DNA replication occurs in all lamina positive micronuclei, but never in lamina negative micronuclei

A micronucleus is a small nucleus-like structure found in the cytoplasm of dividing cells that suffered from genotoxic stress. It is generally hypothesised that micronuclei content is eventually lost from cells, though the mechanism of how this occurs is unknown. If DNA located within the micronucleus is not replicated, it may explain the loss of micronuclei content. Because there had been no compelling evidence for this issue, we have addressed whether DNA located within the micronucleus is replicated this issue. Pulse labelling of bromodeoxyuridine revealed that DNA synthesis takes place in a portion of micronuclei that contain nuclear lamin B protein. By using iodine 3'-deoxyuridine/chlorodeoxyuridine double labelling, we found that all micronuclei containing lamin B are replicated during one cell cycle, whereas micronuclei lacking lamin B are never replicated. This result suggests that the content of lamin B-negative micronuclei is lost during cell division. Furthermore, we simultaneously visualised sites of DNA synthesis, lamin B and the extrachromosomal double minutes chromatin, which contain amplified oncogenes. We found that although the replication timing of double minutes was generally preserved in micronuclei, at times it differed greatly from the timing in the nucleus, which may perturb the expression of the amplified oncogenes. Taken together, these findings uncovered the DNA replication occurring inside micronuclei.