The linker histone H1 seems like an obvious candidate for a condensing activity. But unreplicated sperm chromatids in Xenopus egg extracts look no less condensed when H1 is depleted. The new results suggest that these chromatids do not aptly represent the normal situation. By first replicating the sperm chromatids, the group was able to show that H1 is a condenser.
H1 was enriched on duplicated chromosomes compared with their unreplicated counterparts. It is not clear how H1 is loaded, but it requires passage through interphase, when DNA is replicated. In the absence of H1, other chromosomal proteins, including condensin and cohesin, found their way to chromosomes. But the H1-free chromosomes were longer, failed to align properly on the metaphase plate, and tangled up during anaphase such that segregation failed. Kinetochore function was normal, probably due in part to the presence of CENP-A, a centromere-specific histone variant with some similarity to H1.
The authors propose that, if proteins such as condensin are loaded at specific nucleotide intervals, then the absence of the H1 linker would space them further apart in absolute distance, thus amplifying condensation problems. Chromosomes in cells lacking H1 reached lengths of 60 μm, about twice that of the spindle. Overflowing, unattached arms would be difficult to push to the metaphase plate and then pull to the poles at anaphase.