Chapter 8 The Cell Cycle

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1 What molecule stores your genetic information or determines everything about you? DNA a nucleic acid How are DNA molecules arranged in the nucleus?

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3 As you can see DNA is: Chapter 8 The Cell Cycle 1. Arranged in a double helix conformation 2. The double helix is wound around histone proteins which serve to compact the DNA tightly the resulting structures are called nucleosomes 3. The nucleosomes are coiled into even tighter packages 4. The resulting chromatin is a tangled mass of thin threads of DNA 5. During the process of cell division chromatin condenses into highly compacted structures called chromosomes which are visible under light microscopy

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7 What does a cell have to do to reproduce? The Cell Cycle Phases include: 1. Interphase Preparation phases for mitosis 2. Mitosis Cell division or splitting How do you think the cell has to prepare for the division or splitting step? Interphase 1. G 1 (Growth) 2. S 3. G 2 (Growth)

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9 Interphase (G 1 ) Chapter 8 The Cell Cycle Growth & preparation of the cell for division, including an increase in size & the number of organelles During G 1 an enzyme called S-kinase combines with a protein called S-cyclin. This interaction activates the S-kinase which will phosphorylate (Do you remember what this means?) a protein that will initiate the S phase of interphase Interphase (S) Synthesis or doubling of the DNA Each chromosome is duplicated & the resulting sister chromatids are joined at a pinched region called the centromere Interphase (G 2 ) Continued growth & preparation for mitosis During G2, a second enzyme called M-kinase combines with M-cyclin This interaction phosphorylates a protein that will initiate mitosis

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24 Mitosis 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase 5. Cytokinesis Prophase Chapter 8 The Cell Cycle Centrosomes, composed of centrioles & microtubules, migrate toward opposite ends of the cell The nuclear envelope & nucleolus begin to & will disappear by the end of prophase The chromosomes condense & are visible Spindle fibers (microtubules) will form & specialized ones called kinetochore fibers will attach to the centromeres of the duplicated chromosomes Chromosomes are not arranged in any specific orientation

25 Metaphase Chapter 8 The Cell Cycle The kinetochore fibers push & arranged the duplicated chromosomes along the equator of the cell (metaphase plate) Anaphase The sister chromatids are separated from one another forming two daughter chromosomes that move toward the opposite poles of the spindle The chromatids ride the fibers toward the poles like a train on a railroad track Telophase Reverse prophase Spindle fibers disappear Nuclear envelope & nucleolus reappear Chromosomes de-condense back into chromatin

26 Cytokinesis cytoplasmic cleavage to separate the cell into 2 distinct daughter cells Animal cells Begins in late anaphase with the initiation of a cleavage furrow, which is an indentation of the cell membrane It develops due to the constricting action of a band of actin filaments called the contractile ring The ring continues to pinch off the cell until it is separated into 2 daughter cells Plant cells The cell wall prevents this pinching effect as seen in animal cells Instead the Golgi apparatus produces vesicles which move along the microtubules to the mid-point of the cell These vesicles will fuse forming the cell plate, which will eventually form the new cell membrane of the 2 new daughter cells Cell wall will be formed following cell membrane formation

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28 Why is the diploid # of chromosomes in all stages of mitosis the same rather than being doubled due to the S phase of interphase? Or How do you count chromosomes? You count chromosomes by their centromeres Since the duplicated chromosomes are called sister chromatids connected by their centromeres, the total is 4 based on the # of centromeres, even though you see a total of 8 sister chromatids in the previous figure

29 Cell division in prokaryotic cells Chapter 8 The Cell Cycle What do you think a prokaryotic cell has to due to reproduce? Does it have as many moving parts as a eukaryotic cell? Binary fission Since the prokaryotic cell does not have to worry about a nuclear envelope or membrane bound organelles, reproduction in these cells are simpler. Basically, the single loop of DNA is replicated & each copy is attached to the cell membrane at the membrane fold where binary fission begins As the cell pinches in two, the DNA molecules are pulled apart by the moving cell membrane

30 PRACTICE QUESTIONS 1. What is a nucleosome? 2. How does chromatin differ from chromosomes? 3. What is a karyotype? 4. What are the phases of the cell cycle? 5. What are the phases of interphase? What happens during each phase? 6. What are the phases of mitosis? What happens during each phase? 7. What does a kinase do? 8. How do animal & plant cell telophase differ from one another? 9. What is binary fission?