1. Cell Cycle & Mitosis

2. Cell Cycle
Cell cycle ~ repeating cycle of events in the life of a cell.
Three major phases of the cell cycle:
1. Interphase
2. Mitosis (includes several subphases) = dividing the nucleus
3. Cytokinesis = dividing the cytoplasm

3. Interphase
Interphase ~ period of time between cellular divisions. The cell carries on its normal functions, and grows and matures during this time. It will replicate a full set of the chromosomes towards the end of interphase to prepare for cell division.
Interphase has three phases:
1. G1 ~ (gap) time between cell division and DNA replication. The time when the cell grows & matures.
2. S ~ (synthesis) DNA is replicated
3. G2 ~ cell produces proteins and molecules needed for mitosis.

4. Purpose of Mitosis
1. Development and growth in multicellular organisms.
2. Repair by replacing damaged cells. Injured parts of the body are repaired by the production of new cells.
3. Asexual reproduction ~ producing a new organism involving only mitotic cell division. Used in plant and many unicellular organisms like bacteria.

5. Asexual Reproduction
A. Fragmentation ~ when a colonial organism is broken in two, and the missing cells are replaced by mitosis.
B. Budding ~ producing a new small organism on the side of the parent.
C. Spore ~ a cell or cells with a hard protective covering, which will open when conditions are favorable & the cells will form a new organism by mitosis

8. Mitosis
Mother cell ~ cell that has replicated its DNA and is ready to go through mitosis.
Mitosis ~ process in which the copies of the genetic material are divided into two sets so that each daughter cell is identical to the original cell.

9. Mitosis Four phases of mitosis: I. Prophase
A. Chromatin condenses into chromosomes consisting of sister chromatids joined together at the centromere.
B. Special proteins attach to the centromere to form the kinetochore (point of attachment for spindle fibers).
C. Nuclear membrane disintegrates.
D. Centrosomes (organelle that organizes the microtubules) migrate to opposite poles of the nucleus by motor proteins on microtubules in the cytoplasm. They then form mitotic spindle that will direct movements of the chromosomes.
E. Mitotic spindle begins moving sister chromatids toward the center of the cell.
Two types of spindle fibers:
1. kinetochore fibers ~ extend from centrosomes attach to the kinetochore of the centromere
2. polar fibers ~ extend out from one centrosome to the middle of the cell to join other polar fibers from the opposite centrosome.
Mitosis

10. Mitosis
II. Metaphase ~
A. Sister chromatids line up at the center of the cell, the equatorial plane.
III. Anaphase ~
A. Enzymes break down the proteins in the centromeres, allowing the chromatids to separate.
B. Kinetochore fibers pull the chromosomes toward opposite ends of cell.
C. Polar fibers push the centrosomes apart which elongates the cell.
IV. Telophase ~
A. Chromatids reach the end of the spindle
B. New nuclear membranes form around the chromosomes on separate poles of the cell. (opposite of prophase)
C. Chromosomes begin to uncoil returning to chromatin form (opposite of prophase)
D. Mitotic spindle is deconstructed (opposite of prophase)

11. Cytokinesis
Cytokinesis ~ the final phase, in which the division of the cytoplasmic contents must be divided. Occurs differently in plants cells than animal cells.
Animal cells-
Actin filaments constrict the membrane in the center forming a cleavage furrow which deepens until the cell is pinched into two cells.
Plant cells~
1. Vesicles formed by the Golgi bodies align & fuse to form a membrane-bound cell plate.
2. Cellulose and other components that form the cell wall are produced and form a mature cell wall separating the two daughter cells.

12. Variations in Mitosis
*In some unicellular organisms, all the phases of mitosis occur in the nuclear membrane. The nuclear membrane will be pinched to form 2 nuclei.
*The length of time for mitosis differs for various types of cells.
May take from 10 minutes to 3 hours.
*Embryos have very short interphase, because they must produce cells rapidly.
*Most types of cells can be replaced when damaged or die.
*Most nerve cells in the human brain may grow in size and replace worn-out or damaged cells , but will not divide.
*Most brain cells can’t be replaced when they die.