Tracking the intracellular location of retroviral DNA. To determine how intermediates in the 
replication of retroviruses move to the nucleus, we can use hybridization probes that specifically 
recognize the viral DNA and provide a fluorescent signal. This photomicrograph shows cultured 
rat fibroblasts that have each been infected with about 10 copies of the Moloney murine leukemia 
virus. The green spots indicate the locations of viral DNA molecules. The nucleus is visualized by 
staining with a blue fluorescent dye that binds nonspeciflcally to DNA. An antibody to microtu- 
bules, labeled with a red fluorescent dye, indicates the contours of the cytoplasm. By carrying out 
this kind of analysis at intervals after viral infection, one can track the movement of viral DNA to 
the cell's nucleus. 
Research of Patrick O. Brown. 
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