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Figure 9.14
Model to explain the origin of an internal eliminated segment (IES) by
insertion at a staggered cut (1) in micronuclear DNA of a short segment of AT-rich
DNA (2). After insertion, filling the single-stranded gaps by complementary bases
creates a pair of repeats in the flanking macronuclear destined segment (MDS) (3).
Segments of spacer DNA probably flood the cell transiently during macronu-
clear development when the polytene chromosomes are destroyed. A piece
of blunt-ended spacer DNA is hypothesized to insert at staggered cuts in the
double-stranded micronuclear DNA of a gene as shown in Figure 9.14. (Stag-
gered cutting of DNA is a common phenomenon in molecular genetics.) After
insertion, the single-stranded regions are filled in by DNA repair enzymes,
creating a pair of direct repeats.
IES Migration
Because they are noncoding, mutations in IESs do not affect the organism. Thus,
IESs are free to accumulate mutations, and they do so at a rate many times higher
than do coding regions. These mutations include both changes in nucleotide
sequence and shortening and lengthening of IESs. Mutations that change single
bases are related to a particularly interesting behavior of IESs: IESs change their
positions by migrating along a DNA molecule by the mutational mechanism
illustrated by the IES in Figure 9.15.
1. In Figure 9.15, a mutation changes the first base in the IES from a to
G (a is written in lowercase because it was a part of the IES). This
