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DNA or deoxyribonucleic acid is very complex molecule that stores genetic information in a sequence of base pairs that span the double helix like the rungs of a twisted ladder. In DNA, there are four different types of base and these link together in a strict fashion: guanine pairs with cytosine and thymine pairs with adenine. Because of this defined one to one linkage, each chain carries the same information but coded in complementary bases.
The double helical molecule is about 2.4 nm across (2.4 nanometers or 2.4 x 10-9 meters) and comprises two strands. Each strand is a regular sugar-phosphate-sugar chain with bases projecting from it. The two strands run in opposite directions (referred to as being antiparallel) and each is coiled as a right handed helix. The helix undergoes one complete turn about every ten base pairs (10 bps), which is equivalent to a rise of 3.4 nm up the helical axis (i.e. the helix has a period of 3.4 nm).
The unit molecule of DNA is called a nucleotide and consists of a single sugar-phosphate-base unit. These nucleotides are then strung together to make the chains (polynucleotides) that wrap around each other.
The genetic information for an entire organism is called the genome. The full sequences of genomes have been studied for several organisms, most famously humans in the human genome project. Genes are the inheritance units in the genome and each gene codes for the production of a particular molecule.
The double helical structure of DNA was elucidated over 50 years ago by James Watson and Francis Crick and published in a 1953 letter in NATURE. For this, they shared a Nobel Prize in 1962 with Maurice Wilkins (who, with Rosalind Franklin, furnished the X-Ray diffraction data).