Eukaryotic Chromosome

Due to the compact nature of chromatin in eukaryotic cells, it is virtually impossible to visualize gene expression in vivo. However, there are certain unusual situations in which gene expression can be seen. Such is the case with the polytene chromosomes, which are exceptionally large in comparison to other types of chromosomes.

The salivary glands of Drosophila melanogaster (fruit fly) larvae contain greatly enlarged chromosomes. These are polytene chromosomes, and they result from multiple rounds of replication of a diploid pair of chromosomes joined in parallel. The replicated chromosomes remain attached to one another. Each pair of chromosomes can replicate up to nine times; thus, the resultant polytene chromosome can contain up to 1,024 (2⁹) strands of DNA.

The vast majority (95%) of DNA in the polytene chromosomes is concentrated in chromosomal bands, called chromomeres, which are microscopically visualized through staining. These chromomeres form a pattern that is characteristic for each Drosophila strain. Drosophila polytene chromosomes display roughly 5,000 bands. Since the total number of genes in Drosophila appears to be greater than the number of bands that can be visualized, it is likely that there are multiple genes located within a given band.

The banding pattern of the polytene chromosomes provides a cytological map, or diagrammatic representation, of the physical location of genes at specific sites in the cell. The positions of individual genes can be determined using a technique called in situ hybridization. First, the DNA of an immobilized chromosome preparation is made single-stranded (denatured). A radioactively labeled probe, generally a small piece of DNA corresponding to the gene of interest, is then mixed with the denatured DNA under conditions that permit the radiolabeled DNA to bind to its complementary Scanning electron microscope magnification of one human X chromosome. Genetic information is packed within the "bumps" and loops of material pictured here. DNA strand on the immobilized chromosome preparation. Finally, the chromosomal binding site is determined by way of a procedure called autoradiography, which is used to make the radioactive probe visible on photographic film.

With autoradiography, the sites of gene expression can be visualized along the polytene chromosomes. As DNA decondenses into a more open state, it forms a distinctive swelling known as a chromosomal puff. These puffs are active sites of transcription, or RNA synthesis. Throughout development, they alternately expand and contract, as specific genes are activated or repressed.