This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Basics of genetics, Subject name: Biochemistry
Which of the following gene is constitutively expressed in E.coli? 
Select and explain. 
A. Lac A, B. Lac I, C. Lac Y, D. Lac Z
B: Lac I
Answer: B: Lac I 
Explanation: Genes can be broadly divided into types based on their expression pattern: Constitutively expressed genes Regulated genes House-keeping genes Inducible/repressible Transcribed at a constant level Transcribed depending upon the environmental conditions Eg : Glycolytic enzymes Ribosomal RNA and proteins Eg : UDP-glucuronosyl transferase Lac I gene is constitutively regulated. Lac Z, Lac Y and Lac A genes are regulated by induction/repression.