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In-Depth Information
9
Assembly in Ciliates
David M. Prescott and Grzegorz Rozenberg
Maintenance of normal cell function and structure requires some level of sta-
bility of the cell's DNA—at least the DNA that makes up the genes of the cell.
In most eukaryotes most of the DNA in the genome does not encode genes
and has no known function beyond forming long spacers between successive
genes. For example, the gene density in the germline (micronuclear) genome
of stichotrich ciliates (formerly referred to as hypotrich ciliates) is very low;
only a few percent of the DNA encodes the approximately 27,000 different
genes, and more than 95% is spacer DNA. Powerful DNA repair systems guard
the stability both of nongene and gene DNA in contemporary cells, protecting
it against mutagenesis. Although species survival depends on DNA stability,
cell evolution requires changes in DNA. Presumably, there is a balance be-
tween instability of DNA that allows evolution and a stability that protects
species from mutational extinction. Could cells evolve strategies that change
the balance, allowing a greater rate of DNA change (gene evolution) without
jeopardizing species survival? The stichotrichs may, in fact, have evolved such
a mechanism, dramatically modifying their germline DNA during evolution
to facilitate creation of new genes without reducing the level of cell survival.
The modifications of germline DNA in ciliates, in turn, require dramatic DNA
processing to convert germline DNA into somatic DNA during the life cycle
of the organisms.
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