The compounds of Formula I, Formula Ia, and Formula Ib are described herein along with their analogs, tautomeric forms, stereoisomers, polymorphs, hydrates, solvates, pharmaceutically acceptable salts, pharmaceutical compositions, metabolites, and prodrugs thereof. These compounds inhibit PRMT5 and are useful as therapeutic or ameliorating agent for diseases that are involved in cellular growth such as malignant tumors, schizophrenia, Alzheimer's disease, Parkinson's disease and the like.