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Figure 19.21 Diagram of ld excision of IES 2 in the β TP gene in Figure 9.20. The
molecule folds in such a way that pointers P3 become aligned. Recombination be-
tween pointers P3 excises the internal eliminated segment (IES) together with a hybrid
copy of P3 and ligates macronuclear destined segment (MDSs) 2 and 3 through a
hybrid copy of P3.
two loops so that the two copies of P2 are aligned in one loop, and the two
copies of P4 are aligned in the other loop. Thus, two recombinations take place:
recombination between the two copies of P2 joins MDS 1 to MDS 2 into a
composite MDS through a hybrid copy of P2, and recombination between the
two copies of P4 joins MDS 3 to MDS 4 into another composite MDS through
a hybrid copy of P4. As a result of these two recombinations, IESs 3, 2, and 1
are joined together by the other hybrid copies of P2 and P4. This composite IES
is now interposed between MDSs 2 and 3 and may be removed by a subsequent
operation of ld excision.
The three molecular operations presented above are intramolecular —in each
of these operations a molecule reacts with (folds on) itself (i.e., it folds on itself,
and recombination takes place between some segments within the molecule).
The gene assembly process using the three operations discussed above is pre-
sented in detail elsewhere [2, 3, 10, 13], including the computational properties
of the process. The computational aspects of gene assembly are also discussed
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