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Lactose is a disaccharide present in milk. It is made up of the monosaccharides glucose and galactose. The two monosaccarides join via a condensation reaction forming a β1-4 glycosidic bond. Commonly associated with the Lac Operon which is a model complex for understanding selective gene transcription control in E. coli.

In humans the disaccharide is hydrolysed in humans by the enzyme lactase and by β-galactosidase in bacteria.

Many adults, more commonly outside of Europe, are unable to drink milk as they are unable to hydrolyse the disaccharide into the two monosaccharides. This condition is called lactose intolerance, or hypolactasia. A deficiency of the enzyme lactase is usually the cause of hypolactasia[1].


  1. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.
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