Many single-celled eukaryotes have strangely arranged genomes, and in addition, may carry out peculiar forms of DNA rearrangement or RNA processing. Most of these primitive eukaryotes are still poorly characterized at the molecular level. For practical reasons, the best-investigated protozoans are mostly disease-causing parasites, and it is possible that many of their genetic peculiarities are due to the relaxed constraints of a parasitic lifestyle. However, the ciliates are free-living protozoans, so we will consider their peculiarities first.
The ciliates are a large and widespread group of protozoans that are named after their locomotory organelles, the cilia that cover the cell surface. Perhaps the best known of the ciliates are Paramecium, Tetrahymena and Euplotes. These single-celled
Buchnera Genus of gram-negative bacterial symbionts found in insects that supply their host insect with essential amino acids ciliates Group of free-living protozoans that move by means of cilia attached to the cell surface essential amino acids Those amino acids that animals are unable to synthesize for themselves (Arg, Val, Ile, Leu, Phe, Trp, Thr, Met, Lys, His)
FIGURE 19.08 Amino Acid Synthesis in Aphids and Buchnera
Buchnera live inside aphids and manufacture the amino acids (blue) that aphids cannot manufacture themselves. In return, the aphid provides amino acids that it does manufacture (purple), thus establishing a mutualistic symbiotic relationship.
eukaryotes all contain two types of nucleus, a macronucleus and a micronucleus within the same cell. The numbers of these nuclei vary from one ciliate to another. Paramecium has a single macronucleus and two or more micronuclei, depending on the species. These micronuclei are all diploid and identical because they were derived by mitotic division of the single original micronucleus.
The micronucleus may be regarded as the germline and is used for the sexual exchange of DNA. The genes in the micronucleus are silent and not expressed, whereas, the genes of the macronucleus are transcribed to give mRNA. The macronucleus is thus the somatic component and is polyploid. It may contain from 50 to 1000 copies of the genome (depending on the species) and the copy number may vary depending on environmental conditions. When two cells mate, they exchange haploid micronuclei generated by meiosis. After mating, the haploid micronuclei fuse to give a diploid micronucleus, which then divides. Finally, one of the micronuclei is converted into a new macronucleus and the old macronucleus is disassembled (Fig. 19.09).
Micronucleus DNA must be processed in order for the micronucleus to convert into the macronucleus. The DNA of the micronucleus contains many extra sequences that interrupt the germline genes, called internal eliminated segments (IESs). Most IES are less than 100 bp although they may range from 5 to 900 bp in length, and they are very AT rich (75-100% AT). After removal of the IESs the remaining macronucleus-destined segments (MDSs) are spliced to form uninterrupted genes that can be expressed (Fig. 19.10). There may be from 5 to 50 IESs interrupting a single gene. Each internal eliminated segment (IES) Extra sequences in the DNA of the ciliate micronucleus that are eliminated during conversion of a micronucleus to a macronucleus macronucleus Large somatic nucleus of ciliates that contains multiple copies of genes that are expressed macronucleus-destined segment (MDS) Segments of DNA that remain during conversion of a ciliate micronucleus to macronucleus and are spliced to form uninterrupted genes that can be expressed micronucleus Small germline nucleus of ciliates whose genes are not expressed
Essential amino acids to insect
Essential amino acids to insect
Genes capable of being expressed are assembled by deleting internal sequences during formation of the macronucleus.
Meiosis of (diploid)
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