DNA which stands for Deoxyribonucleic acid is the basic unit of hereditary. DNA is a molecule that consists of the information an organism needs to develop, live and reproduce. It is also known as the life instruction of the cell. These instructions are found inside every cell and are passed down from generation to generation. DNA is mainly found in eukaryotic cells. Most DNA is located in the nucleus of the cell which is also termed as nuclear DNA and small amount of DNA that is located in the mitochondria which are also termed as mitochondrial DNA.
Charles Darwin who was a naturalist and a biologist is the father of theory of evolution. His theory revealed that species survived through a process known as “natural selection”. There those who able to adapt to meet the changes of natural habitat were succeed, while those who failed to adapt and reproduce died off. In his observations he focused on studies of birds, plants and fossils. He noticed that there were similarities among species all over the world, but it varied based on locations, which lead him to believe that the species were evolved from common ancestors (Catania, 1987).
In 1866, an Augustinian monk named Gregor Mendel was the first person who was able to discover that the characteristics are passed down from generations to generations. He is known as the father of genetics. In between 1856 and 1863 Mendel did his experiments using pea plants. He attempted his studies by crossbreeding “true” lines in specific combinations. He was able to identify seven characteristics such as plant height, pod shape, pod color, seed shape, seed color, flower position and color of the flower. His experimental studies resulted that when a yellow pea plant and a green pea plant were allowed to breed, they gave yellow pea plant in their off springs. Anyhow in the next generation it resulted a ratio of 3:1 of yellow: green pea plants. Mendel named the terms ‘dominant’ and ‘recessive’ for the traits to explain this phenomenon. So he identified that yellow was the dominant trait while green was the recessive trait. These laid scientists to conduct experiment and search about more on genetics (Bizzo and El-Hani, 2009).
DNA was first observed by a German physiological chemist named Frederich Miescher. Though he observed it in 1869 many researches and scientists did not realize the importance of this molecule for years. Year 1869 is a landmark year in genetic research. Frederich Miescher first identified “nuclein” which is inside the nuclei of the human White blood cells. His aim was not to discover about the nuclei but to isolate and investigate about the protein components of the white blood cells. Miescher then had arranged a way to collect pus-coated patient bandages. He had washed then and filtered the white blood cells and identified various proteins within the white blood cells. With his investigations he had discovered a substance which had chemical properties other than in proteins, which have also a higher content of phosphorous and resistance to proteolysis too. Miescher was fascinated that he had discovered a new substance (Pray, 2008). Meanwhile, other scientists continued studies to investigate the chemical nature of the molecule formerly name as the “Nuclein”. A Russian biochemist named Pheobus Levene who was earlier a physician and then turned into a chemist was the first one who was able to discover the order of the three major components of a nucleotide, which include phosphate, sugar and the base. And also he was the first scientist who was able to discover the carbohydrate components of RNA and DNA which are ribose and Deoxyribose respectively (Pray, 2008; Dahm, 2005).
Until 1940’s it was not known what carried genetic information from generation to generation. Several researches and scientists suggested that it could be either proteins or DNA that carries information from generation to generation. Early scientists agreed materials that carry genetic information had to possess four characteristics. Those are they had to replicate, they had to store information, they had to be able to express information and they had to be able to show variations via mutations. These characteristics were shown by both proteins as well as by DNA. Prior to 1944 scientists favored proteins as the genetic material. In 1928, Griffith developed the concept of transforming principle. Griffith’s experiments were with bacteria which cause pneumonia. Griffith was interested in what made some bacteria virulent, while others are avirulent. Virulent bacteria are the bacteria which are capable of causing the disease while avirulent bacteria are the bacteria which do not cause the disease. He worked with IIIS as the virulent strains and IIR as the avirulent strains. In his study, first he had injected these virulent and avirulent bacterial strains to mice and observed that the mice die with ingestion of virulent strains while the mice were alive with ingestion of avirulent strains. So then he had heat treated IIIS bacterial strains and then they were injected to healthy mice. So he observed that they were alive showing that the carcasses of the cells do not cause the disease. Finally Griffith took heat killed IIIS bacteria strains and it was mixed with avirulent IIR bacterial strains. So that they were injected to healthy mice and observed that the mice died due to development of pneumonia. Griffith reasoned that avirulent IIR were transformed into virulent IIIS. And also he was able to collect live cells from dead mice, and it consisted of virulent cells. So, Griffith concluded that this transformation was the result of the movement of some information from the living IIR cells to dead IIIS cells. This finding laid an experimental platform to the scientists Avery, McCarty and MacLeod to prove the DNA was the genetic material which transformed (Deichmann, 2004).
