Prescott D M
Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder 80309-0347.
Microbiol Rev. 1994 Jun;58(2):233-67. doi: 10.1128/mr.58.2.233-267.1994.
Ciliates contain two types of nuclei: a micronucleus and a macronucleus. The micronucleus serves as the germ line nucleus but does not express its genes. The macronucleus provides the nuclear RNA for vegetative growth. Mating cells exchange haploid micronuclei, and a new macronucleus develops from a new diploid micronucleus. The old macronucleus is destroyed. This conversion consists of amplification, elimination, fragmentation, and splicing of DNA sequences on a massive scale. Fragmentation produces subchromosomal molecules in Tetrahymena and Paramecium cells and much smaller, gene-sized molecules in hypotrichous ciliates to which telomere sequences are added. These molecules are then amplified, some to higher copy numbers than others. rDNA is differentially amplified to thousands of copies per macronucleus. Eliminated sequences include transposonlike elements and sequences called internal eliminated sequences that interrupt gene coding regions in the micronuclear genome. Some, perhaps all, of these are excised as circular molecules and destroyed. In at least some hypotrichs, segments of some micronuclear genes are scrambled in a nonfunctional order and are recorded during macronuclear development. Vegetatively growing ciliates appear to possess a mechanism for adjusting copy numbers of individual genes, which corrects gene imbalances resulting from random distribution of DNA molecules during amitosis of the macronucleus. Other distinctive features of ciliate DNA include an altered use of the conventional stop codons.
微核和大核。微核作为种系细胞核，但不表达其基因。大核为营养生长提供核RNA。交配细胞交换单倍体微核，新的大核从新的二倍体微核发育而来。旧的大核被破坏。这种转变包括DNA序列的大规模扩增、消除、片段化和剪接。片段化在四膜虫和草履虫细胞中产生亚染色体分子，在具毛类纤毛虫中产生小得多的、基因大小的分子，并添加端粒序列。然后这些分子被扩增，有些扩增到比其他分子更高的拷贝数。核糖体DNA被差异扩增到每个大核数千个拷贝。被消除的序列包括转座子样元件和称为内部消除序列的序列，这些序列中断微核基因组中的基因编码区。其中一些，也许全部，被切除为环状分子并被破坏。在至少一些具毛类中，一些微核基因的片段以无功能的顺序排列混乱，并在大核发育过程中被记录下来。营养生长的纤毛虫似乎拥有一种调节单个基因拷贝数的机制，该机制可纠正大核无丝分裂期间DNA分子随机分布导致的基因失衡。纤毛虫DNA的其他独特特征包括对传统终止密码子的不同使用。
