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Translation I initiation

Like amino acid activation (see p. 248), protein biosynthesis (translation) takes piace in the cytopiasm. it is cataiyzed by compiex nucieoprotein particies, the ribosomes, and mainiy requires GTP to cover its energy requirements. [Pg.250]

Ribosomes consist of two subunits of different size, made up of ribosomal RNA (rRNA) and neariy 80 proteins (the number of proteins appiies to rat iiver ribosomes), it is customary to give the sedimentation coef cients (see p. 200) of ribosomes and their components instead of their masses. For exampie, the eukaryotic ribosome has a sedimentation coefficient of 80 Svedberg units (80 S), whiie the sedimentation coef cients of its subunits are 40 S and 60 S (S vaiues are not additive). [Pg.250]

The smaiier 40 S subunit consists of one moiecuie of 18 S rRNA and 33 protein moie-cuies. The iarger 60 S subunit contains three types of rRNA with sedimentation coef cients of 5 S, 5.8 S, and 28 S and 47 proteins. In the presence of mRNA, the subunits assemble to form the complete ribosome, with a mass about 650 times larger than that of a hemoglobin molecule. [Pg.250]

The arrangement of the individual components of a ribosome has now been determined for prokaryotic ribosomes. It is known that filamentous mRNA passes through a cleft between the two subunits near the characteristic horn on the small subunit. tRNAs also bind near this site. The illustration shows the size of a tRNA molecule for comparison. [Pg.250]

Prokaryotic ribosomes have a similar structure, but are somewhat smaller than those of eukaryotes (sedimentation coef cient 70 S for the complete ribosome, 30 S and 50 S for the subunits). Mitochondrial and chloroplast ribosomes are comparable to prokaryotic ones. [Pg.250]


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Translational initiation

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