Figure 3.

Photographs of the region of the Drosophila X chromosome (band 3C) specifically labeled with colloidally prepared blue PRPs via the in situ hybridization protocol described in Methods. The typical distance between PRPs is ≈2 μm. The polytene chromosome is counterstained with SYBR Green. Both the SYBR Green fluorescence, and the light scattered by the PRPs are photographed simultaneously. The large number of parallel aligned copies of DNA present in the polytene chromosome provides multiple target sites for PRP hybridization, suitable for illustrative purposes, but excessive for the counting of individual PRPs for quantitative analysis. (A) The region of the white gene (w) from the tip of the X chromosome, with band 3C PRP labeled. (B) A close-up of the 3C band from another sample, illustrating the large density of PRPs that can be clustered yet still be readily identified as individual target site labels.