Write short note on nucleus.

The nucleus is the most prominent and essential organelle in eukaryotic cells, serving as the control center that houses the cell's genetic material and directs all cellular activities. Enclosed by a double-layered nuclear envelope perforated with nuclear pores, the nucleus contains chromosomes made of DNA and associated histone proteins. These chromosomes carry genes that encode instructions for protein synthesis and inheritance of traits. The nucleus regulates which genes are expressed at any given time, controlling cell differentiation, metabolism, growth, and reproduction. Within the nucleus lies the nucleolus, a dense region responsible for synthesizing ribosomal RNA and assembling ribosome subunits.

The nuclear envelope's pores selectively regulate the exchange of materials between the nucleus and cytoplasm—allowing mRNA, tRNA, and ribosomal subunits to exit for protein synthesis while permitting enzymes, nucleotides, and regulatory proteins to enter. During cell division, the nuclear envelope temporarily disassembles, allowing chromosomes to segregate properly before reforming in daughter cells. The nucleus essentially functions as the cell's library and administrative center, storing hereditary information and issuing instructions that coordinate all cellular processes. Cells that lose their nucleus, like mature mammalian red blood cells, have limited lifespans and cannot reproduce, demonstrating the nucleus's critical role in long-term cellular function and genetic continuity across generations.