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Nucleus

Distinctive Features, Chromosomal Territories, Interchromatin Compartment, Nucleolus, Subnuclear Bodies, Nuclear Envelope, Nuclear Pores

The largest of the membrane-bound organelles, the nucleus first was described in 1710 by Antoni van Leeuwenhoek using a simple microscope. In 1831 the Scottish botanist Robert Brown characterized the organelle in detail, calling it the "nucleus," from the Latin word for "little nut." The nucleus is the site of gene expression and gene regulation.

Cajal (Coiled) Bodies.

By electron microscopy, Cajal bodies are seen as tangled balls of thread. They number one to ten per nucleus, with more seen in growing cells. They are often found in association with nucleoli or specific chromosomal territories. Although their true function remains unknown, their ability to associate regularly with nucleoli has led to speculation that they are somehow involved in processing either mRNA or rRNA.

Gems.

More tightly coiled, smaller versions of Cajal bodies, gems are frequently seen interacting with Cajal bodies and are distinct structures. They are known to contain a protein called SMN (which stands for "survival of motor neurons") that, when mutated, is responsible for a severe inherited form of a human muscular wasting disease called spinal muscular atrophy. Based upon the known function of the normal SMN protein, it is speculated that gems are involved in trafficking mRNA spliceosome subunits through the nucleus and may indirectly help remove mRNA introns.

PML Bodies.

Nuclei typically have ten to twenty PML bodies (also known as PODs, Kremer bodies, or ND10) that take the shape of dense rings. They contain proteins that, when mutated, have been identified with such disease processes as retinoblastoma and Bloom's syndrome. Their normal pattern is altered in the nuclei of human acute promyelocytic leukemia. When cells are infected with herpes simplex virus type 1, adenovirus, or human cytomegalovirus, PML bodies are disrupted. Although their function remains unknown, the fact that they are altered in diseased or malignant cells suggests that they play an important role in the normal cell, including growth control and apoptosis.

Speckles (Interchromatin Granules).

Speckles are clusters of dense structures seen by electron microscopy that, when stained with fluorescent tags specific to small nuclear ribonucleoproteins (snRNP), give rise to a "speckled" nucleus. Small nuclear ribonucleoproteins are RNA-protein complexes that are subunits of the spliceosome involved in mRNA intron removal. The twenty to fifty speckles per nuclei are typically found in the interchromatin compartment, where mRNA undergoes processing prior to transport through the nuclear pore and into the cytoplasm.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 3