Estimation of Copy Number using SYBR Green: Confounding by AT-rich DNA and by Variation in Amplicon Length

James M. Colborn Departments of Tropical Medicine and Medicine, and the Center for Infectious Diseases, Tulane University Health Sciences Center, New Orleans, Louisiana; Department of Biology, Faculty of Science and Technology, University of Bamako, Bamako, Mali

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Brian D. Byrd Departments of Tropical Medicine and Medicine, and the Center for Infectious Diseases, Tulane University Health Sciences Center, New Orleans, Louisiana; Department of Biology, Faculty of Science and Technology, University of Bamako, Bamako, Mali

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Ousmane A. Koita Departments of Tropical Medicine and Medicine, and the Center for Infectious Diseases, Tulane University Health Sciences Center, New Orleans, Louisiana; Department of Biology, Faculty of Science and Technology, University of Bamako, Bamako, Mali

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Donald J. Krogstad Departments of Tropical Medicine and Medicine, and the Center for Infectious Diseases, Tulane University Health Sciences Center, New Orleans, Louisiana; Department of Biology, Faculty of Science and Technology, University of Bamako, Bamako, Mali

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Although SYBR Green is used to estimate copy number, its fluorescence varies with amplicon length and adenine/thymine (AT) content. As a result, threshold cycle (Ct) values obtained using real-time polymerase chain reaction (PCR) are lower for longer amplicons (P < 0.001) and amplicons with greater AT content (P < 0.001). In contrast, neither amplicon length nor AT content affects the Ct with TaqMan probes or LUX-labeled primers. Because SYBR Green yields lower Cts with AT-rich templates and longer templates, it overestimates copy number for those templates. Therefore, sequence-specific methods such as TaqMan probes or LUX-labeled primers should be considered when using real-time PCR to estimate copy number if the amplicons generated are AT-rich or vary in length.

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