Mitosis: Cell Reproduction
I. The Cell Cycle
A. Cells go through a life cycle (called the cell cycle) that consists of two stages: interphase and mitosis.
1. Most of the life of a cell is spent in interphase.
a. During the first part of interphase the cell grows (it just split from its parent cell and is small).
b. Most of the time in interphase the cell is busy doing its job for the organism.
c. Just before mitosis begins, the DNA makes a copy of itself so that each new cell will have a
complete set of chromosomes.
d. During interphase the chromosomes are thin and threadlike—they are hard to see.
2. When the DNA has finished copying itself, the cell is ready to enter mitosis.
a. mitosis = the process of cell division in which two new nuclei are formed (each having a complete
set of identical chromosomes) and the cytoplasm splits to form two new identical cells.
a. parent cell = the cell that is going to undergo mitosis and divide.
b. daughter cells = the two new cells that form as a result of mitosis.
B. Cells need to reproduce for several reasons:
1. to allow the organism to grow from a single fertilized cell to being multicellular.
2. to replace worn out cells in the organism.
3. to repair damaged tissues.
C. The cells do not all divide at once— the organism would not be able to perform its life functions if all of
its cells were in the process of dividing.
1. In some tissues, once the cells have formed, they do not perform mitosis.
a. Example: Red blood cells are formed from bone marrow cells, carry out their function, and die.
b. Example: In plants, mitosis occurs only in the growth regions (like the tips of the roots) and not
throughout the whole plant.
II. Steps of Mitosis in Animal Cells
A. Mitosis occurs in four steps that are identified by the specific events that happen at each stage.
1. Prophase
a. The chromosomes become shorter and thicker (they become rodlike) and are visible.
b. Each chromosome and its copy are held together at a point called the centromere.
c. The centrioles move to opposite sides of the cell and begin to make spindle fibers.
d. The nuclear membrane disappears.
2. Metaphase
a. The chromosomes and their copies line up along the middle of the cell called the equator.
b. The spindle fibers attach to the centromeres that hold each chromosome to its copy.
3. Anaphase
a. The centromeres split and the chromosome copies separate.
b. The chromosomes are drawn to opposite ends of the cell by the spindle fibers.
c. By the end of anaphase, there is a complete set of chromosomes on each side of the cell.
4. Telophase
a. The spindle fibers disappear.
b. The chromosomes become thin and threadlike again.
c. A new nuclear membrane forms around each set of chromosomes.
d. The cytoplasm and cell membrane pinch in at the middle until the two new, identical cells
separate to form the daughter cells—this is called cytokinesis.
B. After telophase, the new cells enter into interphase, where they will grow and replace organelles and then
perform their job for the organism—the cycle repeats itself.
III. Mitosis in Plant Cells
A. The steps for mitosis in plants are the same as in animal cells with two exceptions:
1. Plants do not have centrioles, but they do have spindle fibers (it is unknown where the spindle fibers
come from in plants).
2. Since plant cells have a rigid cell wall, they cannot “pinch in” as animal cells do.
a. Instead, a cell plate forms between the two new cells, which will eventually become the cell wall.