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Figure 9.16 A model explaining how two internal eliminated segment (IES) recom-
binations change the orthodox order of macronuclear destined segment (MDS) 4-5-
6-7 into the scrambled disorder, MDS 4-6-5-7 in the actin I gene of Sterkiella nova .
(a) MDSs 4, 5, 6, and 7 in the orthodox order. (b) Folded micronuclear DNA and
recombinations between IESs. (c) Scrambled arrangement of MDS created in b.
of the two recombinations, the orthodox MDS order, 4-5-6-7, has changed to
the scrambled order, 4-6-5-7, which is the order observed for these four MDSs
in the micronuclear actin I gene of Sterkiella nova (Figure 9.16c). Additional
IES recombinations create the scrambled arrangement, MDSs 7-9-2-1-8. IES 1
(between MDSs 3 and 4) does not participate in recombination, so that the final
scrambled arrangement is 3-4-6-5-7-9- 2 -1-8. MDS 2 in the micronuclear actin
I gene of Sterkiella nova is a particular case of MDS scrambling because it is
inverted, as indicated by the arrow over the 2. By convention the sequence of
bases in a macronuclear gene is read from left to right in the 5' to 3' direction.
This applies as well to the MDS in a nonscrambled micronuclear gene such as
the seven MDSs in the
TP gene in Figure 9.11. In the actin I gene, eight of
the nine MDSs have maintained their polarity (orientation); they are still read
from left to right (5' to 3') in spite of being scrambled. MDS 2 is in the reversed
(inverted) polarity (3' to 5') as a result of recombination between the two IESs
that flank it. How MDS 2 became inverted is postulated in Figure 9.17. It is
assumed that the micronuclear actin I gene first became scrambled into the
MDS arrangement, 3-4-6-5-7-9-2-1-8, with MDS 2 in the same (noninverted)
polarity as the other eight MDSs (Figure 9.17a). Folding the molecule into a
β
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