In vivo,
the exon regions of eukaryotic genes are transcribed as
part of the heterogeneous
nuclear RNAs (hnRNAs) and are
subsequently spliced together to
form mRNAs. In vitro,
the
process is reversed by reverse transcription of
the complete set of mRNAs present
in any tissue. This yields a library of complementary DNAs (cDNAs)
that correspond to the genes expressed in that sample.
Sequencing a short segment (<100 bases) at one or
both ends of each cDNA produces
a "tag" with enough information to identify the expressed
gene. These expressed sequence tags (ESTs)from a novel organism or
tissue type can be compared with sequences from a
well-characterized genome. This allows identification of the
function of gene loci expressed in the new organism.
