The term nucleic acid, or ribonucleic acid, is a categorical word used to indicate either RNA or DNA, both of which are typically found in the nucleus of a cell. both RNA and DNA are polymers containing either ribose (RNA) or 2'-deoxyribose (DNA) linked together by phosphate bonds. The ribose sugars also have a base attached to them in the 1'-position. For DNA, the bases used are adenine, cytosine, guanosine, or thymine. In RNA, the base thymine is replaced by uracil. The base cytosine my exist in either the methylated or hydroxylated forms, and these modifications are important in the field of epigenetics, which studies multi-generational inheritance.