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

340 Chapter 6: How Cells Read the Genome: From DNA to Protein
H 2 N
H O R 2
C C N C C
R 1 H H O
H
N
H
C C
R 3
O
O
H 2 N
R 4
C C
H
O
O
H 2 N
H O R 2
C C N C C
R 1 H H O
H
N
H O
C C
R 3
N
H
R 4
C C
H
O
O
peptidyl-tRNA attached to
C-terminus of the growing
polypeptide chain
3
4
aminoacyltRNA
tRNA molecule freed from
its peptidyl linkage
OH
4
3
new peptidyl-tRNA molecule
attached to C-terminus of
the growing polypeptide chain
Figure 6–59 The incorporation of an amino acid into a protein. A polypeptide chain grows by the stepwise addition of amino
acids to its C-terminal end. The formation of each peptide bond is energetically favorable because the growing C-terminus
has been activated by the covalent attachment of a tRNA molecule. The peptidyl-tRNA linkage that activates the growing end
is regenerated during each addition. The amino acid side chains have been abbreviated as R 1 , R 2 , R 3 , and R 4 ; as a reference
point, all of the atoms in the second amino acid in the polypeptide chain are shaded gray. The figure shows the addition of the
fourth amino acid (red) to the growing chain.
addition disrupts this high-energy covalent linkage, but immediately replaces it
with an identical linkage on the most recently added amino acid (Figure 6–59).
In this way, each amino acid added carries with it the activation energy for the
addition of the next amino acid rather than the energy for its own addition—an
example of the “head growth” type of polymerization described in Figure 2–44.
The RNA Message Is Decoded in Ribosomes
The synthesis of proteins is guided by information MBoC6 m6.61/6.59
carried by mRNA molecules. To
maintain the correct reading frame and to ensure accuracy (about 1 mistake every
10,000 amino acids), protein synthesis is performed in the ribosome, a complex
catalytic machine made from more than 50 different proteins (the ribosomal
proteins) and several RNA molecules, the ribosomal RNAs (rRNAs). A typical
eukaryotic cell contains millions of ribosomes in its cytoplasm (Figure 6–60). The
large and small ribosome subunits are assembled at the nucleolus, where newly
transcribed and modified rRNAs associate with the ribosomal proteins that have
been transported into the nucleus after their synthesis in the cytoplasm. These two
ribosomal subunits are then exported to the cytoplasm, where they join together
to synthesize proteins.
400 nm
Figure 6–60 Ribosomes in the cytoplasm
of a eukaryotic cell. This electron
micrograph shows a thin section of a small
region of cytoplasm. The ribosomes appear
as black dots (red arrows). Some are
free in the cytosol; others are attached to
membranes of the endoplasmic reticulum.
(Courtesy of Daniel S. Friend.)