= Molecular biology
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<Molecular biology> investigates biological processes through the structures, interactions and transformations of <molecules>. A central set of questions concerns how <DNA> and <RNA> participate in inheritance and <gene expression>, how <biological translation> produces <proteins>, and how those proteins influence cellular function. Molecular composition, sequence and physical interaction provide experimentally distinguishable kinds of evidence.
An important historical example is the1952 <Hershey-Chase experiment>. <A. D. Hershey> and <Martha Chase> compared the behavior of labeled <DNA> and labeled <protein> during infection by <Bacteriophage T2>. <Phosphorus-32> and <sulfur-35> supplied radioactive tracers associated with those different chemical components.<Independent functions of viral protein and nucleic acid in growth of bacteriophage by A. D. Hershey and Martha Chase>[Original Abstract and experimental comparisons of phosphorus- and sulfur-labeled phage]{ref}
The apparatus made physical location measurable. Agitation in a Waring blender detached externally attached phage material from infected <bacteria>; <centrifugation> separated cell-containing material from extracellular label. The investigators then compared label distributions and subsequent phage production. Differential transfer supported distinct roles for viral <DNA> and much of the sulfur-containing coat protein. It did not establish that every organism uses identical molecular machinery.<Independent functions of viral protein and nucleic acid in growth of bacteriophage by A. D. Hershey and Martha Chase>[Original pp.50–53, including p.52: blender treatment, separation and progeny-label comparisons]{ref}
An original fraction-accounting example makes the inference boundary explicit. If a corrected tracer assay gives120 units in one fraction and480 in another, the first contains $120/(120+480)=0.20$ of the recovered readout. That is a distribution of measured label under the assay assumptions. It is not automatically a20-percent contribution to heredity, cellular function or the number of molecules present. Label specificity, recovery and background determine what the measurement means.
The broader strategy is to connect a biological outcome with a manipulable molecular explanation. Sequence comparisons, structural observations and perturbations of <gene expression> answer complementary questions. A molecule can be present without being causally necessary for a tested outcome; changing it and measuring the consequence supplies a different kind of evidence. This combination links the study of <biological cells> with chemistry and quantitative experiments.
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