Ga C Gtcttcgaaggc Ctaggggcc Cttaa

Synthetic polylinker or a similar cofactor. The base-pairing of complementary sticky ends greatly facilitates the ligation reaction (Fig. 9-3a). Blunt ends can also be ligated, albeit less efficiently. Researchers can create new DNA sequences by inserting synthetic DNA fragments (called linkers) between the ends that are being ligated. Inserted DNA fragments with multiple recognition sequences for restriction endonucleases (often useful later as points for inserting additional DNA by cleavage and ligation) are called polylinkers (Fig. 9-3c).

The effectiveness of sticky ends in selectively joining two DNA fragments was apparent in the earliest recombinant DNA experiments. Before restriction endo-nucleases were widely available, some workers found they could generate sticky ends by the combined action of the bacteriophage À exonuclease and terminal transferase (Table 9-1). The fragments to be joined were given complementary homopolymeric tails. Peter Lobban and Dale Kaiser used this method in 1971 in the first experiments to join naturally occurring DNA fragments.

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