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AP Biology Readers Revisit Mitotic Spindle Formation
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by José Vázquez
New York University
New York, New York
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| | Exciting Discoveries in Cell Division
During the 2003 Reading that took place at the University of Nebraska-Lincoln, AP Biology Readers had the opportunity to listen to Dr. Kristen Johansen talk about the structural reorganization of nuclear proteins during mitosis. Dr. Johansen, a professor of biochemistry, cell, and developmental biology at Iowa State University, reminisced about her own experience as an AP Biology student and how the foundations acquired in the course paved the way for a career in biological research.
Her presentation focused on some novel proteins cloned and characterized in her lab that have a crucial role in the mitotic spindle formation and that are part of the spindle matrix, which has been proposed to help in the organization and stability of the spindle during mitosis. Using Drosophila embryos, the protein skeletor shows association with the chromosomes at interphase but eventually redistributes itself into a spindle structure during prophase immediately preceding metaphase. The structure of skeletor does not show any obvious structural motif, which indicates its association in a multi-protein complex fashion.
Further analyses led to the identification of another protein named chromator, which interacts with skeletor and also shows a cell cycle specific pattern. This in turn led to the identification of a third protein, megator, which co-localizes with skeletor during mitosis. Additional studies in Dr. Johansen's lab implicate megator in the composition of the spindle matrix.
At the organism level when homozygous fruit fly embryos show reduced levels of megator they die before hatching. These results suggest that megator is an essential protein for the repeated mitotic events that take place during fruit fly embryogenesis. Moreover, mitosis in mutant embryos leads to defects in skeletor localization and spindle formation, which in turn leads to a variety of abnormal phenotypes. These data point toward a clear identification of three nuclear components that participate in the formation of the spindle matrix required for normal mitosis.
In addition, the proteins characterized in her lab seem to play different roles in later stages of the cell cycle. For instance, it has been observed that some of the chromatin is not involved in the cell division process, and instead is located at the periphery of the spindle matrix. Dr. Johansen concluded her presentation by emphasizing that her results as well as those from other labs are going to change our view about the plasticity of DNA during metaphase and the rest of the cell cycle. She also remarked that ongoing studies might change the information presented in textbooks in the coming years about the nature of events that take place in cell division.
Further Reading
Johansen, K.M. and J. Johansen. "Recent Glimpses of the Elusive Spindle Matrix," Cell Cycle 1 (2002): 312-314.
Walker, D.L., D. Wang, Y. Jin, U. Rath, Y. Wang, J. Johansen, and K.M. Johansen. "Skeletor, a Novel Chromosomal Protein That Redistributes during Mitosis Provides Evidence for the Formation of a Spindle Matrix," Journal of Cell Biology 151 (2002): 1401-1411.
Johansen, K.M., J. Johansen, Y. Jin, D.L. Walker, D. Wang and Y. Wang. "Chromatin Structure and Nuclear Remodeling," Critical Reviews in Eukaryotic Gene Expression 9 (1999): 267-277.
José Vázquez teaches science in the general studies program at New York University. He completed his undergraduate work in biology at the University of Puerto Rico and his graduate work in cell and developmental biology at the University of Pennsylvania. José taught AP Biology for three years in Puerto Rico and another three years at Northwestern University's Center for Talent Development in Evanston, Illinois. In addition to being an AP Reader since 1998, he is the classroom technology reviews editor for American Biology Teacher.
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