Information Technology Reference
In-Depth Information
β
Figure 19.20 Diagram of the micronuclear
TP gene in Sterkiella nova . Pointers at
the ends of macronuclear destined elements (MDSs) are designated by P. An MDS has
an outgoing pointer and an incoming pointer except terminal MDSs (i.e., MDSs 1 and
4). MDS 1 begins with b for “beginning,” and MDS 4 ends with e for “end.” The b
and e are sites to which telomere sequences are added when the gene is finally excised
from its chromosome.
The first of the three operations by which IESs are removed and MDSs
are ligated we call loop, direct repeat excision ,or ld excision for short. This
operation deals with any two MDSs that are in the orthodox order such as any
two MDSs in the micronuclear
TP gene or MDSs 3 and 4 in the actin gene of
Sterkiella nova (Figure 9.19). An ld excision applied to IES 2 in the
β
β
TP gene
in Figure 9.20 is illustrated in Figure 9.21.
The second operation, called hairpin, inverted repeat-excision ,or hi excision ,
applies to MDSs that are inverted, and therefore the two pointers in a pair are
inverted relative to one another. For example, assume that the order of MDSs
in a micronuclear gene is 1-2- 3 -4 (the gene is shown in Figure 9.22). Because
MDS 3 is inverted, its incoming pointer P3 and its outgoing pointer P4 are
also necessarily inverted. Folding of the molecule into a hairpin in Figure 9.22
reinverts MDS 3 and allows the P3 pointer in MDS 2 to align with the P3 pointer
in MDS 3. Recombination between the two P3 pointers then joins MDSs 2 and
3 in a composite MDS with orthodox polarity throughout and shifts the inverted
IES 2 to a position adjacent to the recombined composite MDS. MDS 3 was
reinverted above by only one application of hi excision. This was possible
because its partner for applying hi excision, MDS 2, is separated from it only
by an IES. More commonly, two MDSs that engage in hi excision are separated
by more than just an IES. Therefore, more than one application of hi excision
may be required interspersed by applications of other operations.
The third operation is called double loop, alternating direct repeat-excision ,
or dlad excision . It applies to a molecule with two pairs of pointers in which
the segment of a DNA molecule encompassed by one pair of pointers overlaps
with the segment enclosed by the second pair of pointers.
This is illustrated in Figure 9.23, in which the DNA segment encompassed
by the two copies of P4 overlaps with the segment encompassed by the two
copies of P2 (i.e., one of the copies of P2 resides between the two copies of
P4). To resolve this scrambled state (MDS 1-3-2-4), the molecule folds into
Search WWH ::




Custom Search