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Figure 9.10
Diagram of the generalized structure of a macronuclear DNA molecule.
of replications so that the appropriate gene copy number is achieved. We do
know that as each gene segment is released from chromosomal DNA, a repeat
sequence of 36 nucleotides consisting of 3'GGGGTTTTGGGGTTTT 5', and so
on, called a
telomeric repeat sequence
, is enzymatically added to the 3' end of
both DNA chains. A complementary sequence of 20 bases, 5'CCCCAAAACC-
CCAAAACCCC 3' is added to the 5' end of both chains, producing a telomeric
structure with a 16-base single-stranded overhang, as shown in Figure 9.10.
Special proteins known as
telomere binding proteins
bind to the single-stranded
overhang, protecting the ends of the gene segments from attack by nucleases.
The elimination of the micronuclear spacer DNA removes the burden for the
macronucleus of maintaining and replicating this large mass of nongenetic DNA
that apparently makes no functional contribution to the cell. It permits the cell
to carry an average of 1000 functional copies (in
Sterkiella histriomuscorum
)of
each gene without creating an impractically large nucleus. Thus, the macronu-
cleus represents the evolution of a nucleus with a gene density greater than 80%
of the DNA and maximally streamlined for gene function, which means greatly
increased RNA transcription. Increased transcription supports a higher rate of
protein synthesis and of cell operations in general, which, in turn, promotes a
faster rate of cell reproduction. Stichotrichs undergo cell division every 6-8 h
when food organisms are plentiful. This is a high rate of reproduction for such
very large cells. The strategy of a streamlined macronucleus likely accounts, at
least in part, for the great success of stichotrichs in their ubiquitous and often
dominant occupation of fresh and salt water and of soils throughout the world.
We have described phenomena surrounding the assembly of macronuclear
genes from their encrypted micronuclear form. In the rest of this chapter we
consider in more detail various aspects of the encryption process, the evolution
of encryption, and the mechanism of gene assembly.
MICRONUCLEAR GENES ARE IN AN ENCRYPTED FORM
As striking a phenomenon as the creation of a genome with a very high gene
density and with physically separate, macronuclear genes may be, it is not the
most astonishing process that occurs during the conversion of a micronuclear
genome to a macronuclear genome in stichotrichs.
