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Describe the structure of DNA

DNA is the molecule within cells that contains genetic information, in the form of a sequence of bases that form a code that makes up a protein. DNA is a polymer, meaning that it is a macromolecule (large molecule) made up of many repeating subunits known as monomers. In the case of DNA, the momomers are known as nucleotides, and many nucleotides are linked together form a polynucleotide chain. Each nucleotide is a molecule made up of three individual units: a Phosphate group, the pentose (five carbon) sugar Deoxyribose and a Nitogen-containing base. There are four types of nucleotide base that are named differently due to subtle differences in their chemistry. These are Adenine, Guanine, Cytosine and Thymine. Many mononucleotides join together to form a polynucleotide chain by means of Phosphodiester bonds, that form between the phosphate group of one nucleotide and the deoxyribose group of another. The base then sticks out from this phosphate-sugar backbone. However, the polynucletide chain alone is not enough to form DNA, as the DNA molecules's structure is that of a double helix made up of two (double) polynucleotide chains wound round eachother in a helical shape (helix). The two chains are joind by hydrogen bonds that form between complimentary base pairs. The word complimentary here refers to the fact that bases that will only pair with one other base: Adenine will only pair with Thymine (by means of two hydrogen bonds) and Cytosine will only pair with Guanine (by means of three hydrogen bonds). 

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