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In order to make a "full-length" cDNA library, we
have to device some types of selection procedure for the full-length
cDNA. In order to select the full-length cDNA, the cDNA which contain
both ends of the mRNA should be selected. For that purpose,
the features which are characteristic to the 3 '-end and the 5'-end
of mRNA should be used as "tags". The full-length cDNA could be
selected through the selection steps for both the 3'-end and the
5 '-end "tags".
The polyA stretch is a characteristic feature of the 3'-end
of mRNA. Conventional methods have been using the polyA as a "sequence
tag" to select the 3'-end of mRNA. According to the conventional
methods, the first strand cDNA is usually synthesized from the oligo
dT primer. Because dT primer mostly hybridize at the polyA, most
of the cDNA is selectively synthesized from the 3'-end of the mRNA.
Thus, the conventional methods include the selection step for the
3'-end "tag" of the mRNA. On the contrary, they include no step
to select the 5'-end of mRNA. As a result, the largest part of the
cDNA library is occupied by the cDNA which lack the 5'-end of the
mRNA.
The main reason for this lies, in our view, in the fact that mRNA
does not originally have a "sequence tag" at the 5'-end. The 5'-end
of mRNA also has a characteristic structure, called the cap structure,
but unfortunately it is not a "sequence tag". Unlike the polyA at
the 3'-end, it cannot be used for the hybridization. If the
5'-end "tag" of the mRNA were also a "sequence tag", it would be
easy to use it to select the 5'-end of mRNA.
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