OVERVIEW OF THE CELL CYCLE -Chromosome duplication occurs during S phase (S for DNA synthesis), which requires 10–12 hours and occupies about half of the cell-cycle time in a typical mammalian cell.- After S phase, chromosome segregation and cell division occur in M phase (M formitosis), which requires much less time (less than an hour in a mammalian cell). -M phase comprises two major events: nuclear division, or mitosis, during which the copied chromosomes are distributed into a pair of daughter nuclei; and cytoplasmic division, or cytokinesis, when the cell itself divides in two (Figure 17–2). -At the end of S phase, the DNA molecules in each pair of duplicated chromosomes are intertwined and held tightly together by specialized protein linkages.
Cell-Cycle Control Is similar in all Eukaryotes -Several model organisms are used in the analysis of the eukaryotic cell cycle. oThe budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe are simple eukaryotes (molecular and genetic approaches can be used to identify and characterize the genes and proteins).oThe early embryos of certain animals:frog Xenopus laevis, excellent tools for biochemical dissection of cell-cycle control mechanismsFruitfly Drosophila melanogaster useful for the genetic analysis of mechanisms underlying the control and coordination of cell growth and division in multicellular organisms. Cultured human cells provide an excellent system for the molecular and microscopic exploration of the complex processes by which our own cells divide. Cell-Cycle Progression Can Be studied in Various Ways -To see what stage a cell is in the cell cycle you have to look at living cells with a microscope. -Mammalian cells proliferating in culture reveals that a fraction of the cells have rounded up and are in mitosis. -Looking at budding yeast cells under a microscope is very useful, because the size of the bud provides an indication of cell-cycle stage.-We can gain additional clues about cell-cycle position by staining cells with DNA-binding fluorescent dyes (which reveal the condensation of chromosomes in mitosis) or with antibodies that recognize specific cell components such as the microtubules (revealing the mitotic spindle). -S-phase cells can be identified in the microscope by supplying them with visualizable molecules that are incorporated into newly synthesized DNAoartificial thymidine analog bromodeoxyuridine (BrdU); cell nuclei that haveincorporated BrdU are then revealed by staining with anti-BrdU antibodies.