, transferring, building and operation of all living organisms. This timely discovery has been described by many as “the most significant discovery of the century”, a discovery that won them both Nobel Prize in 1962.

The structure of the DNA is now known to compose four deoxy ribonucleotides, deoxyadenylate (A), deoxyguanylate (G), deoxycytidylate (C) and thymidylate (T). Note that each of these ribonucleotides units contains a nitrogenous base, a ribose sugar, and a phosphate group. These units are linked together to form a polymer of deoxyribonucleoside monophosphate which are covalently linked by 3^l → 5^l-phosphodiester bond. The discovery of the structure and subsequently the details of the DNA molecule allowed medical scientists and biochemists to understand, and to develop cures for, many deadly diseases. Millions of lives have been saved by this singular discovery. Presently, even in courts of law, DNA evidences are commonly admissible. This discovery has also led to the unraveling of the human genome and promises to lead to cures for a wide variety of other serious ailments and birth defects, even cures yet imagined.

Crick’s earlier discoveries relating to the structure of DNA and its function reshaped the study of genetics, and virtually created the field of molecular biology, his ideas and findings gave new direction to a host of endeavors in various fields of medicine.

How Was It Discovered?

The room they occupy, Watson and Crick looked exactly like a tinker toy party gone berserk, really seem like the play room of some hyper active primary two boys. Complex mobiles of wire, coloured beads, and strips of sheet metal, cardboard cut outs, wooden dowels, and wooden balls dangling from the ceiling like a forest of psychedelic stalactites. All sorts of construction supplies, scissors, and tin snips were strewn about the desks and floor, as were pages of complex academic equations, stacks of scientific publications, and photographic sheets of fuzzy X-ray crystallographic images.

The room was really the second floor office shared by graduate students, Francis Crick and James Watson in a 300 year old building on the campus of the University of Cambridge, this was in 1953. The mobiles were not the idle toys of students with too much or extended free time. Rather, they were a frantic effort to win the world wide race to unravel the very core of life and decipher the shape of the novel DNA molecule.

As at 1950 some biochemists had already deduced that DNA present in the nucleus of cells carried genetic information. The key mystery to unlock at the time, was how this huge DNA molecule reproduces (replicate) itself to physically pass this information to a new cell (daughter cell), a new organism (young ones), and a new generation. To answer this question, someone had to first of all figure out what this giant DNA molecule exactly looked like, what this molecule is composed of.

At the University of Cambridge, Francis Crick teamed up with an American biologist James Watson to solve the mysteries locked in this molecule. The two agreed to put their efforts together to construct a model of the DNA molecule while they pursued their separate studies and thesis research.

By 1951 bits and pieces of information about the biologically important DNA molecule were emerging from across the globe. Erwin Chargaff had discovered that a definite ratio of nucleotide sequences could be detected in the DNA bases, suggesting a base pairing relationship. Oswald Avery conducted experiments on bacteria DNA showing that DNA carried genetic information across generational lines. Linus Pauling conceptualized the alpha helix configuration for certain chains of proteins.

Crick and Watson then attempted to combine these separate clues into a single physical structure. Using bits of wire, coloured beads, sheet metal, and card board that were cut out, Crick and Watson hung possible spiral models across their shared office. They correctly surmised that a linking chain of sugar and phosphate formed the back bone of the DNA spiral, and they correctly linked base pairs of peptides. But still they were far from finish, since the model did not fit with available atomic data at the time.

Independent of Crick and Watson’s efforts, but also at the University of Cambridge, Rosalind Franklin used X-ray crystallography to create two-dimensional images of the DNA molecule. In mid-January 1953, Rosalind had redesigned the X-ray cameras she used. X-ray film from these cameras showed the now-famous “X” shape that suggested a helix shape for the DNA molecule.

Tipped off that Franklin had new information from X-ray crystallography, it was reported that Crick made away with one of Rosalind Rosalind’s “X-shaped” X-rays. This, whether it was “borrowed or stolen”, in fact finally put Crick and Watson ahead in the race to solve the structure of DNA. By mid-February they had constructed the first complete, functional, and physical model of a DNA molecule, using the now familiar double helix shape, like two intertwined spiral chains.

Do You Know: If you straightened out each strand of a DNA molecule from each cell in your body and lined them end-to-end, you’d probably have about nine million kilometers of DNA. That’s enough to reach to the moon to the earth and back 13 times.