In 1944, Oswald Avery, C.M. MacLeod and M. McCarty discovered that DNA is the molecule which engages in transforming principle. They put forward steps to discover the biochemical nature of the transforming principle which was observed by Frederick Griffith. For their studies they separated the classes of molecules found in the debris of the dead IIIS cells and tested them for the transforming ability. So, they purified macro-molecules such as DNA, RNA and proteins of heat killed IIIS-strain to determine which macro-molecule converted the avirulent IIR strain into virulent IIIS strain.
They did an experimental demonstration to discover that the transforming material is DNA in an era when it had been thoroughly believed that the transforming material was proteins. In their study virulent IIIS cell suspension were centrifuged and the cell palate was heat-killed. Then those cells were homogenized and the IIIS filtrate was recovered. Thereafter, carbohydrates, lipids and proteins were extracted from the IIIS filtrate and were treated with deoxyribonuclease (DNase), ribonuclease (RNase) and protease separately. Then avirulent IIR cells were added to the treated IIIS filtrates. This was the experiment on assaying for transformation. As results they have observed that only IIR cells were present in IIR cells containing DNase treated IIIS filtrate. That means no transformation had occurred. But they observed that transformation was occurred in the suspension of RNase treated IIIS filtrate with avirulent IIR cells and in Protease treated IIIS filtrate with avirulent IIR cells. Showed that something in heat killed IIIS bacterial cells were able to transform from avirulent IIR bacteria into virulent IIIS bacteria. Their findings showed that proteins and RNA were not responsible for the transformation. But DNA was responsible for the transformation. That means heat treated IIIS bacteria that were treated with DNase were incapable of transforming avirulent IIR to virulent IIIS (Cobb, 2014; Deichmann, 2004; Fry, 2016).
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The work of Avery, MacLeod and McCarty showed that DNA was responsible for the changes they show in bacteria. And an additional work needed to be done in order to prove that DNA was the universal genetic information.
In 1952, two scientists named Alfred Hershey and Martha Chase had conducted series of experiments to confirm that DNA is the genetic material. In their experiments, they used bacteriophages which are also known as virus infected bacteria. Bacteriophages are composed of DNA and proteins. Hershey and Chase showed that when bacteriophages infect bacteria their DNA entered the host bacterial cell but proteins did not enter the host bacterial cell. In their experiments first they labeled the phage DNA with radioactive Phosphorous (32P) and phage protein with radioactive (35S). They used E. coli as the host bacterial cells. Hershey & Chase’s experiments concluded that the radioactively labeled phage DNA were adsorbed into the phage. The radioactively labeled proteins were not adsorbed into the phage. His experimental observation revealed that 32P labeled DNA containing phages were remaining in the solution after allowing the bacteriophages to adsorb to the bacteria showed that the phage DNA were transferred into the bacterial cell. The presence of all radioactive 35S in the solution showed that the protein coat that protects the DNA before adsorption stayed outside the cell. Hershey and Chase concluded that DNA was the genetic material which transformed, not the proteins. They determined though that a protective coat was formed around the bacteriophage, the internal DNA is the one that have the ability to produce progeny inside a bacterium. So DNA was again proved to be the unit of hereditary. (Griffiths et al, 2000)
From all the above discoveries, it was confirmed that DNA is the hereditary unit. After discovering that DNA is the hereditary unit, scientists put forward steps to discover more about DNA to determine its composition as well as its 3D structure through many investigations. DNA is made up of nucleotides. There are four types of nucleotides that involve in making up of DNA. They are Adenine (A), Guanine (G), Cytosine(C) and Thymine (T). In 1950’s 3 separate groups of scientists worked intensively on discovering the structure of DNA. First a scientist named Maurice Wilkins produced X-ray photographs DNA showing the crystalline structure of DNA. These photographs were displayed at a conference in Naples, and this sparked James Watson’s interest in investigating the DNA structure. In 1950, a biochemist named Erwin Chargaff did research and discovered the base composition of DNA. Prior to this discovery it was taught that four bases were present in relatively in equal proportions. The discovery of Chargaff’s findings, were of the arrangement of nitrogen bases in DNA which varied widely, but the amount of certain bases always occurred in a one to one ratio. It was confirmed that there is relatively equal proportions of Adenine and Thymine since Adenine pairs with Thymine and there is a relatively proportions of Guanine and Cytosine since Guanine pairs with Cytosine.
