Whenever we go on a trip, my long-suffering husband quietly puts our luggage next the car and slinks away, trembling and twitching. He knows a mad-woman is ready to pack the trunk, playing luggage-Tetris until it all fits and speaking in tongues. Seriously, though, I’m freaking awesome. That said, I don’t envy the insane packing that a cell must accomplish to jam all of that DNA into neat little chromosomes ready for their own cell division road trip. A recent paper helps us understand how that happens at the centromere.
Centromeres are the regions on chromosomes that bind sister chromatids together and serve as the sites of kinetochore assembly during mitosis. The presence of the protein CENP-A is a hallmark of centromere location, as it is a histone H3 variant that helps package and compact centromeric DNA. It was previously presumed that CENP-A was passed down to daughter cells epigenetically, inherited from previous cell divisions, but a recent paper shows that this is not the case in the nematode worm C. elegans. According to Gassmann and colleagues, pre-existing CENP-A is not required for CENP-A localization to centromeres in subsequent divisions. In fact, CENP-A is unloaded from centromeres at one point in oogenesis, the production of eggs, and later reloaded onto centromeres. By mapping the location of CENP-A in the genome, Gassmann and colleagues found that regions of transcribed genes are regions where CENP-A is excluded, a pattern that changes when germline gene transcription switches to embryonic gene transcription. In the images above, the C. elegans germline is labeled to show chromosomes (top image) and the location of CENP-A (bottom). CENP-A is lost from chromosomes during the pachytene stage of meiosis and later reloaded onto chromosomes during diplotene, and is not found in sperm.
Gassmann, R., Rechtsteiner, A., Yuen, K., Muroyama, A., Egelhofer, T., Gaydos, L., Barron, F., Maddox, P., Essex, A., Monen, J., Ercan, S., Lieb, J., Oegema, K., Strome, S., & Desai, A. (2012). An inverse relationship to germline transcription defines centromeric chromatin in C. elegans Nature, 484 (7395), 534-537 DOI: 10.1038/nature10973
Adapted by permission from Macmillan Publishers Ltd, copyright ©2012