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
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