Cell Growth and Division Chapter 5

Why do cells divide? (3 reasons) DNA “Overload” – cells don’t have extra copies of DNA so large cells don’t have enough instructions Inadequate Cell Surface Area All materials must pass through membrane Distance for diffusion Large cells need more food; can’t absorb enough nutrients to maintain homeostasis Surface area to volume ratio

Why do cells divide? 3. To become a multicellular organism And later to replace worn out cells

How can the problem of “not enough surface area” be solved?

Series of steps to form two daughter cells Cell Division: Series of steps to form two daughter cells

Cell Division DNA must be replicated (copied) Each piece of DNA  chromosome

Cell Division Identical copies of DNA  sister chromatids Held together by a centromere Chromosome

Series of events that cells go through as they grow and divide Cell Cycle Series of events that cells go through as they grow and divide

Four phases in the Cell Cycle M phase G2: doubles organelles G1: growth of cell S phase: DNA replication

Four Phases of the Cell Cycle Mitosis: Division of nucleus Cytokinesis: Division of the cytoplasm G1, S, and G2: make up “Interphase”

Interphase 75% of cell’s life spent here “Inter” = between Centrioles FYI: Cannot see individual chromosomes during interphase Nucleolus

Stages of Mitosis

Prophase First and longest phase of mitosis Spindle Fibers First and longest phase of mitosis Chromosomes shorten and thicken (become visible) Sister Chromatids

Prophase Nucleolus and nuclear membrane disappear Centrioles move apart Chromosomes attach to spindle fibers… push or pull chromosomes into middle

Metaphase Equatorial Plate Second and shortest phase Chromosomes line up across the middle Equatorial Plate FYI: Chromosomes are at their shortest and thickest.

Anaphase Centromeres split Sister chromatids pulled apart  now called chromosomes

Telophase Chromosomes Nuclear membrane reforms reach poles, start to uncoil and lengthen Nuclear membrane reforms Spindle fibers break down Nucleolus reappears Two nuclei with same chromosomes

Cytokinesis Not a stage of mitosis Same time as telophase Division of the cytoplasm and organelles

Cytokinesis Plant Cells Animal cells grows from middle Called the cell plate Animal cells pinch inward – forms cleavage furrow

End Product of Mitosis: Produces 2 identical diploid daughter cells Same number of chromosomes (2N)

Rates of Cell Division Rapid Rarely Skin Digestive tract Bone marrow Replace worn out or broken down cells Rarely Muscle Nerve (slowest) Stay in G0 phase – don’t grow or divide

Controlling Cell Growth Stops growing when touching another cell Can resume when space is found This is called contact inhibition Stops when can’t absorb enough nutrients Cells can ignore signals to stop growing Grow uncontrollably Typical of cancer cells

Checkpoints M Checkpoint Attached to spindle fibers G2 Checkpoint All organelles doubled G1 Checkpoint Checks for enough nutrients Checks for DNA damage

What triggers the cell cycle? Cyclin protein that regulates cell division Internal Regulators External Regulators From outside Received by carbohydrate antennas Transmitted to inside

Abnormal Cells Do not respond to signals May result in cancer Caused by Smoking (chemicals) Radiation Viral infections Defect in the p53 gene that normally tells a cell to stop dividing

Unspecialized Cells Stem cells…stay in G0 Develop into any type of cells Source- bone marrow umbilical cord blood Embryonic cells Potential uses: Repairing spinal cord injuries Repairing organs Growing replacement parts