Deoxyribonucleic acid is a molecule that is shaped like a double helix. There are two complementary strands in the double helix. The molecule is a polymer--containing two long, complementary strands. The shape is like a ladder, which is twisted, much like a spiral staircase. Based on X-ray diffraction pictures from Rosalind Franklin, James Watson and Francis Crick devised the double helix model. A subunit of DNA is known as a nucleotide. A nucleotide contains a phosphate group, a five carbon sugar called deoxyribose and one of four nitrogenous bases--adenine, thymine, cytosine and guanine. A and G are known as purines and C and T are known as pyrimidines. The sides of the ladder are the phosphate and sugar groups. Each "rung" of the ladder consists of two nucleotides, with complementary nitrogenous bases held together in the center by hydrogen bonds. Bases pair according to the rules--adenine pairs with thymine and cytosine pairs with guanine. The two complementary strands in DNA are oppposite in their directions--thus, there is a five prime end and a three prime end. The five prime strand has a terminal phosphate group and the three prime end has a terminal hydroxyl group. DNA contains the hereditary information for an organism to function and its segments are called genes. DNA can replicate and when it does, either half of this double helical molecule can serve as the template for replication. After replication, two exact copies of DNA are produced. This ensures that there will be enough DNA(in chromosomes) for two daughter cells when mitotic cell division occurs. DNA is found in all living organisms.
DNA is the abbreviation for deoxyribonucleic acid. DNA is the key to the identity of organisms, the blueprint for every cell in that organisms body. DNA is composed of subunits that are made of a sugar, a phosphate, and a base. The overall structure of a strand of DNA resembles that of a twisted ladder, with two side rails and rungs, or steps, in between the two rails. The sugar and phosphate parts compose the side rails, while the base part composes the rungs of the ladder. There are four bases that make up the composition of DNA. The bases are cytosine, guanine, adenine, and thymine. The bases pair with each other in that order, cytosine to guanine, and adenine to thymine. DNA may be replicated by a strand splitting down the middle, with each half-strand serving as a template for the assembly of new subunits to make a complementary new DNA strand. This twisted-ladder structure is sometimes referred to as a "spiral double-helix", so when you see this reference, it is a strand of DNA the author is speaking about. The specific DNA sequence that identifies an organism is very long, no two unrelated organisms share the same identical DNA sequence. The only examples that come close are identical twins, or cloned organisms that would be genetically identical.