Content of genomic versus cDNA libraries

(a) — Genomic '—1—' |_l_| I—i—' DNA Gene 1 Gene 2 Gene 3 (expressed (expressed (expressed in eyes) in wings and in eyes and embryogenesis) embryogenesis) (b) Genomic library Embryogenic Eye cDNA library cDNA library 1 — - 2 - 1 1 (2) w - 2 - 1 2 m - 2 - 1 (2) 3 _ - 2 - 1 3 - - 2 1 - -2 _3 All sequences—genic and intergenic—are represented approximately equally. cDNA clones cDNA clones represent only represent only genes 2 and 3. genes 1 and 3. Frequencies reflect levels of gene expression and include only sequences found in mature mRNA (no intron or intergenic sequence).