Monosaccharides are monomeric carbohydrate molecules. Examples include glucose, fructose, galactose, xylose, and ribose. 2-Deoxyribose and ribose are constituents of DNA and RNA, respectively. Many of the carbon atoms attached to hydroxyl groups are chiral centers, giving rise to a number of isomeric forms. Many monosaccharides are unstable over geological time scales. The half-lives for the decomposition of ribose, for example, are 73 min at 100 °C, and 44 years at 0 °C at pH 7. They can be formed in the formose reaction from formaldehyde. Monosaccharide-like compounds have been found in carbonaceous chondrites, and one of the ketoses, dihydroxyacetone (HOCH2COCH2OH), was identified in the extract of Murchison meteorite.