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Chapter 1 Anatomy and Physiology of the Gene 11

rate-limiting components of the translational apparatus, but mRNAs
that are inherently less translatable tend to be translated less efficiently dsRNA
in the face of limited access to other translational components. For
example, the translation factor eIF-4 tends to be produced in higher
amounts when cells encounter transforming or mitogenic events. Dicer
This causes an increase in overall rates of protein synthesis but also
leads to a selective increase in the synthesis of some proteins that were
underproduced before mitogenesis.
Translational regulation of individual mRNA species is critical for
some events important to blood cell homeostasis. For example, as
discussed in Chapter 35, the amount of iron entering a cell is an
exquisite regulator of the rate of ferritin mRNA translation. An
mRNA sequence called the iron response element is recognized by a 21-23 nt siRNA
specific mRNA-binding protein but only when the protein lacks iron.
mRNA bound to the protein is translationally inactive. As iron
accumulates in the cell, the protein becomes iron bound and loses its RISC
affinity for the mRNA, resulting in translation into apoferritin
molecules that bind the iron.
Tubulin synthesis involves coordinated regulation of translation RISC
and mRNA stability. Tubulin regulates the stability of its own mRNA
by a feedback loop. As tubulin concentrations rise in the cell, it
interacts with its own mRNA through the intermediary of an mRNA- AAAA
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binding protein. This results in the formation of an mRNA–protein m G (n)
complex and nucleolytic cleavage of the mRNA. The mRNA is
destroyed, and further tubulin production is halted. m G AAAA
These few examples of posttranscriptional regulation emphasize 7 (n)
that cells tend to use every step in the complex pathway of gene
expression as points at which exquisite control over the amounts of
a particular protein can be regulated. In other chapters, additional
levels of regulation are described (e.g., regulation of the stability, Fig. 1.6 mRNA DEGRADATION BY siRNA. dsRNA is digested into
activity, localization, and access to other cellular components of the 21- to 23-bp siRNAs by the Dicer RNase. These RNA fragments are unwound
proteins that are present in a cell). by RISC and bring the endonucleolytic activity of RISC to mRNA transcripts
in a sequence-specific manner, leading to degradation of the mRNA. dsRNA,
Double-stranded RNA; mRNA, messenger RNA; RISC, RNA-induced silenc-
SMALL INTERFERING RNA AND MICRO RNA ing complex; siRNA, small interfering RNA.
Another posttranscriptional mechanism of gene silencing utilizes
so-called “small RNAs”. One such process is carried out by small miRNA suppresses gene expression, likely either through inhibition
interfering RNAs (siRNAs): short, double-stranded fragments of of protein translation or through destabilization of mRNA. miRNAs
RNA containing 21 to 23 bp (Fig. 1.6). The process is triggered by appear to have essential roles in development and differentiation, and
perfectly complementary double-stranded RNA, which is cleaved by are aberrantly regulated in many types of cancer cells. The identifica-
Dicer, a member of the RNase III family, into siRNA fragments. tion of miRNA sequences, their regulation, and their target genes are
These small fragments of double-stranded RNA are unwound by a areas of intense study.
helicase in the RNA-induced silencing complex (RISC). The anti-
sense strand anneals to mRNA transcripts in a sequence-specific
manner and in doing so brings the endonuclease activity within the ADDITIONAL STRUCTURAL FEATURES OF GENOMIC DNA
RISC to the targeted transcript. An RNA-dependent RNA poly-
merase in the RISC may then create new siRNAs to processively Most DNA does not code for RNA or protein molecules. The vast
degrade the mRNA, ultimately leading to complete degradation of majority of nucleotides present in the human genome reside outside
the mRNA transcript and abrogation of protein expression. structural genes. Structural genes are separated from one another by
Although this endogenous process likely evolved to destroy invad- as few as 1 to 5 kilobases or as many as several thousand kilobases of
ing viral RNA, the use of siRNA has become a commonly used tool DNA. Almost nothing is known about the reason for the erratic
for evaluation of gene function. Sequence-specific synthetic siRNA clustering and spacing of genes along chromosomes. It is clear that
may be directly introduced into cells or introduced via gene transfec- intergenic DNA contains a variegated landscape of structural features
tion methods and targeted to an mRNA of a gene of interest. The that provide useful tools to localize genes, identify individual human
siRNA will lead to degradation of the mRNA transcript, and accord- beings as unique from every other human being (DNA fingerprint-
ingly prevent new protein translation. This technique is a relatively ing), and diagnose human diseases by linkage. Only a brief introduc-
simple, efficient, and inexpensive means to investigate cellular phe- tion is provided here.
notypes after directed elimination of expression of a single gene. The The rate of mutation in DNA under normal circumstances is
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2006 Nobel Prize in Physiology or Medicine was awarded to two approximately 1/10 . In other words, one of 1 million bases of DNA
discoverers of RNA interference, Andrew Fire and Craig Mello. will be mutated during each round of DNA replication. A set of
Micro RNAs (miRNAs, or MIRs) are 22-nucleotide small RNAs enzymes called DNA proofreading enzymes corrects many but not all
encoded by the cellular genome that alter mRNA stability and protein of these mutations. When these enzymes are themselves altered by
translation. These genes are transcribed by RNA polymerase II and mutation, the rate of mutation (and therefore the odds of neoplastic
capped and polyadenylated similar to other RNA polymerase II transformation) increases considerably. If these mutations occur in
transcripts. The precursor transcript of approximately 70 nucleotides bases critical to the structure or function of a protein or gene, altered
is cleaved into mature miRNA by the enzymes Drosha and Dicer. function, disease, or a lethal condition can result. Most pathologic
One strand of the resulting duplex forms a complex with the RISC mutations tend not to be preserved throughout many generations
that together binds the target mRNA with imperfect complementar- because of their unfavorable phenotypes. Exceptions, such as the
ity. Through mechanisms that are still incompletely understood, hemoglobinopathies, occur when the heterozygous state for these