12 DNAの折りたたみ Nucleosomes-H1 linker histone Figure Model for the packing of chromatin and the chromosome scaffold in metaphase chromosomes. In interphase chromosomes, long stretches of 30-nm chromatin loop out from extended scaffolds. In metaphase chromosomes, the scaffold is folded into a helix and further packed into a highly compacted structure, whose precise geometry has not been determined.Fig. 2. Models for the 30-nm fiber. The figure shows a schematic representation of two models that have been proposed to explain the 30-nm fiber, as discussed in the text. The electron micrographs of the 11-nm fiber and solenoid were adapted and modified from Molecular Biology of the Cell (B. Alberts, A. Johnson, J. Lewis, M. Raff, K. Roberts, P. Walter, Garland Science Publishing, London, ed. 4, 2002). The zig-zag electron micrograph was adapted and modified from J. Cell Biol. 83, 403 (1979). Two concentrations of TEA-Cl (trieathanolamine-HCl) are shown, 1 and 5 mM, to emphasize the importance of the salt concentration in the compaction of the nucleosomal arrays. Changing faces of chromatin.htm
37 1. An aminoacyl-tRNA molecule binds to a vacant A-site on the ribosome 2. A new peptide bond is formed at the P-site (LSUずれる)3. The mRNA moves a distance of three nucleotides through the small-subunit chain (SSUが移動)Figure Translating an mRNA molecule. Each amino acid added to the growing end of a polypeptide chain is selected by complementary base-pairing between the anticodon on its attached tRNA molecule and the next codon on the mRNA chain. Because only one of the many types of tRNA molecules in a cell can base-pair with each codon, the codon determines the specific amino acid to be added to the growing polypeptide chain. The three-step cycle shown is repeated over and over during the synthesis of a protein. An aminoacyl-tRNA molecule binds to a vacant A-site on the ribosome in step 1, a new peptide bond is formed in step 2, and the mRNA moves a distance of three nucleotides through the small-subunit chain in step 3, ejecting the spent tRNA molecule and “resetting” the ribosome so that the next aminoacyl-tRNA molecule can bind. Although the figure shows a large movement of the small ribosome subunit relative to the large subunit, the conformational changes that actually take place in the ribosome during translation are more subtle. It is likely that they involve a series of small rearrangements within each subunit as well as several small shifts between the two subunits. As indicated, the mRNA is translated in the 5′-to-3′ direction, and the N-terminal end of a protein is made first, with each cycle adding one amino acid to the C-terminus of the polypeptide chain. The position at which the growing peptide chain is attached to a tRNA does not change during the elongation cycle: it is always linked to the tRNA present in the P site of the large subunit.