CHROMOSOMENSTRUKTUR UND GENETISCHE FUNKTION 



635 



différent types of cells and différent stages of development. The primary product 

 of gene activity in puffed régions seems to be a comparatively large RNA molécule 

 which is subject to complicated transformations in the site of its origin. In some 

 dipteran species a spécial type of puff is found. Here DNA is synthesized locally 

 and apart from the normal replication cycles. The nature of this excess DNA and 

 its physiological significance is not known. 



Another type of structural modification are the loop pairs which are developed 

 by the " lampbrush chromosomes „ in oocytes or spermatocytes of many species. 

 The loops occur in identical pairs and each pair is developed by a chromomere. 

 The loops have an axis of DNA to which are attached ribonucleoproteins. They 

 are formed at sites where RNA is synthesized and they disintegrate after an 

 inhibition of RNA synthesis. The physiological significance of lampbrush loop 

 formation has been further analysed in Drosophila where the Y chromosome 

 develops only a limited number of especially large loop pairs which are ail involved 

 in processes of spermiogenesis. 



The nucleolus is the most common type of chromosomal functional structure. 

 Nucleoli are formed by spécifie chromomeres: the " nucleolus organizers „. Thèse 

 are the sites of the cistrons for ribosomal RNA and they are highly répétitive. 

 An amplification of the cistrons for ribosomal RNA has been found in the oocyte 

 nuclei of many species. In thèse cases, the supplementary DNA may be detached 

 from the nucleolus organizers in the form of rings which consequently give rise 

 to free nucleoli. Thus, oocytes seem to be able to increase their synthetic capacity 

 for ribosomal RNA by a locally restricted amplification of the corresponding 

 gènes. 



Puffs, lampbrush loops, and nucleoli are developed by the same chromosomal 

 units, the chromomeres. There is évidence that chromomeres act as functional 

 units not only during the transcription of the genetic message, but also during 

 the replication of the DNA. The elucidation of the chromomeric organization 

 has, therefore, become of main importance for cytogenetic work. 



RÉSUMÉ 



Les changements de la structure des chromosomes et leurs relations 

 avec l 'activité des gènes 



On connaît plusieurs types de variation de la structure des chromosomes 

 dont on a pu démontrer la corrélation avec l'activation de facteurs génétiques. 



L'une de ces variations, connue depuis longtemps, est l'héterochromatine qui 

 est de la chromatine qui reste à l'état condensé pendant l'interphase mitotique. 

 Dans les expériences de génétique, les parties héterochromatiques se comportent 

 comme si elles ne contenaient pas de facteurs mendéliens et par autoradiographie 



