nucleolus

The nucleolus is a compartment within the nucleus of eukaryotic cells in which ribosomal components are assembled. Nucleoli comprise proteins and ribosomal DNA components. The size of nucleoli depends upon the ribosomal requirements of the type of cell in which they are found. In cells that produce large amounts of protein, and thus require significant numbers of ribosomes, the nucleolus sometimes occupyies as much as 25 percent of the total volume of the nucleus.

Nucleoli lie within the nucleoplasm, surrounded by condensed chromatin, and appear as dark dots on microscopy. They contain granular and fibrillar components, DNA, and an ill-defined matrix. (tem, labelled tem, plant tem, pink nor, tem, color tem, tem, image) Fibrillarins are small nuclear ribonucleoproteins (snRNPs) involved in ribosomal RNA processing, which lie centralized within the sub-organelle. Nucleoli form at chromosomal sites usually termed nucleolus organizer regions (NORs), which disappear at the onset of mitosis. Post-mitosis, the new nucleolus develops from the NORs. The DNA found at chromosomal NORs encodes the genes for ribosomal RNA (rRNA). The rDNA serves as template for the transcription of pre-rRNA by RNA polymerase I. The synthesis of rRNAs is not achieved by simple transcription of the individual rRNA species, rather it requires a complex series of post-transcriptional processing steps.

. animation - nucleolus heterochromatin : animation - spinning nucleus :art - nuclear membrane and nuclear pores : Virtual Cell Textbook - Cell Biology :

• A • adhesion • C • cell membranes • cellular adhesion molecules • cellular signal transduction • centrioles • chemotaxis • chloroplast • cilia & flagella • communication • concentration gradients • cytokine receptors • cytoplasm • cytoskeleton • E • energy transducers • endoplasmic reticulum • endosomes • exosome • F • flagella & cilia • G • Golgi apparatus • GPCRs • H • hormones • I • ion channels • L • lysosome • M • meiosis • microtubules • mitosis • mitochondrion • N • Nitric Oxide • neurotransmission • neuronal interconnections • nuclear membrane • nuclear pore • P • pinocytosis • proteasome • pumps • R • receptor proteins • receptor-mediated endocytosis • S • second messengers • signaling gradients • signal transduction • spindle • structure • T • transport • two-component systems • V • vacuole • vesicle •