Hypothetical explanations for the difference in frequency of poly A/T tracts within the two libraries. The black boxes represent the 3' UTR of hypothetical transcripts. On the right are the calculated percentages of ESTs containing a polyA or polyT tract that would result from the creation of ESTs from the hypothetical mRNA shown. a: Differential digestion of the initial cDNAs. ESTs resulted from the amplification of cDNA fragments that have been digested by RsaI, each RsaI fragment having an equal chance of being amplified and cloned. If the adult cDNAs were more completely digested than the juvenile cDNAs, the adult cDNAs generated a higher number of ESTs (4 instead of 3 in this case), a lower proportion of which would contain polyA tracts. b and c: Differential mRNA length in vivo. Adult cDNAs were, on average, longer than juvenile cDNAs, so each adult cDNA produced more ESTs (4 instead of 3), yielding a lower proportion of ESTs containing the polyA tract. Adult cDNAs could be longer because they have, on average, longer coding sequences (b) or longer 3'UTRs (c).