Information Technology Reference
In-Depth Information
Figure 9.13
Diagram of alignment of the repeats in Figure 9.12 to create a loop of the
internal eliminated segment (IES), which is removed by recombination.
switch configuration so that the IES, with one copy of the repeat, is removed as
a circular molecule, and the two MDSs are joined through a copy of the repeat.
Pairs of repeats are present without exception at the ends of MDSs that are
destined to be joined. These pairs of repeat sequences flanking different IESs
differ in length, ranging from 3 to 20 bp, and can be any sequence of the bases
A, T, G, and C. Thus, to join the seven MDSs in the
TP gene and to clear it of
IESs, six separate loops must be formed in the DNA of this micronuclear gene
by aligning six pairs of repeat sequences.
β
ORIGIN OF IESs
To try to understand the significance of IESs, we have addressed questions
of their origin and behavior during evolution. A proposed mechanism for the
creation of IESs from spacer DNA is shown in Figure 9.14. The DNA of IESs
consists of 75% to 100% A
T nucleotides. This is the same as the nucleotide
composition of spacer DNA, but contrasts with the 48% to 68% A
+
+
T nucleo-
tides in the coding region of stichotrich genes.