With the knowledge of the proportions of the bases scientists put forward steps to discover the structure of the DNA. In 1948, Linus Pauling discovered that many proteins take the shape of an alpha helix, spiraled like a spring coil. These discoveries were an important foundation for the later description of DNA. In 1953, James Watson and Francis Crick used Chargaff’s data on composition of DNA and x-ray diffraction pictures of DNA to determine the double helix structure, twisted later structure of DNA. This was a great milestone in Molecular biology and a huge amount of ground work had led to this massive discovery. In 1951 a scientist named Rosalind Franklin made a series of X-ray diffraction images of DNA. She took high quality photographs of DNA. Her images supported only for a helical structure of DNA, and she was unable to propose a specific model for DNA. But during her time, she was able to discover two forms of DNA. Those are “A” form which was also known as dehydrated form and “B” form which was also known as hydrated form. Rosalind Franklin focused on the crystalline “A” form while Wilkins was interested in “B” form, as it was the biological form. In November, 1951 Alec Stokes solved basic mathematics of helical diffraction theory and proposed Wilkin’s X-ray diffraction data to indicate a helical structure of DNA (Schindler, 2008).
Unlike Franklin and Wilkins who favored data collection, measureable results and interpretation, Watson and Crick utilized the model building approach.
In late 19th century a German biochemist was able to find that nucleic acids/ long -chain polymers of nucleotides were made up of sugar, phosphoric acid and several nitrogen containing bases. Later on it was found that the sugar can be a ribose or a deoxy- ribose giving two forms as RNA and DNA respectively.
In 1953, Watson and Crick were able to discover a specific structure for DNA. That is they were able to discover a double helix structure for DNA. They assumed the double helix structure of the DNA. In their double helix structure, two strands of the DNA are coiled with a right handed turn. The two chains were arranged antiparallel, in which 3’-5’ orientation was in opposite directions. In this the nitrogenous bases are stacked inside of the double helix. Sugar phosphate chains are outside the double helix in antiparallel directions. It is arranged in a way that Adenine pairs with Thymine forming two Hydrogen bonds, and Guanine pairs with Cytosine forming three hydrogen bonds. The double helix structure consists of two groves that are named as minor and major grooves. The major groove is wider than the minor groove. In 1962, Watson and Crick won the Nobel Prize for medicine for this discovery (Crick, 1954; Schindler, 2008).
With the discovery of the double helix structure of DNA, there were no direct evidences to substantiate the hypothesis of Watson and Crick. Wilkins and his group developed the quality of X-ray diffraction pictures of DNA. The data on structure of the DNA was too low. So several groups of scientists took steps to investigate the structure of DNA in stereo chemically and energetically. Their investigations resulted that the sugar phosphate chains did not intertwine. The structure was called as the SBS structure which was denoted as the Side-by-side and RL which is denoted as Right left, since they were consisted of right handed and left handed blocks. The calculated patterns of X-ray diffraction were consistent with the experimental data and the calculated pattern of the double helix of Watson and Crick (Kypor and Vorlickova, 1985).
References
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