The Mercurial Germline Genome of Hypotrichous Ciliates
A hypotrichous ciliate contains both a germline nucleus and a somatic nucleus. The germline nucleus, called the micronucleus, contains chromosomes with very high molecular weight DNA. It divides mitotically and undergoes meiosis at cell mating. Its genes are transcriptionally silent during vegetative growth. The somatic nucleus, called the macronucleus, contains only gene-sized DNA molecules ranging in size from ~400 bp to ~15,000 bp with an average of 2200 bp in Oxytricha species. Each different sized molecule is present in the macronucleus in ~1000 copies. There are 24,000 different sized molecules, or a total of 24 × 106 molecules per macronucleus (Lauth et al., 1976). A gene-sized molecule contains a single transcription unit. The ~66 gene-sized, macronuclear molecules characterized for hypotrichs so far consist of a 5 ′ DNA leader, a transcription unit or open reading frame (ORF) for protein-encoding molecules, and a 3′ DNA trailer (Hoffman et al., 1995. All the molecules possess terminal repeats of 3′ dG4T4 with a 14-base or 16-base 3′ tail of the repeat sequence.
KeywordsPolytene Chromosome Ciliated Protozoan Somatic Nucleus Repeat Pair Nova Gene
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