Ribosomes electron micrograph

The 70 S ribosome itself, as depicted in Fig. 6, appears to be rather symmetric, with the 30 S subunit lying somewhat obliquely across the 50 S subunit with the head of the 30 S subunit near the LI shoulder on the 50 S subunit. The platform region of the 30 S subunit faces the 50 S subunit providing an interesting shielded area where the ribosomal function apparently occurs. It appears from electron micrographs that the structure of the subunits is not greatly altered upon their association to form the 70 S ribosome. 

Fig. 7. (a) Crystals of E. colt 70 S ribosomes, (b) and (c) Electron micrographs of sections through three-dimensional crystals shown in (a) in two orthogonal directions (Wittmann et al., 1982). (d) and (e) Crystals and computed filtered image of a section through a crystal of Bacillus stearothermophilus 50 S ribosomal subunits (Yonath et al., 1982a,b Leonard et al., 1983). (d) and (e) are related to two different crystal forms. Reproduced with permission from Wittmann (1983). 

Euchromatin generally corresponds to looped 30-nm fibers. Heterochromatin is more highly condensed. Figure 1-1-14 shows an electron micrograph of an interphase nucleus containing euchromatin, heterochromatin, and a nucleolus. The nudeolus is a nuclear region spedalized for ribosome assembly (discussed in Chapter 3). 

The chemical and physical properties of ribosomes are well characterized (for reviews see The exact understanding of their function, however, still lacks a detailed molecular model. Appropriate methods such as image reconstruction from electron micrographs of two-dimensional sheets, or X-ray structure analysis, all depend on the crystallizability of the material. 

FIGURE 27-1 Ribosomes and endoplasmic reticulum. Electron micrograph and schematic drawing of a portion of a pancreatic cell, showing ribosomes attached to the outer (cytosolic) face of the endoplasmic reticulum (ER). The ribosomes are the numerous small dots bordering the parallel layers of membranes. 

Rapid Translation of a Single Message by Polysomes Large clusters of 10 to 100 ribosomes that are very active in protein synthesis can be isolated from both eukaryotic and bacterial cells. Electron micrographs show a liber between adjacent ribosomes in the cluster, which is called a polysome (Fig. 27-27). The connecting strand... 

The electron micrograph in Fig. 28-5 shows RNA polymerase complexes apparently moving along a DNA strand with ribosomes assembled on the RNA and... 

Figure 28-5 Electron micrograph showing transcription from an unidentified operon in E. coli. Note the DNA duplexes (horizontal) and the mRNA chains with ribosomes attached. The mRNA chains are shorter at the right side where transcription begins and larger to the left where transcription has proceeded for a longer time. From O. L. Miller, Jr.85...

The many thousands of proteins present in each cell are made within the ribosomes, which are able to read each specific mRNA that comes their way. While ribosomes appear as little more than blurred dots in most electron micrographs, the 15,000 ribosomes of one cell of E. coli represent one-fourth of the total mass of the cell. Eukaryotic cells contain many times more of these little molecular machines. When ribosomes were first observed in the early 1950s,1 3 nobody could imagine either their composition or their function. 

Polyribosomes. Under suitable conditions ribosomes isolated from cells are found to sediment together in clusters, often of five or more. These polyribosomes (or polysomes), which can be seen in electron micrographs (Fig. 28-5), are held together by chains of mRNA. Polyribosomes arise because a single mRNA molecule is being translated by several ribosomes at once. As the 5 terminus of the mRNA emerges from one ribosome, it may soon combine with another and initiate translation of a second peptide chain, etc. The length of the mRNA determines how many ribosomes are likely to be associated in a polyribosome. 

The current view of the overall morphology of ribosomes is based largely on electron-micrographic studies of the subunits of E. coli. From this analysis it appears that both subunits are asymmetrical (fig. 28.6). 

Figure 4.7. Electron micrograph of a milk fat globule in the alveolar lumen that has a large cytoplasmic crescent entrained between the globule and the surrounding membrane. This crescent contains secretory vesicles, ribosome-studded vesicles of apparent endoplasmic reticulum origin, and an abundant amount of particulate material. Bar = 2 pm.

Microsomes are small, spherical, membranous vesicles with attached ribosomes. During differential sedimentation, they sediment only in the late stages of a preparation, when very high centrifugal forces are used. They don t appear in electron micrographs of a cell. From whence do they arise ... 

Figure 1. Morphological analysis of dog pancreatic rough ER microsomes. (A) Electron micrograph of dog pancreatic rough microsomes. Arrows indicate ribosomes at the surface of ER microsomal membranes. (B) Contact mode Atomic Force Microscopy micrograph of purified ER microsomes fused on mica. Scale bars are 1 jxm.

Figure 11.21. Golgi Complex and Endoplasmic Reticulum. The electron micrograph shows the Golgi complex and adjacent endoplasmic reticulum. The black dots on the cytoplasmic surface of the ER membrane are ribosomes. [Micrograph courtesy of Lynne Mercer.]...

Figure 29.15. Ribosomes at Low Resolution. Electron micrographs of (A) 308 subunits, (B) 508 subunits, and (C) 708 ribosomes. [Courtesy of Dr. James Lake.]...

Polysomes, (a) Electron micrograph of an E. coli polysome. (Courtesy of Barbara Hamkalo.) (b) Diagram showing relative movement of the VOS ribosome and the mRNA and growth of the protein chain. 

Fig. 3-14 Electron micrograph showing polyribosomes from the bacterium Escherichia coli. Filaments are DNA molecules. Ribosomes are attached to mRNA which they are translating. Bar = 0.1 /rm. Photograph courtesy of Dr. O. L. Miller, Jr. (from Miller ef a/., 1970).

A FIGURE 16-2 Electron micrograph of ribosomes attached to the rough ER in a pancreatic acinar cell. Most of the proteins synthesized by this type of ceil are to be secreted and are formed on membrane-attached ribosomes. A few membrane-unattached (free) ribosomes are evident presumably, these are synthesizing cytosoiic or other nonsecretory proteins. [Courtesy of G. Paiade.l... 

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