5. DNA

Deoxyribonucleic Acid, known as DNA, is the genetic material found in the cells of nearly all living organisms. DNA is the fundamental building blocks of life. Nearly every cell (with a nucleus) in a person's body has the same DNA. Most DNA is located in the cell nucleus (nuclear DNA), but DNA can also be found in the mitochondria (mitochondrial DNA or mt-DNA) and in chloroplast (chloroplast DNA or ctDNA). In 1929 Phoebus Levene at the Rockefeller Institute identified the components that make up a DNA Molecule. The information in DNA is made up of four bases which combine to form chains. These bases include two purines (Adenine and Guanine) and two pyrimidines (Cytosine and Thymine). These are commonly referred to as A, G, C and T, respectively. Each base is attached to a Sugar (S) molecule and a Phosphate (P) molecule (Figure 21.4.A-C). Sugar and phosphate are back bone of nucleotides (Figure 21.5 A).Together, a base and a sugar are called a nucleoside (Figure 21.5B). Together, a base, sugar, and phosphate are called a nucleotide (Figure 21.5C). Nucleotides are arranged in two long strands that form a spiral called a double helix (Figure 21.6 A). The structure of the double helix is somewhat like a ladder, with the base pairs (Figure 21.6B) forming the ladder's rungs and the sugar and phosphate molecules forming the vertical side pieces of the ladder. He showed that the components of DNA were linked in the order phosphate-sugar-base. He called each of these units a nucleotide and suggested the DNA molecule consisted of a string of nucleotide units linked together through the phosphate groups, which are the 'backbone' of the molecule. However Levene thought the chain was short and that the bases repeated in the same fixed order. Torbjorn Caspersson and Einar Hammersten showed that DNA was a polymer. This was only accepted after the structure of DNA was elucidated by James D. Watson and Francis Crick in their 1953 Nature publication. Watson and Crick proposed the central dogma of molecular biology in 1957, describing the process whereby proteins are produced from nucleic DNA. In 1962 Watson, Crick, and Maurice Wilkins jointly received the Nobel Prize for their determination of the structure of DNA. The number of purine bases in DNA is equal to the number of pyrimidines. This is due to the law of complimentary base pairing; where Thymine (T) can only pair with Adenine (A), and Guanine (G) can only pair with Cytosine (C).  Knowing this rule, we could predict the base sequence of one DNA strand if we knew the sequence of bases in the complimentary strand.

Figure 21.4: Components of nucleotides and nucleic acids