Chapter 9 Study Guide - Study guide 9 UNIT II: - Cell...

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Study guide 9 UNIT II: - Cell Structure & Function - Respiration and Fermentation - Photosynthesis - Cell division Cell Division The first events of cell division to be discovered, in the 1880's, were those of mitosis. For many years, the dramatic changes in chromosome morphology and the separation of duplicate chromosomes were all that were known about how cells divide. And mitosis only takes about one hour out of 20 in the life cycle of a cell! In fact, the cell cycle consists of several very active phases that include time for growth and for DNA replication as well as preparation for division . . Bacterial Cell Division In contrast to the eukaryotic cell cycle, the events of cell division in bacteria are not separated into discrete steps ( Box 11.2, Fig. 11.8 ); the processes of DNA synthesis, growth and division are continuous and overlapping in actively growing bacterial cultures, and cell doubling time can be as short as 20 minutes. There is no separation in time between the replication of DNA and the actual division of the cell ( binary fission ) into two daughter cells. The newly synthesized double stranded DNA molecules are separated into the daughter cells because they are attached to the cell membrane on either side of the growing points of a dividing cell. . The Eukaryotic Cell Cycle In an elegant experiment, cultured cells were exposed to radioactive thymidine for a few minutes (a ‘ pulse ’). The cells were then spread on a slide, coated with a radiation-sensitive film, exposes and developed to produce an autoradiograph . Fig. 11.4 shows that only a few cells were making DNA during the time the ‘hot ’ thymidine was present, and these cells were not in the process of mitosis. If after short pulse of exposure to the hot thymidine, the label was removed and the cells were allowed to keep growing, it was possible to show that the DNA made during the pulse labeling period showed up in mitosing cells only after 4 hours of ‘chase’, meaning that there was a 4 hour gap between the end of DNA synthesis and the beginning of mitosis. Similar experiments showed that cells synthesized DNA for about 9-10 hours of their cycle ( Fig. 11.4 ). For a cell that divided every 20 hours, the phases of the cell cycle could be deduced: S phase (9 hr of DNA synthesis) followed by G2 phase (a 4 hour gap with no DNA synthesis) followed M & C (1-2 hr of M ytokinesis) followed by the G1 phase (a gap of 5-6 hours between the end of cell division and the start of DNA synthesis). A typical cell cycle is shown in
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This note was uploaded on 11/22/2009 for the course BIO SCI 150 taught by Professor Geraldbergstrom during the Spring '08 term at Wisconsin Milwaukee.

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Chapter 9 Study Guide - Study guide 9 UNIT II: - Cell...

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