19. True or False The start codon is always AUC and codes for the amino acid methionine. 20. True or False A polyribosome is the combination of a mRNA strand with several attached ribosomes. 21. Enzymes that are constantly produced in a bacterial cell at a fixed rate are called _________________________ enzymes. Their genes are turned on all the time. 22. True or False Repression inhibits gene expression and decreases the synthesis of enzymes. It is mediated by regulatory proteins called repressors. 23. True or False In 1931, Francois Jacob and Jacques Monad studied the induction of the enzymes for lactose catabolism in E. coli. These studies later became known as the lac operon. . 24. Which of the following statements is false regarding the lac operon? A. The lac operon is composed of a combination of structural genes and a control region. B. There are three lac structural genes. C. The control region is upstream of the structural genes. D. The control region contains a promotor and an operator site. E. None of the above 25. Which of the following statements is false regarding the lac operon? A. The structural gene Z codes for the enzyme -galactosidase. B. The structural gene Y codes for the enzyme permease. C. The structural gene X codes for the enzyme transacetylase. D. When no lactose is present, the repressor protein binds to the operator site. E In the presence of lactose, allolactose will bind with the repressor protein, a