Expressed Sequence Tags (ESTs)

    In vivo, exon regions of the eukaryotic genome are expressed as transcribed heterogeneous nuclear RNAs (hnRNAs) that are spliced together to form mRNAsIn vitro , reversal of this process by reverse transcription of the mRNAs present in any tissue sample 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 gives enough information to identify the gene.  These short sequences ("tags") are called expressed sequence tags (ESTs).  Comparison of the ESTs  from a lesser-known species with those from a species whose genome is well-characterized allows identification of the function of gene loci in the new organism. Development of a complete EST library is an important step in mapping of a novel genome.

Figure modified after Griffiths et al. 2002; text material ©2014 by Steven M. Carr