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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.

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