The genetic information carried in the molecule called DNA determines every inherited physical characteristic of every living thing. DNA—more formally known as deoxyribonucleic acid—is found inside almost every cell. It controls how the cell replicates and functions, and what traits are inherited from previous generations. DNA determines traits that can be seen—such as eye color in animals—as well as traits that are not visible, such as blood type. DNA is also found in some viruses (see virus).
DNA molecules consist of two strands of biochemical compounds called nucleotides linked together by chemical bonds (see molecule). Nucleotides are composed of three molecules: a phosphate, a sugar, and a nitrogen-containing base. In DNA, the sugar is deoxyribose, and the base is either adenine, guanine, cytosine, or thymine (see biochemistry, “DNA Carries Heredity”). Adenine and guanine (symbolized for convenience as A and G, respectively) are classified as purines; cytosine (C) and thymine (T) are pyrimidines.
Within each strand the nucleotides are connected to each other by covalent bonds linking the phosphate of one nucleotide with the sugar of the next (see inorganic chemistry). This forms a chain from which the bases protrude. The bases of one strand are linked to bases of the second strand by hydrogen bonds. This base pairing is very specific—because of their structures, adenine can only pair with thymine, and cytosine can only pair with guanine. Thus joined, the ladder-shaped DNA strands are coiled around each other, forming a spiral, or double helix. This configuration makes the DNA molecule very stable.
Within the cell, DNA and various proteins are organized into dense units called chromosomes. In prokaryotes—organisms such as bacteria whose cells lack a true nucleus—these float freely in the cell’s cytoplasm. In eukaryotes, chromosomes are located within the nucleus, though some DNA is found also in mitochondria and in chloroplasts (see genetics; living things, “Simple and Complex Cells”).
The History of DNA
DNA was first discovered in 1869, but its role in genetics was not clear until the 1940s. In 1953, scientists James Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin determined the double-helix structure of DNA, as well as its method of replication. Their work earned a Nobel prize in 1962 (see Franklin, Rosalind).
Source : Encyclopedia Britanica
The Discovery of the Molecular Structure of DNA – The Double Helix.
Source : http://www.nobelprize.org
Play the DNA – The Double Helix Game