In a double helix the strands are antiparallel. These asymmetric bonds mean each strand of DNA has a direction. The pentoses are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. The backbone of the DNA strand is made from alternating phosphate and 2-deoxy-ribose, a pentose. The DNA chain is 22 to 26 Ångströms wide (2.2 to 2.6 nanometres), and one nucleotide unit is 3.3 Å (0.33 nm) long.
DNA is a long polymer made from repeating units called nucleotides.
