Condensation des chromosomes associée à la mitose

Abstract

Y a-t-il une relation directe entre la condensation des chromosomes et la synthèse de l’ADN ? Pour répondre à cette question, on a localisé l’ADN néosynthétisé et analysé la structure de la chromatine au cours de la mitose divisée en 11 stades. La synthèse de l’ADN a toujours lieu le long de nucléofilaments composés de rangées de nucléosomes larges de 11 nm, libres dans le nucléoplasme, compactés dans l’hétérochromatine ou attachés à sa surface. Au cours de la première étape de la condensation des chromosomes, les réplicons qui ont synthétisé leur ADN s’assemblent pour former d’abord des nucléofilaments compactés, puis des amas de chromatine et des protubérances. Au cours de la deuxième étape, qui dure moins de cinq minutes lors de la prophase, les amas et les protubérances s’agrègent et se transforment en chromomères. Au cours de la troisième étape, enfin, les chromomères s’approchent les uns des autres et se fondent pour donner naissance aux chromosomes mitotiques.

The DNA-specific osmium-ammine technique has been used to examine the distribution of DNA in a stable cell, the hepatocyte, and in a cyclic cell, the cylindrical cell of duodenal crypts, in the adult mouse. In both, the DNA is present in nucleofilaments composed of rows of 11 nm-wide nucleosomes. In hepatocytes at all times and in crypt cells just before and at the very beginning of the S phase, nucleofilaments are either 'free' in the nucleoplasm, 'compacted' in the heterochromatin or 'attached' at the surface of the heterochromatin. During the long S phase of crypt cells, the DNA organization changes gradually, as two kinds of chromatin accumulations, referred to as 'aggregates' and 'bulges', appear and grow; they are composed of compacted nucleofilaments and have attached nucleofilaments at their surface. The use of bromodeoxyuridine for the detection of the sites of DNA synthesis has revealed that this synthesis takes place along free and attached but not along compacted nucleofilaments. Within an hour, the replicons of the free and attached nucleofilaments whose DNA has been synthesized associate to form aggregates and bulges. The elaboration of these structures constitutes the first step in the condensation of chromosomes. The second step of the condensation, in less than five minutes at prophase, consists in some confluence of over 1000 aggregates and bulges giving rise to less than 300 chromomeres; these line up in a number of rows. At the third step of the condensation, which coincides with the prometaphase and lasts about five minutes, the chromomeres of each row move closer to one another and finally coalesce. Thus arise the mitotic chromosomes. [References: 16]