Spradling A C
Cell. 1981 Nov;27(1 Pt 2):193-201. doi: 10.1016/0092-8674(81)90373-1.
Drosophila chorion genes are located in two clusters, one on the X chromosome at 7F1-2 of the polytene chromosome map and a second on the third chromosome at 66D11-15. Genes in both regions undergo amplification in ovarian follicle cells prior to their expression late in oogenesis. Analysis of cloned genomic segments derived from these chromosomal sites revealed that each cluster contains two tandemly transcribed chorion protein genes separated by only 1-2 kb. At least two other regions complementary to ovary RNA are located within 5 kb of these genes. During oogenesis, the transcribed sequences within each cluster, as well as the spacer sequences that separate them, are amplified equally. Sequences adjacent to the transcribed regions also replicate differentially but to a lesser extent, giving rise to gradients of decreasing amplification involving 40-50 kb of flanking chromosomal sequences. Differences between the restriction maps of unamplified and amplified DNA could not be detected in genomic DNA within either of the 90-100 kb domains of amplification. These observations suggest a model of amplification in which additional rounds of replication are specifically initiated within the central gene-containing regions, followed by bidirectional replication in the absence of discrete termination sites.
果蝇绒毛膜基因位于两个基因簇中，一个位于多线染色体图谱X染色体的7F1 - 2处，另一个位于第三条染色体的66D11 - 15处。这两个区域的基因在卵子发生后期表达之前，会在卵巢卵泡细胞中进行扩增。对源自这些染色体位点的克隆基因组片段的分析表明，每个基因簇都包含两个串联转录的绒毛膜蛋白基因，它们之间仅相隔1 - 2 kb。在这些基因的5 kb范围内至少还有另外两个与卵巢RNA互补的区域。在卵子发生过程中，每个基因簇内的转录序列以及分隔它们的间隔序列会等量扩增。与转录区域相邻的序列也会有差异地复制，但程度较小，从而形成涉及40 - 50 kb侧翼染色体序列的递减扩增梯度。在90 - 100 kb的任何一个扩增区域内的基因组DNA中，未扩增和扩增后的DNA的限制性图谱差异均无法检测到。这些观察结果提示了一种扩增模型，即在含基因的中央区域内特异性地启动额外的复制轮次，随后在没有离散终止位点的情况下进行双向复制。
