DNA

DNA is the building block of human life . Its double helical structure is deceptively simple, yet the rules encoded within this structure specify the form and function of all cells within an organism. DNA consists of two long strands of polydeoxyribonucleotides that twist around each other clockwise to form an unbroken double helix. Alternating deoxyribose-phosphate groups form the backbone of the helix, with the phosphate group making a 5′-3′ phosphodiester bond between the fifth carbon of one pentose ring and the third carbon of the next pentose ring . Nucleic acid bases attached to the sugar groups of each strand face each other within the helix, perpendicular to the strand axis. The order of the nucleic acids specifies the eventual sequence of the protein product of the gene. Only four bases exist: the purines adenine and guanine (A and G) and the pyrimidines cytosine and thymine (C and T). During assembly of the double helix, a purine can pair only with a pyrimidine, and a pyrimidine with a purine. Each base pair (bp) forms one of the rungs in the twisted ladder of the DNA molecule, which can be millions of bases long. The two strands of DNA, which are held together by hydrogen bonds between complementary base pairs, have opposite chemical polarities. One strand is oriented in a 5′ to 3′ direction, while the other is in a 3′ to 5′ direction. Enzymes that recognize specific DNA sequences also recognize the polarity of the strand. An enzyme “reads” the nucleotide sequences on the two strands in opposite directions. Because the structure of the helical backbone is invariant, enzymes responsible for DNA copying, cleavage, and repairing strand breaks can act anywhere along the length of the DNA strand.