 Understand why cells divide  Describe the events of the cell cycle  Draw diagrams representing the stages of the cell cycle

 To heal or repair tissue  Growth of multi-cellular organisms  Reproduction in unicellular organisms  To keep cell sizes small for increased efficiency

 DNA overload › Extra copies are not created with growing cell  Ratio of surface area to volume › Smaller cells = greater ratio › More “stuff” can move in and out of the cell

 Requires › Division of the nucleus (genetic material) = MITOSIS › Division of cytoplasm = CYTOKINESIS

 Takes up most of cell cycle ~ 90%  Divided into three phases – G 1, S, G 2  When finished G 2, cell is ready to enter M phase = Mitosis

 G 1 – Cells do most of their growing › Produce new proteins & organelles  S – DNA replication occurs (very important!)  G 2 – Shortest part of interphase  Organelles & molecules associated with cell division are produced Cell is now ready to enter ‘M phase’ = Mitosis

 Divided into 4 phases › Prophase › Metaphase › Anaphase › Telophase

 First phase of mitosis › ~ 50-60% of total mitosis time › Chromatin becomes condensed & visible = chromosomes › Centrioles move to opposite sides of nucleus & form spindle

› Nucleolus disappears › Nuclear envelope breaks down

 Very short, often lasts just a few minutes  Chromosomes line up across center of cell = equator or metaphase plate  Microtubules connect centromere of each chromosome to the two poles of the spindle

 Centromeres split, allowing sister chromatids to separate into individual chromosomes.  Ends when chromosomes stop moving near the poles of the spindle

 Final stage of mitosis › Nuclear envelope reforms › Chromosomes begin to unwind › Spindle begins to break apart › Nucleolus becomes visible

 Usually occurs at the same time as telophase  Cytokinesis = division of cytoplasm  Different process in animal cells and plant cells

 Animal cells › Membrane gets drawn inward until cytoplasm is pinched in two  Plant Cells › Cell plate forms between two nuclei  Plate gradually develops into a separating membrane › New cell wall begins to appear in the cell plate

Animal Cell Plant Cell

 Assignment: How long is each phase relative to the others?

 Controls on cell division › Cyclins (1980’s) – a group of closely related proteins that regulate the timing of the cell cycle › More recently, other “regulatory proteins” have been discovered that help regulate the cell cycle  Internal regulators  External regulators

 Internal regulators – proteins that respond to events inside the cell. › Allow cell cycle to proceed only when certain processes have happened  Example:  Certain proteins ensure cycle doesn’t continue until all chromosomes have been duplicated  Another protein prevents that start of anaphase until all chromosomes are attached to a spindle

 External regulators – proteins that respond to events outside the cell › Direct cells to either speed up or slow down the cell cycle › Growth factors - especially important during embryonic development and wound healing › Molecules on nearby cells have opposite effect – cause cells to slow or stop cell cycle.  Prevent excessive growth and prevent tissues from disrupting one another.

 Cancer › A lot of cancer cells have defect in gene which normally halts the cell cycle until all DNA has been replicated › Cells lose the ability to control growth which causes the cells to grow uncontrollably How would the perfect cancer drug function?

 Test: › Wednesday, November 2, 2011 › Review: Mitosis Dry Lab – Due Wed morning