Abstract: Compositions and methods for a nucleic acid amplification and diagnostic methods based therein, are disclosed. The compositions include purified Thermus aquaticus DNA polymerase (Taq Pol) and Moloney Murine Leukemia Virus Reverse Transcriptase (MMLV-RT), in proportions and under reaction conditions that allow one-step amplification of a nucleic acid of interest. In particular, the compositions are useful in one-step RT-qPCR and RT-LAMP methods. The method includes sampling the specimen from a subject, extracting and purifying RNA from the sample. The compositions and methods may be used to produce, analyze, quantitate, detect and otherwise manipulate nucleic acid molecules. For example, disclosed are methods of detecting the presence of a viral nucleic acid in a sample from a subject. Methods of diagnosing a subject as being infected with a viral pathogen are also provided. A preferred virus or pathogen is SARS-CoV-2.