12.1 Introduction 12.2 Replicons can be linear or circular 12.3 Origins can be mapped by autoradiography and electrophoresis 12.4 The bacterial genome is a single circular replicon 12.5 Each eukaryotic chromosome contains many replicons 12.6 Isolating the origins of yeast replicons 12.7 D loops maintain mitochondrial origins 12.8 The problem of linear replicons