The most extensively studied polytene chromosomes. During larval development, the cells of the salivary gland undergo 9 or 10 cycles of endomitotic DNA replications to produce chromosomes that contain 1,000–2,000 times the haploid amount of DNA. The cytological map of the chromosomes of D. melanogaster contains slightly over 5,000 bands. It is divided into 102 divisions, distributed as illustrated. The solid circles represent the centromeres. Each division is subdivided into subdivisions lettered A–F, and the subdivisions contain varying numbers of bands. Genes have been localized within these bands by studying overlapping deficiencies and, more recently, by in situ hybridization with labeled probes. Since the number of genes in the euchromatin of Drosophila is known to be 13,000, the average band in a giant chromosome must contain two or three genes. The insertion of a transposable element (q.v.) can generate new bands and interbands in the salivary chromosomes. The cells of the larval salivary gland are in interphase, and within each nucleus the chromosomes show a typical orientation. The telomeres tend to be on the surface of the nuclear envelope, opposite the portion of the envelope nearest to the nucleolus, where all the centromeres are located. The arms of each autosome remain close together, whereas the relative positions of the arms vary. Different chromosomes are never entangled. The dominant chromosome folding motif is a right-handed coil. See Chronology, 1933, Painter; 1935, Bridges; 1968, Semeshin et al.; 1988, Sorsa; biotinylated DNA, chromosomal puff, deficiency loop, Drosophila virilis, heterochromatin, insulator DNAs, Rabl orientation, salivary gland chromosomes.
Subjects: Genetics and Genomics.