Deoxyribonucleic AcidIt is a nucleic acid containing the genetic instructionsused in the development and functioning of all knownliving organisms (with the exception of RNA viruses).The DNA segments carrying this genetic informationare called genes. Likewise, other DNA sequences havestructural purposes, or are involved in regulating theuse of this genetic information. Along with RNA andproteins, DNA is one of the three majormacromolecules that are essential for all known formsof life.
DNA consists of two long polymers of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds. These two strands run in opposite directions to each other and are therefore anti-parallel. Attached to each sugar is one of four types of molecules called nucleobases (informally, bases).
DNA is a long polymer made from repeating unitscalled nucleotides. As first discovered by James D.Watson and Francis Crick, the structure of DNA of all speciescomprises two helical chains each coiled round the sameaxis, and each with a pitch of34 Ångströms (3.4 nanometres) and a radius of10 Ångströms (1.0 nanometres). According to another study,when measured in a particular solution, the DNA chainmeasured 22 to 26 Ångströms wide (2.2 to 2.6 nanometres),and one nucleotide unit measured 3.3 Å (0.33 nm) long.
The DNA double helix is stabilizedprimarily by two forces: hydrogenbonds between nucleotides and base-stacking interactions amongthe aromatic nucleobases. In theaqueous environment of the cell, theconjugated π bonds of nucleotide basesalign perpendicular to the axis of theDNA molecule, minimizing theirinteraction with the solvation shell andtherefore, the Gibbs free energy. Thefour bases found in DNAare adenine (abbreviatedA), cytosine (C), guanine (G)and thymine (T). These four bases areattached to the sugar/phosphate to formthe complete nucleotide, as shownfor adenosine monophosphate.
Nucleobases The nucleobases are classified into two types: the purines, A and G, being fused five- and six- membered heterocyclic compounds, and the pyrimidines, the six-membered rings C and T. A fifth pyrimidine nucleobase, uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring. Uracil is not usually found in DNA, occurring only as a breakdown product of cytosine. In addition to RNA and DNA a large numbe of artificial nucleic acid analogues have also been created to study the proprieties of nucleic acids, or for use in biotechnology.
Grooves Twin helical strands form the DNA backbone. Another double helix may be found by tracing the spaces, or grooves, between the strands. These voids are adjacent to the base pairs and may provide a binding site. As the strands are not directly opposite each other, the grooves are unequally sized. One groove, the major groove, is 22 Å wide and the other, the minor groove, is 12 Å wide.
Alternate DNA structuresFrom left to right, the structures of A, B and Z DNA
DNA exists in many possible conformations thatinclude A-DNA, B-DNA, and Z-DNA forms, although, onlyB-DNA and Z-DNA have been directly observed infunctional organisms.The conformation that DNA adoptsdepends on the hydration level, DNA sequence, theamount and direction of supercoiling, chemicalmodifications of the bases, the type and concentration ofmetal ions, as well as the presence of polyamines insolution.
Quadruplex structuresAt the ends of the linear chromosomes arespecialized regions of DNA called telomeres. Themain function of these regions is to allow the cellto replicate chromosome ends using theenzyme telomerase, as the enzymes that normallyreplicate DNA cannot copy the extreme 3′ ends ofchromosomes.These specialized chromosomecaps also help protect the DNA ends, and stopthe DNA repair systems in the cell from treatingthem as damage to be corrected.
EvolutionDNA contains the genetic information that allows all modern living things tofunction, grow and reproduce. However, it is unclear how long in the 4-billion-year history of life DNA has performed this function, as it has been proposedthat the earliest forms of life may have used RNA as their geneticmaterial. RNA may have acted as the central part of early cell metabolism as itcan both transmit genetic information and carry out catalysis as partof ribozymes. This ancient RNA world where nucleic acid would have beenused for both catalysis and genetics may have influenced the evolution of thecurrent genetic code based on four nucleotide bases.