Micromechanical studies of mitotic chromosomes.
We review micromechanical experiments on mitotic chromosomes. We focus on work where chromosomes were extracted from prometaphase amphibian cells, and then studied by micromanipulation and microfluidic biochemical techniques. These experiments reveal that chromosomes have well-behaved elastic response over a fivefold range of stretching, with an elastic modulus similar to that of a loosely tethered polymer network. Perturbation by microfluidic 'spraying' of various ions reveals that the mitotic chromosome can be rapidly and reversibly decondensed or overcondensed, i.e. that the native state is not maximally compacted. Finally, we discuss microspraying experiments of DNA-cutting enzymes which reveal that the element which gives mitotic chromosomes their mechanical integrity is DNA itself. These experiments indicate that chromatin-condensing proteins are not organized into a mechanically contiguous 'scaffold', but instead that the mitotic chromosome is best thought of as a cross-linked network of chromatin. Preliminary results from restriction-enzyme digestion experiments indicate a spacing between chromatin 'cross-links' of roughly 15 kb, a size similar to that inferred from classical chromatin-loop-isolation studies. We compare our results to similar experiments done by Houchmandzadeh and Dimitrov (J Cell Biol 145: 215-213 (1999)) on chromatids reconstituted using Xenopus egg extracts. Remarkably, while the stretching elastic response of the reconstituted chromosomes is similar to that observed for chromosomes from cells, the reconstituted chromosomes are far more easily bent. This result suggests that reconstituted chromatids have a large-scale structure which is quite different from chromosomes in somatic cells. More generally our results suggest a strategy for the use of micromanipulation methods for the study of chromosome structure.
Poirier
M G
MG
Department of Physics, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607-7059, USA.
Marko
J F
JF
D000818:Animals; D001696:Biomechanical Phenomena; D002843:Chromatin / Q000187:drug effects / Q000502:physiology; D002875:Chromosomes / Q000187:drug effects / Q000502:physiology*; D004262:DNA Restriction Enzymes / Q000494:pharmacology; D005057:Eukaryotic Cells / Q000166:cytology; D006801:Humans; D007477:Ions / Q000494:pharmacology; D008836:Micrococcal Nuclease / Q000494:pharmacology; D008846:Micromanipulation / Q000295:instrumentation* / Q000379:methods*; D008938:Mitosis; D008954:Models, Biological
D002843:Chromatin; D007477:Ions; D004262:DNA Restriction Enzymes; D008836:Micrococcal Nuclease