2000B: Anasomes
It seems that the spindle microtubules would have to initially attach to the kinetochores on the individual chromosomes in order to pull the sister chromosomes apart. If our observations are correct, and there is indeed no penetration of microtubules into the interior of the anasome, it appears that the microtubules must detach from the individual chromosomes after the initial separation, and re-attach to the nuclear membrane after the membrane has enclosed the chromosome cluster. This needs to be investigated at higher resolution.
Discussion
We have not yet published these results.
No previous study has reported this structure in C. elegans. However, in a retrospective analysis of images of early C. elegans embryos double-stained for microtubules and DNA, the microtubules appear to stop just short of the chromosome cluster in anaphase cells, suggesting the presence of an intervening structure (Keating and White, 1998).
Keating, H. H. and White, J. G. (1998). Centrosome dynamics in early embryos of Caenhorabditis elegans. J.Cell Sci. 111, 3027-3033.
Interestingly, studies of early embyogenesis in other organisms, including vertebrates, have found that nuclear membrane fragments surround individual chromosomes during anaphase and early telophase. These individual "karyomeres" then coalesce during telophase to form the nuclear envelope.
Richards, A. (1917). The history of chromosome vesicles in Fundulus and the theory of genetic continuity of chromosomes. Biol.Bull. 32, 249-290. [Review]
Wilson, E.B. (1925). The cell in development and heredity (New York: Macmillan Co.). [Book]
Montag, M., Spring, H., and Trendelenburg, M. F. (1988). Structural analysis of the mitotic cycle in pre-gastrula Xenopus embryos. Chromosoma 96 (3), 187-196.
Since the anasome surrounds the entire cluster of chromosomes rather than individual chromosomes, it appears to be distinctly different from karyomeres, but we can't rule out that structures analogous to karyomeres form earlier in anaphase in young C. elegans embryos and then later coalesce. Previous studies have found that DNA replication begins during anaphase in karyomeres. The authors of these studies speculated that allowing DNA replication to begin during mitosis is a specialization of rapidly dividing early embryo cells (pre-mid-blastula stage). We hypothesize that anasomes may serve a similar purpose in C. elegans.
Ito, S., Dan, K., and Goodenough, D. (1981). Ultrastructure and 3H-thymidine incorporation by chromosome vesicles in sea urchin embryos. Chromosoma 83 (4), 441-453.
Lemaitre, J.-M., Géraud, G., and Méchali, M. (1998). Dynamics of the genome during early Xenopus laevis development: Karyomeres as independent units of replication. J.Cell Biol. 142 (5), 1159-1166.
Discussion
We have not yet published these results.
No previous study has reported this structure in C. elegans. However, in a retrospective analysis of images of early C. elegans embryos double-stained for microtubules and DNA, the microtubules appear to stop just short of the chromosome cluster in anaphase cells, suggesting the presence of an intervening structure (Keating and White, 1998).
Keating, H. H. and White, J. G. (1998). Centrosome dynamics in early embryos of Caenhorabditis elegans. J.Cell Sci. 111, 3027-3033.
Interestingly, studies of early embyogenesis in other organisms, including vertebrates, have found that nuclear membrane fragments surround individual chromosomes during anaphase and early telophase. These individual "karyomeres" then coalesce during telophase to form the nuclear envelope.
Richards, A. (1917). The history of chromosome vesicles in Fundulus and the theory of genetic continuity of chromosomes. Biol.Bull. 32, 249-290. [Review]
Wilson, E.B. (1925). The cell in development and heredity (New York: Macmillan Co.). [Book]
Montag, M., Spring, H., and Trendelenburg, M. F. (1988). Structural analysis of the mitotic cycle in pre-gastrula Xenopus embryos. Chromosoma 96 (3), 187-196.
Since the anasome surrounds the entire cluster of chromosomes rather than individual chromosomes, it appears to be distinctly different from karyomeres, but we can't rule out that structures analogous to karyomeres form earlier in anaphase in young C. elegans embryos and then later coalesce. Previous studies have found that DNA replication begins during anaphase in karyomeres. The authors of these studies speculated that allowing DNA replication to begin during mitosis is a specialization of rapidly dividing early embryo cells (pre-mid-blastula stage). We hypothesize that anasomes may serve a similar purpose in C. elegans.
Ito, S., Dan, K., and Goodenough, D. (1981). Ultrastructure and 3H-thymidine incorporation by chromosome vesicles in sea urchin embryos. Chromosoma 83 (4), 441-453.
Lemaitre, J.-M., Géraud, G., and Méchali, M. (1998). Dynamics of the genome during early Xenopus laevis development: Karyomeres as independent units of replication. J.Cell Biol. 142 (5), 1159-1166.