mitosis & meiosis

    Cards (42)

    • Mitosis is part of a precisely controlled process known as the cell cycle.
    • The cell cycle is the regulated sequence of events that occurs between one cell division and the next.
    • The cell cycle has three phases: interphase, nuclear division (mitosis), and cell division (cytokinesis).
    • The length of the cell cycle is very variable depending on environmental conditions, the cell type and the organism.
    • Onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly).
    • The movement from one phase to another in the cell cycle is triggered by chemical signals called cyclins.
    • During Interphase the cell increases in mass and size and carries out its normal cellular functions such as synthesising proteins and replicating its DNA ready for mitosis.
    • Nuclear division (mitosis) follows interphase and is referred to as the M phase.
    • Cell growth stops during the M phase.
    • Cytokinesis follows M phase and once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells.
    • In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells, a new cell wall is formed.
    • Interphase consists of three phases: G1 phase, S phase, and G2 phase.
    • It is at some point during the G1 phase a signal is received telling the cell to divide again.
    • The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids) during the S phase.
    • The S phase is relatively short.
    • The gap between the previous cell division and the S phase is called the G1 phase.
    • Cells make the RNA, enzymes and other proteins required for growth during the G1 phase.
    • Between the S phase and next cell division event the G2 phase occurs.
    • During the G2 phase, the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired.
    • Other preparations for cell division are made (eg production of tubulin protein, which is used to make microtubules for the mitotic spindle) during the G2 phase.
    • Interphase = G1 + S + G2.
    • Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus, having the same number of chromosomes as the parent cell.
    • Mitosis can be divided into four main stages: Prophase, Metaphase, Anaphase, and Telophase.
    • Most organisms contain many chromosomes in the nuclei of their cells, for example, humans have 46.
    • During Prophase, chromosomes condense and are now visible when stained.
    • The chromosomes consist of two identical chromatids called sister chromatids, each containing one DNA molecule, that are joined together at the centromere.
    • The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles.
    • Spindle fibres (protein microtubules) begin to emerge from the centrosomes.
    • The nuclear envelope (nuclear membrane) breaks down into small vesicles during Prophase.
    • The nucleolus disappears during Prophase.
    • During Metaphase, centrosomes reach opposite poles.
    • Spindle fibres (protein microtubules) continue to extend from centrosomes during Metaphase.
    • Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles during Metaphase.
    • Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres during Metaphase.
    • The attachment of chromosomes to spindle fibres during Metaphase involves specific proteins called kinetochores.
    • During Anaphase, the sister chromatids separate at the centromere (the centromere divides in two).
    • Spindle fibres (protein microtubules) begin to shorten during Anaphase.
    • The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules) during Anaphase.
    • Telophase occurs after Anaphase, during which chromosomes arrive at opposite poles and begin to decondense.
    • Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes during Telophase.
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