The Cell Cycle :The Cell Cycle BIOLOGY


Key Terms :Key Terms Genome—the total complement of an organism’s genes Chromatin—network of DNA and fibrils in a nucleus that is not dividing. Chromosome—structure of DNA that transmits genetic information to the next generation. Chromatids—two copies of the same chromosome attached together Centromere—constriction where sister chromatids of a chromosome are held together. Chromatin in a developing salamander ovum


Chromosome Numbers :Chromosome Numbers Somatic cells (body cells)— diploid (2N) 2 of each chromosome Gametes (sex cells)— haploid (N) 1 of each chromosome Examples: Body cells Sex Cells Humans 46 23 Fern 1,262 631 Drosophila 8 4 Dog 78 39


Mitosis :Mitosis A diploid cell divides to produce diploid daughter cells. Ensures that each daughter cell contains the exact same chromosomes as the original mother cell. Mitosis can occur in either diploid or haploid cells; maintaining either a 2N or N chromosome number.


Interphase :Interphase Most of the cell cycle is spent in interphase. G1 - organelles double; accumulates materials for DNA synthesis. S - DNA replication. G2 - Cell synthesizes proteins necessary for cell division.


Prophase & Prometaphase :Prophase & Prometaphase Prophase Chromatin has condensed. Nucleolus disappears. Nuclear envelope disintegrates. Spindle begins to assemble. Prometaphase Kinetochores develop on centromere. Attach sister chromatids to spindle fibers.


Figure 12.5 The stages of mitotic cell division in an animal cell: G2 phase; prophase; prometaphase :Figure 12.5 The stages of mitotic cell division in an animal cell: G2 phase; prophase; prometaphase


Metaphase and Anaphase :Metaphase and Anaphase Metaphase Chromosomes, attached to kinetochore fibers, are in alignment at center of cell. Anaphase Sister chromatids split, producing daughter chromosomes. Daughter chromosomes are pulled to opposite poles.


Telophase :Telophase Telophase Spindle disappears as new nuclear envelopes form around the daughter chromosomes. Chromosomes become diffuse chromatin again. Nucleolus appears in each daughter nucleus.


Cytokinesis :Cytokinesis Animal Cells: Cleavage furrow begins as anaphase nears completion. Deepens when a contractile ring forms a constriction between the two daughter cells. Mitosis without cytokinesis results in a multinucleated cell.


Cytokinesis :Cytokinesis Plant Cells: Rigid cell wall surrounding plant cells does not permit cytokinesis by furrowing. Begins with formation of a cell plate which eventually becomes new plasma membrane between the daughter cells.


Figure 12.9 Mitosis in a plant cell :Figure 12.9 Mitosis in a plant cell


Figure 12-09x Mitosis in an onion root :Figure 12-09x Mitosis in an onion root


Cell Cycle Control :Cell Cycle Control Protein (cyclin) increases & decreases as cell cycle continues. Must be present for the cell to proceed from the G2 stage to the M stage and from G1 stage to S stage. Allows time for any damage to be repaired.


The Cell Cycle and Cancer :The Cell Cycle and Cancer Cancer is a growth disorder that results from the mutation of genes regulating the cell cycle. Lack differentiation. Have abnormal nuclei. Form tumors. Loss of contact inhibition. Undergo metastasis (spread).


Apoptosis :Apoptosis Often defined as programmed cell death. Cells routinely harbor enzymes (caspases) necessary for apoptosis to occur. Ordinarily held in check by inhibitors, but can be unleashed by internal or external signals. Mitosis increases and apoptosis decreases the number of somatic cells.