Biochemical Genetics 6:293-313 (1972) Genome Amplification and Gene Expression in

The focus of this review is on the micronucleus and macronucleus in the ciliated protozoa nd the organization and function of the DNA molecules within them. We present (1) some of the structural and functional differences which are known, (2) the genetic evidence for macronuclear units, (3) two hypotheses for the organization of the DNA molecules in the macronucleus to explain these units, and (4) experiments designed to discriminate between these hypotheses. We conclude that the size of the genome is not reduced in the macronucleus and that there are 45 copies of the haploid genome present in the macronucleus of normal strains ofTet rahymena pyriformis and 800 copies in the macronucleus of Paramecium aurelia. The ciliate genome is relatively simple in terms of repeated sequences. However, not all copies of the genes present in the macro-nucleus may be identical since fractions of differing thermal stability appear after renaturation