First consider the "D" of DNA: the "deoxyyribose-methyl-phosphate" chains that make up its outer spirals. Looking slightly from above, you can see here that these chains consist of three repeating units. Moving downward along the right chain:
First is a deoxyribose ring. Ribose is a common sugar. Dexoyribose is that sugar with one of its normal oxygen’s removed. The bases in DNA’s core are attached to these ribose rings (where you now see incomplete bonds sticking inward).
Second is a methyl group (C + 2 H’s).
Third is a phosphate group ( P + 4 O’s). This unit forms with one unpaired electron. To pair that electron it grabs an electron from surrounding water, thereby acquiring a net negative charge. This, in turn, gives DNA its negative charge. Which allows for DNA’s manipulation via electric fields - as in the technology of gel electrophoresis (link).