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Cellular Computing$
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Martyn Amos

Print publication date: 2004

Print ISBN-13: 9780195155396

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195155396.001.0001

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Encrypted Genes and Their Assembly in Ciliates

Encrypted Genes and Their Assembly in Ciliates

Chapter:
9 Encrypted Genes and Their Assembly in Ciliates
Source:
Cellular Computing
Author(s):

David M. Prescott

Grzegorz Rozenberg

Publisher:
Oxford University Press
DOI:10.1093/oso/9780195155396.003.0015

Maintenance of normal cell function and structure requires some level of stability 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 between 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. The ciliate strategy rests on the evolution of nuclear dimorphism: the inclusion both of a germline nucleus (micronucleus) and a somatic nucleus (macronucleus) in the same cell (Figure 9.1; for a general review, see Prescott [6, 7]). Like the example in Figure 9.1, most stichotrich species contain two or more micronuclei and two or more macronuclei per cell. The multiple micronuclei are genetically identical to each other, and the multiple macronuclei are genetically identical; these multiplicities of nuclei have no bearing on the issues addressed in this chapter. The micronucleus is used only in cell mating, and its genes are silent. Hence, micronuclear genes do not support the maintenance, growth, or division of the cell.

Keywords:   Actin I, Codon, Enzyme, Homologous pairing, Intron, Ligation, Macronucleus, Nuclear dimorphism, Phylogeny, Repeat sequence

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