Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

FROM RNA TO PROTEIN
341
70S
80S
MW 2,500,000
MW 4,200,000
50S (large) subunit
30S (small) subunit
60S (large) subunit
40S (small) subunit
MW 1,600,000 MW 900,000
MW 2,800,000
MW 1,400,000
5S rRNA
23S rRNA
16S rRNA
5S rRNA 28S rRNA 5.8S rRNA 18S rRNA
120
nucleotides 2900
nucleotides
1540
nucleotides
120
nucleotides
4700
nucleotides
160
nucleotides 1900
nucleotides
34 proteins 21 proteins
BACTERIAL RIBOSOME
~49 proteins ~33 proteins
EUKARYOTIC RIBOSOME
Figure 6–61 A comparison of bacterial and eukaryotic ribosomes. Despite differences in the number and size of their
rRNA and protein components, both bacterial and eukaryotic ribosomes have nearly the same structure and they function
similarly. Although the 18S and 28S rRNAs of the eukaryotic ribosome contain many nucleotides not present in their bacterial
counterparts, these nucleotides are present as multiple insertions that form extra domains and leave the basic structure of the
rRNA largely unchanged.
Eukaryotic and bacterial ribosomes have similar structures and functions,
being composed of one large and one small subunit that fit together to form a
complete ribosome with a mass of several million daltons (Figure 6–61). The small
subunit provides the framework on which the tRNAs are MBoC6 accurately m6.63/6.61 matched to
the codons of the mRNA, while the large subunit catalyzes the formation of the
peptide bonds that link the amino acids together into a polypeptide chain (see
Figure 6–58).
When not actively synthesizing proteins, the two subunits of the ribosome
are separate. They join together on an mRNA molecule, usually near its 5ʹ end,
to initiate the synthesis of a protein. The mRNA is then pulled through the ribosome,
three nucleotides at a time. As its codons enter the core of the ribosome, the
mRNA nucleotide sequence is translated into an amino acid sequence using the
tRNAs as adaptors to add each amino acid in the correct sequence to the growing
end of the polypeptide chain. When a stop codon is encountered, the ribosome
releases the finished protein, and its two subunits separate again. These subunits
can then be used to start the synthesis of another protein on another mRNA molecule.
Ribosomes operate with remarkable efficiency: in one second, a eukaryotic
ribosome adds 2 amino acids to a polypeptide chain; the ribosomes of bacterial
cells operate even faster, at a rate of about 20 amino acids per second.
To choreograph the many coordinated movements required for efficient translation,
a ribosome contains four binding sites for RNA molecules: one is for the
mRNA and three (called the A site, the P site, and the E site) are for tRNAs (Figure
6–62). A tRNA molecule is held tightly at the A and P sites only if its anticodon