DNA. Computer model showing the structure of DNA (deoxyribose nucleic acid), the carrier of hereditary information in cell nuclei. The structure is a double helix. The 'backbones' of the double helix (shown here as ribbons) are made from alternating phosphate and sugar groups connected to atoms of carbon in pentose molecules. Between the two backbones are the bases: adenine, guanine, cytosine and thymine. The hydrogen bonding between bases is such that only two base pairings are allowed: adenine to thymine and cytosine to guanine. This specific pairing is the key to DNA replication.

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