This assay is used when a large number of people happen to be exposed as in a disastrous situation as for instance an atom bomb is dropped as it was done at Hiroshima and Nagasaki in 1945. This assay is used as a screening procedure when a large number of people happen to be exposed as scoring of micronuclei under the microscope is much easier compared to scoring dicentric chromosomes. This assay is however comparatively less sensitive than dicentric measurement.
Formation of micronucleus: Micronucleus is also induced by radiation exposure. The interaction of radiation with chromosome will result not only the formation of dicentric chromosome but also acentric fragments. Since acentric fragments do not have a centromere they are not pulled towards the daughter nuclei at the time of nuclear division. These acentric fragments are left in the cytoplasm which appear as micronuclei. The frequency of micronuclei provides an estimation of radiation exposure.
Method: The procedure for micronucleus assay and dicentric assay is almost same except the addition of cytochalasin-B in place of colcemid. In the case of dicentric chromosome the cells are arrested at metaphase stage where as for micronuclei the cells are arrested at a stage called cytokinesis. At this stage the nucleus has just divided but not the cytoplasm.
Dose Estimation: Using micronuclei frequency the dose is estimated using the dose-response curve constructed in this laboratory. In this laboratory the dose response curve has been constructed with high (1 Gy/min) as well as with low (0.0014 Gy/min) dose rates of gamma radiation.