Life on this earth is under the control of DNA which is called the “Blueprint of Life” and this DNA is controlled by the creator of this Universe. Whatever may be our belief about how life came to be on this earth, looking at the systematic and fool proof functions of a living cell we have to admit that the creator is indeed a genius and we have spent our lives in understanding these pathways by delving into the nano-scale level. Just as the understanding of the vastness of the universe makes us awestruck at how big celestial objects may appear so also we are dumbfounded by the meticulous nature of the cell and its autonomy at the micro level. We marvel at way in which an unicellular organism – be it a bacterium or an alga or a fungus carries on its metabolic activity without having to be told to behave in a certain way. All the information required for its functioning is built into its system. This information is in the form of a language of three letter words or codons made using only four alphabets A (Adenine), T (Thymine), G (Guanine) and C (Cytosine) . This information is contained in the DNA (Deoxy ribonucleic acid) located inside the nucleus of the cell.
DNA is in the form of a highly coiled double stranded chain and each strand has a sequence of the three letter words in linear order and these words are called as the nucleotides. each nucleotide is a complex having a nitrogen base, a pentose sugar called de-oxyribose sugar and a phosphoric acid and many such nucleotides are linked with one another by phospho di-ester bonds forming a polynucleotide chain (many nucleotides linked together). The Nitrogen bases are of Two types: 1. Purines which two in number : Adenine (A) and Guanine (G) and 2. Pyrimidines which are also two in number : Thymine (T) and Cytosine (C). Both of these are made of heterocyclic ring structure with C and N occupying the corners. Purines are generally nine-membered rings (formed by the fusion of a six-membered pyrimidine ring and a five membered imidazole ring). The pyrimidines are six-membered rings.
There are two such strands that run in opposite directions (Antiparallel) . One of the strands is designated as 5′-3′ and the other is called 3′-5′. the two strands are kept together by hydrogen bonds between adjacent nucleotides of both strands. It is a rule to see that if on one strand there is A then on the opposite strand at corresponding position there is T and if there is a T then on the other strand it would be an A. So also if there is a G on one strand then opposite to it on the other strand it will be a C and so also again if it is a C on one strand on the other one it will be a G. It follows therefore that a purine and a pyrimidine nucleotide is paired on the two strands with the condition that when the purine is A then the pyrimidine is T and if the Purine is G then the pyrimidine is always C. This is called Complementary base pairing and the if the information on one strand is known the other strand can be worked out. These rules were popularly called as the Chargaff’s Base pairing rules after the proponent of these rules. Moreover based on the structural variations of these nucleotides we find that when A and T pair there are two hydrogen bonds formed between them while pairing between G and C shows the formation of three Hydrogen bonds.
Both the strands are coiled about an imaginary axis making an angle of 36 degrees with the axis and hence the DNA molecule is seen to possess a double helical structure.Based on the data from bio-physical experiments, X-ray diffraction studies and base pairing rules James Watson and Francis Crick proposed the Double Helical Model of DNA structure that holds good to this day. The X-ray Diffraction data of DNA was first studied by Rosalind Franklin and Maurice Wilkins and hence they too should receive credit for this flawless presentation of the model of the DNA Double Helix. Each strand of the Helix has genetic information coded on it in the from of triplet codons (three alphabets) and the genetic code has a set of 64 unique codons which are meant for the 20 aminoacids that make up all the proteins that are required for the structure and function of living organisms.