The Cell Cycle

  • PHASES OF THE CELL CYCLE

  • REGULATION OF THE CELL CYCLE, CANCER, APOPTOSIS

OVERVIEW

  • cell cycle is important in the growth, repair, and reproduction of cells in living organism

  • In eukaryotes, cells divide and transmit genetic information via the cell cycle, a highly regulated series of events for the growth and reproduction of cells

PHASES OF THE CELL CYCLE

  • interphase (G1, S, and G2)

  • mitosis (M phase)

  • cytokinesis

  • nondividing cells will leave the cell cycle and enter G0

INTERPHASE

  • longest phase of the cell cycle

  • cell grows so that it has enough material to divide between two daughter cells

  • cell also replicates its genetic material (DNA)

    • G1

      • cell grows and prepares for the replication of DNA and some cellular organelles are replicated

    • S (synthesis)

      • DNA is replicated

      • when it begins, each chromosome consists of one chromatid

      • after the replication, each chromosome has two identical chromatids held together by one centromere

      • at the end of S stage, the cell contains x2 the amount of DNA it had at the end of G1

    • G2

      • the cell continues to grow and prepares the materials needed for mitosis

        • such as the proteins that will make up the spindle fibers

MITSOSIS (M phase)

  • goal is to make sure there is an accurate transfer of a parent cell’s complete genome to each of the two resulting daughter cells

  • mitosis plays a role in growth, tissue repair, and asexual reproduction

  • mitosis alternates with interphase in the cell cycle.

  • 4 stages: prophase, metaphase, anaphase, and telophase

    • prophase

      • the nuclear membrane dissolves and the chromosomes condense and become visible

      • spindle fibers begin to form

    • metaphase

      • spindle fibers have fully attached to the centromeres of each chromosome

      • chromosomes are then aligned along the “equator” of the cell in a single column

      • metaphase plate

        - the center of the mitotic spindle

    • anaphase

      • each chromosome splits as its centromere as opposing spindle fibers begin to shorten

      • the identical chromatids are pulled toward opposite ends of the cell

      • each chromatid no has its own centromere and is considered a separate chromosome

      • in the end, the cell has x2 the number of chromosomes that it had originally

    • telophase

      • two nuclear membranes form

      • each of the two nuclei now contain the same number of chromosomes and the same genetic information as the parent cell

CYTOKINESIS

  1. the division of the cytoplasm and all of its cellular contents between the two daughter cells

  2. in animal cells, a cleavage furrow is formed, which parts the cytosol and its contents between the two new cells

  3. in plant cells, a cell plate is built within the dividing cell

    • providing new cell wall material for each daughter cell

NONDIVING CELLS

  • some cells may stop dividing temporarily or permanently

  • they stop dividing when they reach their mature, fully differentiated state or when environmental conditions are not favorable for continued growth

  • the nondividing cells have exited the cell cycle and are in G0

  • it can reenter the cell cycle in response to appropriate cues

  • nondividing cells may exit the cell cycle or be held at a particular stage in the cell cycle.

REGULATION OF THE CELL CYCLE, CANCER, AND APOPTOSIS

  • proper regulation of the cell cycle is important to appropriate growth, repair, and reproduction of cells

  • use checkpoints to regulate

  • some checkpoints are controlled by interactions between cyclins and cyclin-dependent kinases

  • cyclin-dependent kinases

    • present at constant levels throughout the cell cycle

    • these kinases add phosphate groups to other molecules activating those molecules

    • inactive until they are bound to cyclin proteins

      • levels of cyclin proteins vary during the cell cycle

      • reaches maximum just before mitosis starts


DISRUPTIONS TO CELL CYCLE

  • disruptions to the cell cycle may result in cancer and/or apoptosis (programmed cell death)

  • cancer

    • uncontrolled cell growth

    • cancer cells are not regulated so they can continue to grow and divide under conditions that would cause normally functioning somatic cells to stop dividing

  • apoptosis

    • sometimes a living organism’s survival depends on some cells dying and not reproducing

    • triggered when a cell acquires a mutation that could cause cancer