what is dna

DNA, or deoxyribonucleic acid, is a molecule that carries genetic information for the development, functioning, growth, and reproduction of all known organisms. It is a long polymer made from repeating units called nucleotides. DNA is composed of two polynucleotide chains that coil around each other to form a double helix. Both chains are coiled around the same axis and have the same pitch of 34 ångströms (3.4 nm) . The pair of chains has a radius of 10 Å (1.0 nm) . The backbone of the DNA strand is made from alternating phosphate and sugar groups. The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. DNA contains four basic building blocks or "bases": adenine (A), guanine (G), cytosine (C), and thymine (T) . The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences. DNA bases pair up with each other, A with T and C with G, to form the "rungs" of the DNA "ladder". Each base pair is joined together by hydrogen bonds. DNA usually occurs as linear chromosomes in eukaryotes and circular chromosomes in prokaryotes. The set of chromosomes in a cell makes up its genome; the human genome has approximately 3 billion base pairs of DNA arranged into 46 chromosomes.