Figure 1

Schematic outline of a typical EST manufacture process. After a sample of interest (A) is collected all transcribed copies of expressed genes (mRNAs) are isolated (B). Each mRNA is reverse-transcribed into a complementary DNA (cDNA) (C). Note that cDNA copies may have different lengths due to the polymerase processivity. The copy number of those fragments is increased when they are inserted into a host cell (D). The resulting population of host cells, containing cDNA fragments from the sample of interest is called a cDNA library (E). Typically a few thousand clones are randomly picked from the cDNA library to produce single-pass reads from 3', more rarely from the 5' ends and sometimes from the random location in the middle (F). The resulting collection represents fragments of different lengths starting from the polyadenylation site at 3' end and partially overlapping fragments from 5' end, starting at different points. Although 3' and 5' ESTs rarely overlap, they usually share the same clone ID in the annotation.