in other vertebrates. Degradation Multicellular organisms replace worn-out cells through cell division. In some animals, however, cell division eventually halts. In humans this occurs, on average, after 52 divisions, known as the Hayflick limit. The cell is then referred to as senescent. With each division the cells telomeres, protective sequences of DNA on the end of a chromosome that prevent degradation of the chromosomal DNA, shorten. This shortening has been correlated to negative effects such as age-related diseases and shortened lifespans in humans. Cancer cells, on the other hand, are not thought to degrade in this way, if at all. An enzyme complex called telomerase, present in large quantities in cancerous cells, rebuilds the telomeres through synthesis of telomeric DNA repeats, allowing division to continue indefinitely. History A cell division under microscope was first discovered by German botanist Hugo von Mohl in 1835 as he worked over the green alga Cladophora glomerata. In 1943, cell division was filmed for the first time by Kurt Michel using a phase-contrast microscope. See also Cell fusion gametic fusion Cell growth Cyclin-dependent kinase Labile cells, cells that constantly divide Mitotic catastrophe References Further reading Morgan HI. (2007). "The Cell Cycle: Principles of Control" London: New Science Press. J.M.Turner Fetus into Man (1978, 1989). Harvard University Press. Cell division: binary fission and mitosis McDougal, W. Scott, et al. Campbell-Walsh Urology Eleventh Edition Review. Elsevier, 2016. The Mitosis and Cell Cycle Control Section from the Landmark Papers in Cell Biology (Gall JG, McIntosh JR, eds.) contains commentaries on and links to seminal research papers on mitosis and cell division. Published online in the Image & Video Library of The American Society for Cell Biology The Image & Video Library of The American Society for Cell Biology contains many videos showing the cell division. The Cell Division of the Cell Image Library Images : Calanthe discolor Lindl. - Flavon's Secret Flower Garden Tyson's model of cell division and a Description on BioModels Database WormWeb.org: Interactive Visualization of the C. elegans Cell Lineage - Visualize the entire set of cell divisions of the nematode C. elegans Cell cycle Cellular processes Telomeres 1835 in science 1835 in Germany