Holmquist G P
Am J Hum Genet. 1987 Feb;40(2):151-73.
Late-replicating chromatin in vertebrates is repressed. Housekeeping (constitutively active) genes always replicate early and are in the early-replicating R-bands. Tissue-specific genes are usually in the late-replicating G-bands and therein almost always replicate late. Within the G-bands, however, a tissue-specific gene does replicate early in those cell types that express that particular gene. While the condition of late replication may simply be coincident with gene repression, we review evidence suggesting that late replication may actively determine repression. As mammals utilize a developmental program to Lyonize (facultatively heterochromatinize) whole X chromosomes to a late-replicating and somatically heritable repressed state, similarly another program seems to Lyonize individual replicons. In frogs, all genes begin embryogenesis by replicating during a very short interval. As the developmental potency of embryonic cells becomes restricted, late-replicating DNA gradually appears. This addition to the repertoire of gene control--i.e., repression via Lyonization of individual replicons--seems to have evolved in vertebrates with G-bands being a manifestation of the mechanism.
脊椎动物中晚期复制的染色质受到抑制。管家（组成型活跃）基因总是早期复制，位于早期复制的R带中。组织特异性基因通常位于晚期复制的G带中，并且几乎总是在晚期复制。然而，在G带内，特定组织特异性基因在表达该特定基因的细胞类型中确实早期复制。虽然晚期复制的状态可能仅仅与基因抑制同时发生，但我们回顾了一些证据，表明晚期复制可能积极地决定抑制作用。由于哺乳动物利用一种发育程序使整个X染色体莱昂化（兼性异染色质化），进入晚期复制且体细胞可遗传的抑制状态，类似地，另一种程序似乎使单个复制子莱昂化。在青蛙中，所有基因在胚胎发育开始时都在非常短的时间间隔内进行复制。随着胚胎细胞的发育潜能受到限制，晚期复制的DNA逐渐出现。这种基因控制方式的增加——即通过单个复制子的莱昂化实现抑制——似乎是在脊椎动物中进化而来的，G带就是这种机制的一种表现。
