© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

8.5.4.5 Nucleolus

The largest and most prominent "nuclear organelle" is the nucleolus, a highly coiled structure associated with numerous particles but not surrounded by a membrane.¹¹⁴¹ The nucleolus is a ribosome-manufacturing machine. Assembly of precursor ribosomal subunits within the nucleolus requires ~1800 sec, while the complete assembly of a large ribosomal subunit (needing only protein to make a completed ribosome) takes ~3600 sec.⁵³¹

The nucleolus is composed of DNA, RNA, and proteins. It also has a granular component (each granule ~150 nm thick) and a fibrillar component, and a variable internal structure.¹¹⁴¹ The granular component consists of ~15-nm particles that are ribosomal subunits in the process of maturation. The fibrillar component consists of rRNA molecules that have already become associated with proteins to form fibrils with a thickness of ~5 nm. The size of the nucleolus correlates with its level of activity. In cells characterized by a high rate of protein synthesis and hence by the need for many ribosomes, the nucleolus can occupy 20-25% of nuclear volume (3-5 micron diameter in a 20-micron cell), mostly comprised of the granular component. In less active cells, the nucleolus is much smaller -- as small as 0.5 micron in a mature lymphocyte.¹¹⁴¹ Nucleoli are frequently located at or near the nuclear envelope, adhering directly to the nuclear lamina or attaching to it by a pedicle. In nuclei having a centrally located nucleolus, the nuclear envelope is folded to form a nucleolar canal that is in direct contact with the nucleolus.¹¹⁴¹

Most human nuclei contain only one nucleolus, except for liver cell nuclei which may contain more than one nucleolus⁹³⁵ and cultured HeLa (cancer) cells which may have up to six.¹¹⁴¹ The number of nucleoli in a eukaryotic cell nucleus normally is determined by the number of chromosomes with secondary constrictions, or nucleolus organizer regions (NORs). The human genome contains five NORs per haploid chromosome set, or 10 NORs per diploid nucleus, each located near the tip of a chromosome. However, instead of 10 separate nucleoli, the typical human nucleus contains a single large nucleolus representing the fusion of loops of chromatin from the 10 separate chromosomes with NORs (Fig. 8.48). The DNA from the remaining diploid chromosomes is distributed in specific regions throughout the nucleoplasm (Fig. 8.47). During mitosis, the chromosomes condense into a more compact form and the nucleolus shrinks, then disappears altogether. A cell undergoing mitosis thus has no nucleolus and synthesizes no rRNA. Once mitosis is complete, the nucleolus reappears. As rRNA synthesis resumes, ten tiny nucleoli appear, one near the end of each of 10 different chromosomes; these enlarge, eventually fusing into the single large nucleolus characteristic of the interphase human nucleus.

Last updated on 20 February 2003