8 1 The Basics of Cellular Reproduction 8 1 The Basics of Cellular Reproduction cont One element of the cell theory is that all cells arise from pre existing cells Cellular reproduction is also the process by which unicellular organisms reproduce The cells of multicellular organisms are produced by trillions of episodes of cellular reproduction originating from a single cell Reproduction through cellular reproduction is called asexual reproduction Cellular reproduction also replaces wornout or damaged cells in the body 8 1 The Basics of Cellular Reproduction cont Chromosomes Cellular reproduction involves two important processes Growth Cell division Growth involves duplication of the cell s DNA and contents Cell division involves the separation of the cell s DNA and content into daughter cells Chromosomes cont The passage of DNA to daughter cells is a critical aspect of cellular reproduction Because the DNA of cells is extremely long the DNA is packaged with proteins into easily managed units called chromosomes Chromatin to Chromosomes When a cell is not dividing the DNA and its associated proteins are uncoiled in thin strands called chromatin Within chromatin the DNA is coiled around cores of eight proteins called histones Each DNA protein unit is called a nucleosome 1 Chromatin to Chromosomes Chromatin to Chromosomes cont cont Prior to the division of a cell several events occur The chromatin condenses to form chromosomes The chromosomes undergo DNA replication producing an identical copy of the DNA The two halves the original DNA and the copy are attached via the centromere 8 2 The Cell Cycle The process of cellular reproduction involves an ordered series of steps called the cell cycle The cell cycle spans the period of time from the production of a daughter cell to the cellular reproduction of that cell to produce two new daughter cells Interphase Interphase A cell spends most of its existence in a phase of the cell cycle called interphase Most of interphase involves the normal activities and functions of that cell Part of interphase is spent preparing the cell for cell division Interphase cont Interphase has three stages During the G1 phase the cell doubles the number of organelles and accumulates resources for DNA replication DNA replication begins during the S phase of interphase to create the duplicate DNA strand The original and duplicate DNA strand are called sister chromatids During the G2 phase the cell synthesizes the proteins needed for cell division 2 M Mitotic Stage The division of the cell occurs during the M mitotic stage of the cell cycle The division of the nuclear material DNA is called mitosis The division of the cytoplasm and its contents is called cytokinesis 8 3 Mitosis and Cytokinesis The separation of the sister chromatids during mitosis produces two genetically identical daughter chromosomes The daughter cells produced by mitosis have the same number of chromosomes as the original cell each of which has the identical content 8 3 Mitosis and Cytokinesis The Spindle cont In most eukaryotic cells the separation of identical chromatids during mitosis utilizes spindle fibers Spindle fibers are assembled from the microtubule proteins of the cytoskeleton The spindle fibers are organized by the centrosome The Spindle cont In animal cells centrosomes consists of two parts Two centrioles An array of microtubules called an aster The centromeres migrate prior to mitosis to opposite poles of the nucleus A spindle attaches to each centrosome and stretches across the nucleus overlapping at the spindle equator The Spindle cont The spindles attach to each duplicated chromosome to facilitate the separation of the sister chromatids The separation of the sister chromatids during mitosis occurs in four phases Prophase Metaphase Anaphase Telophase 3 Phases of Mitosis in Animal Cells cont Phases of Mitosis in Animal Cells Although divided into four phases the separation of sister chromatids during mitosis is a continuous process Cytokinesis the separation of the cytoplasm and its contents occurs after mitosis The cell cycle is completed after cytokinesis Phases of Mitosis in Animal Cells cont Cytokinesis in Animal and Plant Cells Cytokinesis does not always occur after mitosis If cytokinesis does not occur the cell will be multinucleated have multiple nuclei When cytokinesis does occur it occurs differently in animal and plant cells Cytokinesis in Animal Cells cont Cytokinesis in Animal Cells Cytokinesis in animal cells begins during anaphase as a cleavage furrow an indentation of the membrane Actin filaments constrict to deepen the furrow until the cytoplasm is separated between the two daughter cells 4 Cytokinesis in Plant Cells Cytokinesis in Plant cells cont Plant cells undergo cytokinesis by forming a new cell wall between the daughter cells The Golgi apparatus produces vesicles that fuse to form the cell plate The cell plate expands until the cytoplasm is divided 8 4 The Cell Cycle Control System Cell Cycle Checkpoints The cell cycle is tightly regulated to insure that it is completed correctly The cell cycle has checkpoints that control the progression of the cell cycle The control system of the cell cycle insures that the stages of interphase occur sequentially in the correct order The G1 checkpoint is important because passing this point commits the cell to division If a cell does not pass the G1 checkpoint it may be held in G0 Cell Cycle Checkpoints cont Cell Cycle Checkpoints cont The G2 checkpoint is the point at which the cell cycle pauses until DNA replication has been completed A checkpoint also occurs during mitosis when division pauses until the chromosomes are distributed accurately to the daughter cells 5 8 5 The Cell Cycle and Cancer Cancer is a disease of the cell cycle The regulation of the cell cycle is lost and uncontrolled cell division occurs Cancers are classified by their location Carcinomas are cancers of the organs Sarcomas are cancers of the muscles Leukemias are cancers of the blood Characteristics of Cancer Cells Carcinogenesis is the development of cancerous cells with specific characteristics Cancer cells lack differentiation meaning that they have failed to acquire the specialized structure or function that the cell should have Cancer cells have abnormally large nuclei and or nuclei with an abnormal number of chromosomes Characteristics of Cancer Cells Characteristics of Cancer Cells cont cont Cancer cells form a mass of dividing cells