The DNA double helix. (A) Space-filling model of 1 1/2 turns of the DNA double helix. The coiling of the two strands around each other creates two grooves in the double helix. As indicated in the figure, the wider groove is called the major groove, and the smaller, the minor groove. (B) A short section of the double helix viewed from its side. Four base pairs are shown. The nucleotides are linked together covalently by phosphodiester bonds through the 3?hydroxyl (-OH) group of one sugar and the 5?phosphate (P) of the next. Thus, each polynucleotide strand has a chemical polarity; that is, its two ends are chemically different. The 3?end carries an unlinked -OH group attached to the 3?position on the sugar ring; the 5?end carries a free phosphate group attached to the 5?position on the sugar ring. Figure 6-6 from Essential Cell Biology, by Alberts et al., 1998, Garland Publishing Inc