The larval salivary glands of Drosophila species and other dipteran insects contain enlarged interphase chromosomes that are visible in the light microscope. When fixed and stained with a dye that stains DNA, these polytene chromosomes are characterized by a large number of reproducible, well-demarcated bands, which have been assigned standardized numbers (Figure 1a). The densely staining bands represent regions where the chromatin is more condensed, and the light interband areas are regions where the chroma tin is less condensed. Although the molecular mechanisms that control the formation of bands in polytene chromosomes are not yet understood, the highly reproducible banding pattern seen in Drosophila salivary gland chromosomes provides an extremely powerful method for locating specific DNA sequences along the chromosomes of this species. Not only are chromosomal translocations and inversions readily detectable in polytene chromosomes, but specific chromosomal proteins can be localized on interphase polytene chromosomes by immunostaining with specific antibodies raised against them. Insect polytene chromosomes offer one of the only experimental systems in all of nature in which such immunolocalization studies on decondensed interphase chromosomes are possible.

EXPERIMENTAL Fig1. Banding on Drosophila polytene salivary gland chromosomes. (a) In this light micrograph of Drosophila melanogaster larval salivary gland chromosomes, four chromosomes can be observed (X, 2, 3, and 4), with a total of approximately 5000 distinguishable bands. The banding pattern results from reproducible patterns of DNA and protein packing within each site along the chromosome. Dark bands are regions of more highly compacted chromatin. The centromeres of all four chromosomes often appear fused at the chromocenter. The tips of chromosomes 2 and 3 are labeled (L = left arm; R = right arm), as is the tip of the X chromo some. (b) The pattern of amplification of chromosome 4 during five replications. Double-stranded DNA is represented by a single line. Telomere and centromere DNA are not amplified. In salivary gland polytene chromosomes, each parental chromosome undergoes about 10 replications (210 = 1024 strands). See C. D. Laird et al., 1973, Cold Spring Harbor Symp. Quant. Biol. 38:311. [Part (a) courtesy of Joseph Gall, Carnegie Institution for Science.]

A generalized amplification of DNA gives rise to the polytene chromosomes found in the salivary glands of Drosophila. This process, termed polytenization, occurs when the DNA repeatedly replicates everywhere except at the telomeres and centromere, but the daughter chromosomes do not separate. The result is an enlarged chromosome com posed of many parallel copies of itself, 1024 resulting from ten such replications in Drosophila melanogaster salivary glands (Figure 1b). The amplification of chromosomal DNA greatly increases gene copy number, presumably to supply sufficient mRNA for protein synthesis in the massive salivary gland cells. The bands in Drosophila polytene chromosomes each represent some 50,000–100,000 bp, and the banding pattern reveals that the condensation of DNA varies greatly along these relatively short regions of an interphase chromosome.

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مواضيع ذات صلة

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