How worm chromosomes are held together
Copies of each of our chromosomes are held together at one short site; but a worm’s chromosomes each have ~100 of these sites.
During cell division, the chromosomes in the original cell must be replicated and these ‘sister chromosomes’ must then be divided equally between the two new daughter cells. At first, the sister chromosomes are held together near a region called the centromere, which is important because the microtubules that pull the sister chromosomes apart attach themselves to the centromere. In many cases, the centromere is a small region near the middle of the chromosomes, which produces a classic X shape. However, in some organisms centromeres span the entire length of the chromosomes. There are at least 13 plant and animal lineages with such holocentromeres.
Inside the nucleus of cells, DNA is wrapped around molecules called histones. There are five major families of histones, and histones belonging to one of these families — the H3 histones — are replaced by cenH3 variant histones at both conventional centromeres and holocentromeres. There are many unanswered questions about holocentromeres. In particular, do holocentromeres truly extend along the full length of the chromosomes, or are they found at a large number of specific sites?
Now Florian Steiner and Steven Henikoff have studied the distribution of cenH3 in the genome of the worm C. elegans to investigate holocentromeres in greater detail. These experiments showed that the holocentromeres in C. elegans are actually made of about 700 individual centromeric sites distributed along the length of all the chromosomes. Each of these sites contains just one nucleosome that contains cenH3, and these sites are likely to be the sites that microtubules attach to during cell division. Surprisingly, the same sites can also act as so-called ‘HOT–sites’: these sites are bound by many proteins that are involved in regulating the process by which genes are expressed as proteins, which suggests a link between centromeres and these regulatory proteins.
The work of Steiner and Henikoff describes how centromeric nucleosomes are distributed across the genome, but why and how cenH3 ends up at these particular 700 sites remains an open question.
To find out more
Read the eLife research paper on which this story is based: “Holocentromeres are dispersed point centromeres localized at transcription factor hotspots” (April 8, 2014).
eLife is an open-access journal that publishes outstanding research in the life sciences and biomedicine.
