It is from this numbering system of the sugar group that DNA gets its polarity. The linkages between nucleotides occur between the 5' and 3' positions on the sugar group. One end has a free 5' end and the other has a free 3' end.
Attached to the remaining free carbons at the 1', 3' and 5' positions is an oxygen-containing hydroxyl group (-OH). The DNA sugar is called a deoxyribose because it is lacking a hydroxyl group at the 2' position. Instead there is just a hydrogen (see ).
A phosphate group consists of a central phosphorous surrounded by four oxygens. The phosphorous is single-bonded to three of the oxygens and double-bonded to the fourth. Due to the nature of the chemical bonds, there is a negative charge on each oxygen that has only one bond coming off of it. This negative charge accounts for the overall negative charge on the phosphate backbone of a DNA molecule.