Cellular Structure, Proteins, and Metabolism
61
tide may be split at different points in different cells depend-
ing on the specifi city of the hydrolyzing enzymes present.
Carbohydrates and lipid derivatives are often covalently
linked to particular amino acid side chains. These additions
may protect the protein from rapid degradation by proteolytic
enzymes or act as signals to direct the protein to those loca-
tions in the cell where it is to function. The addition of a fatty
acid to a protein, for example, can lead the protein to anchor
to a membrane as the nonpolar portion of the fatty acid inserts
into the lipid bilayer.
The steps leading from DNA to a functional protein are
summarized in
Table 3–2
.
Although 99 percent of eukaryotic DNA is located
in the nucleus, a small amount is present in mitochondria.
Mitochondrial DNA, like bacterial DNA, does not contain
introns and is circular. These characteristics support the
hypothesis that mitochondria arose during an early stage of
evolution when an anaerobic cell ingested an aerobic bacte-
rium that ultimately led to what we know today as mito-
chondria. Mitochondria also have the machinery, including
ribosomes, for protein synthesis. However, the mitochondrial
DNA contains the genes for only 13 mitochondrial proteins
and a few of the rRNA and tRNA genes. Therefore, additional
components are required for mitochondrial protein synthesis,
and most of the mitochondrial proteins are coded by nuclear
DNA genes. These components are synthesized in the cyto-
plasm and then transported into the mitochondria.
Regulation of Protein Synthesis
As noted earlier, in any given cell only a small fraction of the
genes in the human genome are ever transcribed into mRNA
and translated into proteins. Of this fraction, a small number of
genes are continuously being transcribed into mRNA. The tran-
scription of other genes, however, is regulated and can be turned
on or off in response either to signals generated within the cell or
to external signals the cell receives. In order for a gene to be tran-
scribed, RNA polymerase must be able to bind to the promoter
region of the gene and be in an activated confi
guration.
Transcription of most genes is regulated by a class of
proteins known as
transcription factors,
which act as gene
switches, interacting in a variety of ways to activate or repress
the initiation process that takes place at the promoter region
of a particular gene. The infl uence of a transcription factor on
transcription is not necessarily all or none, on or off; it may
simply slow or speed up the initiation of the transcription
Ribosome
mRNA
Translation of mRNA
into single protein
Protein 1
ab
Protein 2
b
Protein 3
Posttranslational
splitting of protein 1
Posttranslational
splitting of protein 3
Protein 4
Protein 5
b
a
c
c
c
Figure 3–23
Posttranslational splitting of a protein can result in several smaller
proteins, each of which may perform a different function. All these
proteins are derived from the same gene.
Table 3–2
Events Leading from DNA to Protein
Synthesis
Transcription
1. RNA polymerase binds to the promoter region of a gene
and separates the two strands of the DNA double helix in
the region of the gene to be transcribed.
2. Free ribonucleotide triphosphates base-pair with the
deoxynucleotides in the template strand of DNA.
3. The ribonucleotides paired with this strand of DNA
are linked by RNA polymerase to form a primary RNA
transcript containing a sequence of bases complementary to
the template strand of the DNA base sequence.
4. RNA splicing removes the intron-derived regions in
the primary RNA transcript, which contain noncoding
sequences, and splices together the exon-derived regions,
which code for specifi c amino acids, producing a molecule
of mRNA.
Translation
5. The mRNA passes from the nucleus to the cytoplasm,
where one end of the mRNA binds to the small subunit of
a ribosome.
6. Free amino acids are linked to their corresponding tRNAs
by aminoacyl-tRNA synthetase.
7. The three-base anticodon in an amino acid–tRNA complex
pairs with its corresponding codon in the region of the
mRNA bound to the ribosome.
8. The amino acid on the tRNA is linked by a peptide bond
to the end of the growing polypeptide chain.
9. The tRNA that has been freed of its amino acid is released
from the ribosome.
10. The ribosome moves one codon step along mRNA.
11. Step 7 to 10 are repeated until a termination sequence is
reached, and the completed protein is released from the
ribosome.
12. In some cases, the protein undergoes posttranslational
processing in which various chemical groups are attached
to specifi c side chains and/or the protein is split into several
smaller peptide chains.
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