Essential Cell Biology 5th edition

From RNA to Protein
243
nuclear
envelope
exon
junction
complex
cap-binding
protein
5′ cap
initiation factors
TRANSLATION
for protein synthesis
AAAAAA
poly-A-binding
protein
nuclear pore
complex
AAAAAA
NUCLEUS
CYTOSOL
PROTEIN
EXCHANGE
AAAAAAA
Figure 7–25 A specialized set of RNA-binding proteins signals that a completed mRNA is ready for export
to the cytosol. As indicated on the left, the 5’ cap and poly-A tail of a mature mRNA molecule are “marked” by
proteins that recognize these modifications. Successful splices are marked by exon junction complexes (see Figure
7−22). Once an mRNA is deemed “export ready,” a nuclear transport receptor (discussed in Chapter 15) associates
with the mRNA and guides it through the nuclear pore. In the cytosol, the mRNA can shed some of these proteins
and bind new ones, which, along with poly-A-binding protein, act as initiation factors for protein synthesis, as we
discuss in the next section of the chapter. ECB5 e7.23/7.25
Each mRNA molecule is eventually degraded into nucleotides by ribonucleases
(RNAses) present in the cytosol, but the lifespans of mRNA
molecules differ considerably—depending on the nucleotide sequence of
the mRNA and the type of cell. In bacteria, most mRNAs are degraded
rapidly, having a typical lifespan of about 3 minutes. The mRNAs in eukaryotic
cells usually persist longer: some, such as those encoding β-globin,
have lifespans of more than 10 hours, whereas others stick around for
less than 30 minutes.
These different lifespans are in part controlled by nucleotide sequences
in the mRNA itself, most often in the portion of RNA called the 3ʹ untranslated
region, which lies between the 3ʹ end of the coding sequence and
the poly-A tail (see Figure 7−17). The lifespans of different mRNAs help
the cell control how much protein will be produced. In general, proteins
made in large amounts, such as β-globin, are translated from mRNAs
that have long lifespans, whereas proteins made in smaller amounts, or
whose levels must change rapidly in response to signals, are typically
synthesized from short-lived mRNAs.
The synthesis, processing, and degradation of RNA in eukaryotes and
prokaryotes is summarized and compared in Figure 7−26.
FROM RNA TO PROTEIN
By the end of the 1950s, biologists had demonstrated that the information
encoded in DNA is copied first into RNA and then into protein. The debate
then shifted to the “coding problem”: How is the information in a linear
sequence of nucleotides in an RNA molecule translated into the linear
sequence of a chemically quite different set of subunits—the amino acids
in a protein? This fascinating question intrigued scientists from many different
disciplines, including physics, mathematics, and chemistry. Here
was a cryptogram set up by nature that, after more than 3 billion years
of evolution, could finally be solved by one of the products of evolution—
the human brain! Indeed, scientists have not only cracked the code but
have revealed, in atomic detail, the precise workings of the machinery by
which cells read this code.