Cell Division
The cells in a human infant are the same size as the cells in an adult. Why?
1. If a cell were to get bigger as an organism grows, eventually the cell membrane
would not be able to handle the transport of materials into and out of the cell.
2. The cell would need too many raw materials and would make too much waste,
thus causing the cell to die.
3. Organisms deal with growth by increasing the number of cells, not the size of the
cells.
In order to increase the number of cells, the existing cells must go through cell division.
One cell divides into two identical cells, or daughter cells.
In order for the parent cell to produce two new identical daughter cells, the genetic
material from the parent cell nucleus must be passed on to the nuclei of the daughter
cells.
Cell division occurs in a series of phases.
Phase 1: Chromosomes are copied (Interphase)
A. Chromosomes are duplicated.
B. Normal chromosome number in the cell is doubled.
C. Each chromosome attaches to the centromere.
Phase 2: Mitosis Begins (Prophase)
A. The nucleus divides into two nuclei (process called mitosis).
B. In animal cells the centrioles line up on either end of the cell and a spindle forms
between them.
C. Nuclear membrane begins to break down and nucleolus and nucleus disappear.
Phase 3: Chromosomes attach to the spindle (Metaphase)
A. Chromosomes attach to the middle of the spindle
B. Each chromosome attaches at the centromere
Phase 4: Chromosomes begin to separate (anaphase)
A. The chromosomes break apart at the centromere.
B. Each chromatid migrates along the spindle to opposite ends of the cell.
Phase 5: Two new nuclei form (telophase)
A. A nuclear membrane forms around the chromatin.
B. A nucleus and nucleolus reappear.
C. Mitosis is now complete
Phase 6: Two daughter cells form (cytokinesis)
A. Cytoplasm of the cell divides
B. The membrane of the cell moves inward until the cytoplasm is pinched into two
equal parts.
C. The cell membrane forms to produce to identical daughter cells.