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The nucleotides found at the ends of a linear piece of DNA have properties that are biochemically important and useful for orienting the DNA strand. At one end of the DNA chain, a nucleotide will have a phosphate attached to its 5′ carbon that does not connect it to another nucleotide. This end of the strand is called the 5′ end or the 5′ phosphate end (Figure 1.3B). On the other end, the last nucleotide lacks a phosphate at its 3′ carbon. Because it has only a hydroxyl group (the OH in Figure 1.3B), this end is called the 3′ end or the 3′ hydroxyl end.

Figure 1.2 Chemical structures of deoxyribonucleotides, showing the bases and sugars and how they are assembled into a deoxyribonucleotide.

Figure 1.3 (A) Schematic drawing of a DNA chain, showing the 3′-to-5′ attachment of the phosphates to the sugars, forming phosphodiester bonds. (B) Two strands of DNA bind at the bases in an antiparallel arrangement of the phosphatesugar backbones.