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Individual nucleotides are joined together to form oligonucleotides or nucleic acids. The linkage is from the sugar of one nucleotide to the sugar of the next nucleotide through the phosphate group. This linkage is called a phos-phodiester link. The phosphate between the sugars is linked to the 5' position of one sugar molecule and the 3' position of the second molecule. This repeating sugar-phosphate sequence forms the backbone of the nucleic acid. The five different bases, attached to the sugars at the 1' position, stick out away from the sugar-phosphate backbone (Figure 13.4). By convention, the end of the nucleic acid containing the free 5' hydroxyl or phosphate group is written to the left and is called the head, while the end with the free 3 ' phosphate is the tail and is written to the right.

It is the sequence of different bases in the oligonucleotide that is of functional importance and therefore the nucleic acid chain is usually expressed simply as its base sequence, e.g.

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