DNA has a negative charge due to the negative charge of its phosphate component. The other two components of DNA consist of a 5-carbon sugar and a nitrogen base.
The phosphate are found in the ribose-phosphate backbone of DNA. Phosphate links the sugars, called deoxyribose and from which DNA or deoxyribonucleic acid gets its name, to make up the backbone of each strand of DNA. Each sugar is linked to the next by a phosphate group.
RNA, made up of ribose rather than deoxyribose, or both sugars, is similarly negatively charged. The difference between the two sugars is that deoxyribose is missing one of the hydroxyl groups.
The negative charge of both DNA and RNA is used to separate them along with proteins from compounds using a test called gel electrophoresis. The test involves introducing the test material into a gel medium with a positive charge at one end and a negative charge at the other. Because of their negative charge, DNA and RNA will move towards the positive phase. DNA molecules can be separated out according to their size. The molecules will move through the gel at speeds corresponding to their size; the smallest will move fastest.The electrophoresis results will show banded areas and those that moved the farthest are the smallest DNA molecules.