Nicotiana otophora的異染色質

Abstract

將固定之單倍體Nicotiana otophora （n＝12）根尖以福根試劑染色，分析核型。其12條染色體依大小可分為兩組：5條長的和7條短的。5條長染色體是屬於俱近端中節，且長臂上各有一大塊異染色質。再依大塊異染色質的位置及附隨體的有無，可細分為3組。7條短的染色體是屬於中央中節或次中央中節，無法再依染色體大小及中節位置做進一步區分。以氚—胸腺嘧啶核?短時間標記根尖細胞染色體DNA，於生長不同時間後固定，做放射自顯術，發現生長2?4小時之前中期染色體，只有5大塊異染色質有標記；生長5小時，始見各染色體之中節兩側異染色質有標記；而生長7小時以後，大塊異染色質不再有標記，銀粒子只出現在?染色質上。故知長染色體上之大塊異染色質是晚複製，而?染色質最早複製；中節兩側的異染色質雖亦是晚複製，但不似大塊異染色質那麼遲。

Root tips of haploid plants of Nicotiana otophoroa (n=12) were fixed and stained by the Feulgen squash method, and karyotype was analyzed. The 12 chromosomes fall into two distinct size groups: 5 are large and 7 are small. All the large chromosomes are acrocentric and have a large piece of heterochromatin on the long arm. However, they can be further divided into three groups according to the position of the large heterochromatin and the presence of a satellite. The 7 small chromosomes are metacentric or submetacentric; they cannot be distinguished by size and position of the centromere. Haploid plants were treated with 3H-thymidine for 30 min and then transferred to culture medium without 3H-thymidine for further growth. After various time of incubation, root tips were fixed and stained by the Feulgen squash method. Chromosome preparations were subjected to autoradiography and the position of silver grains on prometaphase chromosomes were analyzed. Among the materials collected after 2-4 hrs of incubation, only the five large pieces of heterochromatin were labeled. Centromeric heterochromatin was labeled after 5 hrs and after 7 hrs only euchromatin was labeled. These observations indicate that the large heterochromatin on the long chromosomes is the latest to replicate and euchromatin is the earliest to replicate in the chromosome complement. Centromeric heterochromatin perhaps also replicates late but not as late as the large heterochromatin bodies.