In biology, a nucleus is the cellular organelle that contains the organism’s genetic material and controls cellular functions, earning it the designation as the cell’s command center. The nucleus contains nuclear lamina, the structural framework of protein filaments that supports its contents; the nuclear envelope, a double-walled membrane that protects the genetic material and prevents it from randomly interacting with molecules in the cell’s cytoplasm; and nucleoplasm,in which the chromosomes and other structures are suspended.
Chromosomes are the intertwined strands of complex proteins and strands of DNA, or deoxyribonucleic acid, that form the cell’s genome. The nuclear envelope contains pores that allow the transport of smaller molecules necessary to carry energy and messages in and out of the nucleus. The largest structure in the nucleus is the nucleolus, which is the site of ribosome production.
Ribosomes are the protein-ribonucleotide machines that carry out a variety of functions that include protein synthesis and the synthesis of amino acids and other molecules to form molecules of RNA, or ribonucleic acid. RNA molecules come in “transfer” and “messenger” types that untwist, decode and copy segments of DNA molecules by finding, selecting and assembling the polypeptides in the correct order that forms genes and provides the molecule’s double helix structure. The ribosomes also act as escorts for other molecules entering the nucleus, such as ATP, the energy packets that fuel cellular activity. The nucleolus also produces and contains proteins that act as signal receptors that trigger particular cellular responses.