Chromatid macrocoiling and chromosome compaction

H aapala , O. 1984. Chromatid macrocoiling and chromosome compaction. — Hereditas 100: 17–27. Lund, Sweden. ISSN 0018–0661. Received March 17, 1983 The process of chromosome compaction was studied by the observation of the number of macrocoil gyri within a variety of chromosome lengths towards metaphase. The number of gyri was found to remain almost unchanged from the onset of macrocoiling until compact metaphase codiration, suggesting that chromatid contraction by coiling is not accompanied by gyre elimination. The specific helical dimensions of chromosome 1 macrocoils were measured and the lengths of helical chromonemata were calculated. The lengths varied between 18.1 and 23.9 mUm and, accordingly, the chromonemal lengths of 192–249 mUm for the autosome set were derived. The coiling pattern of the individual chromatid arms indicated that chromatid arms do not spiralize, in respect to the coil direction (left‐ or right‐handed helix) they generate, randomly within a chromosome. Present results, along with theoretical considerations, make possible a better interpretation of the relationship of chromatid substructures emerging during mitotic condensation of chromosomes.