I love mitotic spindles, so of course I love early fruit fly development and its rapid, synchronized syncytial divisions. Watching these mitotic spindles perform a synchronized swimming routine, complete with tiny little swim caps and nose plugs, is always a treat. Today’s image is from a paper that helps to define the relationship between DNA replication, chromosome condensation, and mitotic progression.
During the cell cycle, DNA strands are replicated. After proper DNA replication, the very long strands are compacted in order to allow efficient and accurate separation of chromosomes during mitosis. When chromosome condensation doesn’t occur correctly, the progress through mitosis is disrupted. This may be due to the well-studied spindle assembly checkpoint, or there may be a checkpoint that monitors chromosome condensation. A recent paper describes results showing the effects of certain inhibitors on chromosome condensation and mitotic progression, marked by entry into anaphase. Fasulo and colleagues found that the inhibitors that severely disrupted chromosome condensation also disrupted anaphase onset. These delays occurred through disruption of the Wee1 kinase, and not due to the spindle assembly checkpoint. By using the early fruit fly embryo, Fasulo and colleagues could track many synchronized cell divisions at once, allowing for fast and direct analysis of the effects of the various inhibitors used. The cartoon and images above show the different steps during mitosis of these divisions. Both chromosomes (top row of images, green in merged) and microtubules (middle row of images, red in merged) are fluorescently tagged.
Fasulo, B., Koyama, C., Yu, K., Homola, E., Hsieh, T., Campbell, S., & Sullivan, W. (2012). Chk1 and Wee1 kinases coordinate DNA replication, chromosome condensation, and anaphase entry Molecular Biology of the Cell, 23 (6), 1047-1057 DOI: 10.1091/mbc.E11-10-0832