The Cell Cycle Chapter 12

Cell Division (Mitosis) Why does it occur? Renewal and repair; replacing cells that die Allows organisms to grow and develop from a single cell (zygote) A means of reproduction (asexual) for some organisms such as amoebas and plants Regeneration Budding Vegetative reproduction Produces 2 identical cells

The Cell Cycle

The Cell Cycle Interphase M Phase G1 Mitosis S G2 Cytokinesis Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis BioFlix: Mitosis

Cell Control Systems Based on the cyclical concentrations of a set of molecules in the cell that both triggers and coordinates key events in the cell cycle Works on its own according to a “built-in clock” The clock has specific checkpoints where the cell cycle stops until a “go-ahead” signal is received Go-ahead signals are transmitted by signal transduction pathways

Checkpoints A “go-ahead” will only be given if crucial cellular processes that should have occurred by that point in the cycle have indeed occurred Checkpoints are found in the G1, G2, and M phases For mammalian cells the G1 checkpoint is the most important If the checkpoint is not passed the cell will NOT continue and go into a G0 phase (a nondividing state)

G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint

Cell Cycle Control Molecules Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) As the concentrations of clyclins fluctuate, the activity of Cdks fluctuate The G1 checkpoint is controlled by at least 3 Cdk proteins and several different cyclins MPF (maturation-promoting factor) is a cyclin- Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

See Fig. 12.17

Cell Cycle Control (Internal Signals) The M phase checkpoint ensures that all the kinetochores are properly attached to the spindle at the metaphase plate before anaphase. A signal to delay anaphase originates at kinetochores that have not yet attached to spindle microtubules.

Cell Cycle Control (External Signals) Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide For example, platelet- derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture

Cell Cycle Control (External Signals) In a living organism, platelets release PDGF in the vicinity of an injury. The resulting proliferation of fibroblasts help heal the wound.

Cell Cycle Control (External Signals) Another example of an external signal is density-dependent inhibition, in which crowded cells stop dividing Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide

Cancer Cells Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence

Loss of Cell Cycle Controls in Cancer Cells Cancer cells do not respond normally to the body’s control mechanisms Why? They may make their own growth factor They may convey a growth factor’s signal without the presence of the growth factor They may have an abnormal cell cycle control system

Cancer Cell Growth A normal cell is converted to a cancerous cell by a process called transformation Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue If abnormal cells remain at the original site, the lump is called a benign tumor Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors

Breast Cancer