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Figure 9.23
The dlad excision operation unscrambles macronuclear destined
segments (MDSs) in which a segment of DNA encompassed by one pair of point-
ers (in this case P2) overlaps with a DNA segment encompassed by another pair of
pointers (P4). The molecule folds into a double loop that allows pointer pairs P2 and
P4 to align. Recombination between pointers joins MDS 1 to MDS 2 and MDS 3 to
MDS 4. A subsequent ld excision operation removes the composite internal elimi-
nated segment (IES) 3-2-1 and joins MDS 2 with MDS 3.
orthodox order. The order of MDSs in the micronuclear actin I gene in
Sterkiella
nova
is 3-4-6-5-7-9-
2
-1-8. Table 9.1 gives the incoming and outgoing pointers
for each of the MDSs, which are listed in their order in the scrambled gene.
The pointers shown in Table 9.1 identify how MDSs are to be joined in the
actin I gene of
Sterkiella nova
. Thus, for example, MDS 7 is separated from
MDS 8 by MDSs 9-
2
-1 and four IESs. The right end (the 3' end) of MDS 7
contains the outgoing pointer P8, CTTTGGGTTGA, and the left end of MDS
8 (the 5' end) to which it must join contains the incoming pointer, P8. The
following succession of operations will assemble the micronuclear actin I gene
