Results in the formation of two identical cells consists of nuclear division

Results in the formation of two identical cells

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Results in the formation of two identical cells, consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis) Process visible under microscope Nuclear Division (Mitosis) has four stages i. Prophase: chromatin fibres condense and shorten into chromosomes preventing entangling of DNA; each prophase chromosomes consists of a pair of identical strands called chromatids; a region called the centromere holds the chromatids together; at the outside of the chromatid is the kinetochore, a protein complex; a mitotic spindle is formed by tubules in pericentriolar material of the centrosomes and connects
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to the kinetochore of the the chromatids; as the microtubules lengthen the push the centrosomes to the poles of the cell making the spindle extend from pole to pole of the cell thus the spindle is responsible for separating chromatids to opposite poles; at this point the nucleus disappears and the nuclear envelope breaks down ii. Metaphase: microtubules of the spindle align the centromeres of the chromatid pairs at the exact centre of the spindle; the midpoint region is known as the metaphase plate iii. Anaphase: centromeres split; separated chromatid pairs move towards poles of cell; chromatids now separated and referred to as chromosomes pull the microtubules of the spindle causing them to appear V- shaped as the centromeres lead the way iv. Telophase: begins after chromosomes stop moving; the chromosomes uncoil and revert to threadlike chromatin form; nuclear envelope forms around each chromatin mass; the nucleoli reappear in the identical nuclei and the mitotic spindle breaks up Cytoplasmic Division (Cytokinesis) o Begins during anaphase with the formation of a cleavage furrow, a slight indentation of the plasma membrane o It is completed after telophase o Actin microfilaments in the plasma membrane form a contractile ring that pulls the membrane inward and ultimately pinches it in two o The furrow is always perpendicular to the mitotic spindle ensuring the chromosomes end up in separate cells o When cytokinesis is complete interphase begins Control of Cell Destiny Cell has 3 possible destinies: 1. Remain alive and function without dividing 2. Grow and divide 3. Die Homeostasis is maintained when these destinies are balanced Within cells there are enzymes called cyclin-dependent protein kinases that transfer a phosphate group from ATP to a protein to activate the protein while other enzymes remove the phosphate group to deactivate it - this activation and de-activation at the appropriate time are crucial to cell division Cellular proteins called cyclins are responsible for switching on and off the cdk, cyclin is important for determining the timing and sequence of events Necrosis is a pathological type of cell death that results from tissue injury Reproductive Cell Division The mechanism that produces gametes and consists of a special two-step division called meiosis in which a number of chromosomes in the nucleus is reduced by half Gametes contain a single set of 23 chromosomes and thus are haploid cells, only fertilization restores the diploid number of chromosomes Meiosis occurs in two successive stages: meiosis I and
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