. The biosynthesis of proteins. Proteins -- Synthesis. 44 THE BIOSYNTHESIS OF PROTEINS antibody from ribonucleoprotein particles of lymph nodes or spleen of immunized animals, by destroying the RNA with ribonuclease. All the experiments reviewed above point to the microsomes and more especially to the ribonucleoprotein they contain as important centres of synthesis of soluble cell proteins within the living cell. It must be em- phasized that 'microsome' is not the name of any intact cell structure. This term simply refers to a certain type of small particles which can be separated from cell ho
. The biosynthesis of proteins. Proteins -- Synthesis. 44 THE BIOSYNTHESIS OF PROTEINS antibody from ribonucleoprotein particles of lymph nodes or spleen of immunized animals, by destroying the RNA with ribonuclease. All the experiments reviewed above point to the microsomes and more especially to the ribonucleoprotein they contain as important centres of synthesis of soluble cell proteins within the living cell. It must be em- phasized that 'microsome' is not the name of any intact cell structure. This term simply refers to a certain type of small particles which can be separated from cell homogenates by high speed centrifugation in an appropriate medium. Microsome pellets are jelly-like, transparent and slightly coloured. When redispersed in aqueous medium they form an opalescent suspension in which regular light microscopy or even phase contrast does not reveal 700 P iipoprotein/' / -o 400 300 200 r y / ribo osome-| nucleopr otein / / /Soluble protein of cell -0 ^ ^^ (a). 20 0 5 10 15 20 25 0 5 10 15 Time, min Time, min Fig. 19. In vivo incorporation of I'^C leucine into microsomes of rat liver fractionated with sodium deoxycholate. (a) Saturating dose of ^'^C leucine. (b) Tracer dose of ^'^C leucine. (Littlefield et al., 1955). any structure. Dark field examination shows a great number of particles animated with intense brownian motion. No such particles can be observed in living cells and it has long been suspected that microsomes form after disruption of the cell, during the dispersion of the cell contents in the extracting medium (Brachet and Jeener, 1944; Claude, 1947; Porter, 1953). The fact that they contain most of the cellular RNA indicated that microsomes originate from the ergasto- plasm, also called cytoplasmic ground substance. Thanks to the development of electronmicroscopy of ultrathin tissue sections (Porter and Blum, 1953; Sjostrand, 1953), knowledge of the structure of cytoplasm made great progress during the last few years. A network of lamella
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