Stack S M
J Cell Sci. 1984 Oct;71:159-76. doi: 10.1242/jcs.71.1.159.
A combined light- and electron-microscopic examination of chromosomes from two angiospermous plants, Plantago ovata and Lycopersicon esculentum, and a mammal, Mus musculus, was performed. From this investigation three observations have been made that may be relevant to the observed lack of crossing over in heterochromatin. (1) Differential staining indicates that heterochromatin represents a smaller fraction of the length of pachytene chromosomes than it represents in the length of mitotic metaphase chromosomes. Since the synaptonemal complex (SC) runs throughout the length of these pachytene chromosomes, it is under-represented in heterochromatin. Considering the evidence for a rough correlation between the length of SC and the amount of crossing over, this could result in less crossing over in heterochromatin than expected on the basis of its length in mitotic metaphase chromosomes. (2) Electron microscopy indicates that, unlike the SC in euchromatin, the SC in heterochromatin is densely ensheathed in highly compact chromatin. If crossing over occurs in the SC or even in the surrounding chromatin, the compaction of the chromatin may prevent the penetration of enzymes needed in recombination. (3) Finally, a difference in the structure of SCs in euchromatin versus heterochromatin was observed that could be associated with the lack of crossing over in heterochromatin.
对两种被子植物（卵叶车前和番茄）以及一种哺乳动物（小家鼠）的染色体进行了光镜和电镜联合检查。通过这项研究得出了三项观察结果，这些结果可能与在异染色质中观察到的交叉缺失现象有关。（1）差异染色表明，与有丝分裂中期染色体长度相比，异染色质在粗线期染色体长度中所占比例更小。由于联会复合体（SC）贯穿这些粗线期染色体的全长，所以在异染色质中其代表性不足。考虑到有证据表明SC的长度与交叉发生量之间存在大致的相关性，这可能导致异染色质中的交叉发生量比根据其在有丝分裂中期染色体中的长度所预期的要少。（2）电子显微镜显示，与常染色质中的SC不同，异染色质中的SC被高度致密的染色质紧密包裹。如果交叉发生在SC甚至周围的染色质中，染色质的致密化可能会阻止重组所需酶的穿透。（3）最后，观察到常染色质与异染色质中SC结构的差异，这可能与异染色质中交叉缺失有关。
