Essential Cell Biology 5th edition

250 CHAPTER 7 From DNA to Protein: How Cells Read the Genome
Figure 7–34 Ribosomes are located in
the cytoplasm of eukaryotic cells. This
electron micrograph shows a thin section of
a small region of cytoplasm. The ribosomes
appear as small gray blobs. Some are free
in the cytoplasm (red arrows); others are
attached to membranes of the endoplasmic
reticulum (green arrows). (Courtesy of
George Palade.)
endoplasmic reticulum
400 nm
QUESTION 7–4
In a clever experiment performed in
1962, a cysteine already attached to
its tRNA was chemically converted
to an alanine. These “hybrid” tRNA
molecules were then added to a cellfree
translation system from which
the normal cysteine-tRNAs had
been removed. When the resulting
protein was analyzed, it was found
that alanine had been inserted at
every point in the polypeptide chain
where cysteine was supposed to be.
Discuss what this experiment tells
you about the role of aminoacyltRNA
synthetases and ribosomes
during the normal translation of the
genetic code.
prokaryotes and eukaryotes, the machine that gets the job done is the
ribosome—a large complex made from dozens of small proteins (the
ribosomal proteins) and several RNA molecules called ribosomal RNAs
(rRNAs). A typical eukaryotic cell contains millions of ribosomes in its
ECB5 e7.31/7.34
cytosol (Figure 7–34).
Eukaryotic and prokaryotic ribosomes are very similar in structure and
function. Both are composed of one large subunit and one small subunit,
which fit together to form a complete ribosome with a mass of several
million daltons (Figure 7–35); for comparison, an average-sized protein
+
+
+
~49 ribosomal proteins + 3 rRNA molecules ~33 ribosomal proteins + 1 rRNA molecule
+
large subunit
small subunit
MW = 2,800,000
MW = 1,400,000
Figure 7–35 The eukaryotic ribosome
is a large complex of four rRNAs and
more than 80 small proteins. Prokaryotic
ribosomes are very similar: both are formed
from a large and small subunit, which only
come together after the small subunit has
bound an mRNA. The RNAs account for
most of the mass of the ribosome and give
it its overall shape and structure.
large
subunit
small
subunit
complete eukaryotic ribosome
MW = 4,200,000
~82 different proteins +
4 different rRNA molecules