Nucleus is the controlling centre of a cell as it contains chromosomes and genes. Chromosomes contain the DNA which by transcription prepare RNA which in turn leads to the formation of proteins by translation. These proteins regulate the various metabolic processes going in the cell. Besides this, nucleus contains genetic information for reproduction, development \& behaviour of an organism.
Nuclear membrane is double layered, semipermeable and is perforated by minute, circular nuclear pores. The pore controls the passage of macromolecules like tRNAs, mRNAs, ribosomal proteins, enzymes etc. during different processes of transcription, translation etc.