細(xì)胞生物學(xué)11核糖體-文庫吧在線文庫
【正文】 polyribsome (polysome). (b) Electron micrograph of a grazing section through the outer edge of a rough ER cisterna. The ribosomes are aligned in loops and spirals, indicating their attachment to mRNA molecules to form polysomes. (c) Electron micrograph of metalshadowed polysomes isolated from reticulocytes engaged in hemoglobin synthesis. Most of these polysomes have between four and six ribosomes. mRNA + ① ② ③ In step 1, initiation of translation begins with the association of the 30S ribosomal subunit with the mRNA at the AUG initiation codon, a step that requires IF1 and IF3. The 30S ribosomal subunit binds to the mRNA at the AUG initiation codon as the result of an interaction between a plementary nucleotide sequence on the rRNA and mRNA. In step 2, a ternary (threemembered) plex consisting of formylmethionyltRNAIF2GTP bees associated with the mRNA30S ribosomal subunit plex acpanied by the release of IF3. In step 3, the 50S subunit joins the plex with the acpanying release of IF1, IF2, and the products of GTP hydrolysis (GDP and Pi). The initiation of protein synthesis in prokaryotes (a) (b) Steps in the elongation of the nascent polypeptide during translation in prokaryotes (a) In step 1, an aminoacyltRNA whose anticodon is plementary to the second codon of the mRNA enters the empty A site of the ribosome. The binding of the tRNA is acpanied by the release of GDPTu. In step 2, peptide bond formation is acplished by the transfer of the nascent polypeptide chain from the tRNA in the P site to the aminoacyltRNA of th A site, forming a peptidyltRNA in the A site and a deacylated tRNA in the P site. The reaction is catalyzed by a part of the 28S rRNA acting as a ribosome. In step 3, the binding of factor G and the hydrolysis of its associated GTP results in the translocation of the ribos
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