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

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

8.5.3.4 Ribosomes

Ribosomes are the smallest and most numerous "organelle" (macromolecular assembly) in the human cell. A typical liver cell contains 10⁷ ribosomes constituting ~5% of the total dry mass of the cell. Cells less actively involved in protein synthesis have correspondingly fewer ribosomes. Some ribosomes float freely in the cytosol, making proteins for intracellular use. Others are attached to membranes or to the cytoskeleton, synthesizing proteins destined for membranes or for export from the cell.

Ribosomes are ~25 nm in diameter. Each of the several types of ribosome (containing rRNA) is constructed of two subunits that fit snugly together. A typical ribosome might have a mass of 4.2 million daltons, comprised of 2.8 million daltons for the large ribonucleoprotein 60S subunit (~23 nm diameter) and 1.4 million daltons for the small ribonucleoprotein 40S subunit (~9 nm diameter).^938,996 There is no intrinsic difference between free cytosolic ribosomes (50% or more of the total) and membrane-bound ribosomes -- it is the "signal" sequence on the end of the protein being synthesized that directs a free ribosome to become a bound ribosome.

The ribosome is an ATP-powered protein-assembling machine (Fig. 2.2). Amino acids drawn from the cytosol are presented to the ribosome, which incorporates them one by one into polypeptides at ~20 Hz.⁵³¹ The complete synthesis of an average-sized protein takes 20-60 seconds. Even during this short period, multiple initiations take place, with a new ribosome hopping onto the 5' starting end of an mRNA (messenger) molecule almost as soon as the preceding ribosome has translated enough of the amino acid sequence to get out of the way, thus allowing the assembly of many copies of the same protein to proceed almost in parallel.⁵³¹ Thus, under physiological conditions, actively translated mRNA is found in polyribosomes, or polysomes, formed by gangs of multiple ribosomes spaced as close as 80 nucleotides apart along a single messenger molecule (Fig. 8.38). Further discussion of ribosomes may be found in Volume II.

Last updated on 20 February 2003