DNA elimination and its relation to quantities in the macronucleus of Tetrahymena

Abstract

The macronucleus of Tetrahymena contains a large number of DNA molecules of subchromosomal size. They belong to about 270 species each one occurring at an average number of 45 copies. Macronuclei divide unequally and nothing is known of segregation control. This and the elimination and degradation of DNA during macronuclear amitosis make the clonal stability of macronuclei a problem of qualitative and quantitative control on a subchromosomal level. We studied the contribution of DNA elimination to the quantitative composition of the macronucleus cytophotometrically in single cells of different strains. This was done under standard conditions and under conditions known to influence the amount of macronuclear DNA. The following results were found: Elimination of DNA occurs at almost every division. The size of the elimination body is highly variable but still positively correlated with the macronuclear DNA content. In T. thermophila the amount of eliminated DNA is 2.5% of the G2 content and is not dependent on the growth state. It varies with species, amounting to as much as 8% in T. pigmentosa. During conditions which increase the macronuclear DNA content, very little DNA is eliminated. On the other hand, large amounts are eliminated under other conditions causing the macronuclear DNA content to decrease. DNA to be eliminated at division is synthesized at the same time as bulk DNA. We developed a computer program which helps us study the effects of DNA elimination and unequal divisions upon the copy numbers of subchromosomal DNA classes.(ABSTRACT TRUNCATED AT 250 WORDS)