Download ONE GENE, MANY PROTEINS

ONE GENE, MANY PROTEINS
The classical view of gene expression was simple and
consistent: a gene’s DNA sequence is first transcribed by
cellular machinery into an RNA version, then that primary
transcript is edited to remove “junk” stretches called introns,
and only the information-bearing exons are included in
the final messenger-RNA that is translated into a protein.
As it turns out, these rules do not always apply. The genes
of complex organisms can have their exons and introns
mixed and matched, discarded and retained, in alternative
combinations during editing, permitting one gene to
encode a variety of different proteins, depending on how its
information is edited.
DNA gene
CLASSIC GENE EXPRESSION
A DNA sequence is transcribed into
a single-strand copy made of RNA.
Cellular splicing machinery then
edits this primary transcript to
remove introns, each of which is
defined by distinctive nucleotide
sequences at its beginning and end,
known, respectively, as the 5 (“fiveprime”) and 3 (“three-prime”) splice
sites. The exons are then spliced
into a messenger-RNA version of the
original gene that will be translated
into a protein by the cell.
Exon
Intron
Transcription
5’
3’
Primary RNA transcript
Splicing
Intronic RNA
Messenger RNA
Translation
Discarded
Protein
ALTERNATIVE SPLICING
A gene’s primary transcript can
be alternatively edited in several
different ways, shown at right,
where splicing activity is indicated
by dashed lines. An exon may be
left out (a). Splicing machinery may
recognize alternative 5 ’ splice sites
for an intron (b), or alternative 3 ’
splice sites (c). An intron may be
retained in the final mRNA transcript
(d). Exons may be retained on a
mutually exclusive basis (e).
a) EXON SKIPPING
RESULTING mRNA
b) ALTERNATIVE 5 ’ SPLICE SITES
c) ALTERNATIVE 3’ SPLICE SITES
d) INTRON RETENTION
Exon always spliced in
Exon alternatively spliced
e) MUTUALLY EXCLUSIVE
CREDIT
Intron
w w w. s c ia m . c o m
SCIENTIFIC A MERIC A N
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