Information Technology Reference
In-Depth Information
Figure 9.7 Electron micrograph of several
polytene chromosomes isolated from a devel-
oping macronucleus of a stichotrich. Bar =
10 µ m. Courtesy of Dr. K. G. Murti.
Transection of DNA has been confirmed by DNA purification at this stage;
the very long micronuclear DNA molecules in the polytene chromosomes have
been cut into shorter molecules, but they are still many times longer than the
gene-sized molecules of a mature macronucleus. After transecting the inter-
bands, the proteinaceous septa expand around each band, forming a string of
vesicles (Figure 9.9).
If the developing macronucleus is broken open at this stage, the vesicles
immediately disperse, each vesicle containing 64 copies of a short segment of
the micronuclear DNA molecules. Most of each of these segments in a vesicle
consists of spacer DNA, which is now destroyed, leaving short, gene-encoding
molecules. Next, the vesicle walls dissolve, and the short DNA molecules go
through (in Sterkiella histriomuscorum , for example) five rounds of replica-
tion, yielding a fully developed macronucleus with approximately 2000 copies
Figure 9.8 Electron micrograph of
a section through a short segment of
two polytene chromosomes showing
transecting septa through interbands.
Bar = 1 µ m. Courtesy of Dr. K. G.
Murti.
Search WWH ::




Custom Search