DNA is a polymer whose repeating unit contains a phosphate group (PO4), a ribose ring, and a DNA base, which can be either Adenine (A), Thymine (T), Guanine (G), or Cytosine (C); the phosphate group and ribose ring don't vary. The Carbon atoms in the base are numbered 1 through however many that base has, and those in the ribose are numbered 1' through 5'. The 5' carbon of the ribose ring is covalently bonded to one of the oxygens of the phosphate group, while the 3' carbon of the ribose ring is bonded to (a different) one of the oxygens of the next phosphate group; hence biologists commonly refer to the "5' end" and "3' end" of a DNA strand. The phosphate at the 5' end of a strand "normally" just won't have anything bound to it (although sometimes the 5' end of a DNA or RNA strand will get modified for the purpose of some cellular process); while 3' carbon of the base at the 3' end of a strand will have an OH group instead of the O of the next phosphate.

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