Be sure to visit these websites for valuable animations of these concepts:
Practice Cell Division before the test at: http://www.biologyinmotion.com/]
Chapter 8: Cell Division
I. Cells divide to replace old cells or because they have grown too large. There
are definite limits to how big a cell can grow.
II. What limits the size of a cell?
A. Transport issues arise when cells get too large. As the cell grows, its
volume increases faster than its membrane. The cell cannot keep up with
the transport demands (diffusion of gases, active transport of ions and
food molecules) that come with the increase in cell volume. A cell that
grows too large will starve or poison itself.
B. DNA content also limits size. As the amount of cytoplasm increases, so
does the demand for more protein production (i.e. enzymes).
III. The Cell Cycle is the sequence of growth and cell division that repeats itself
in cells. (See Cells Alive! http://www.cellsalive.com/cell_cycle.htm
A. The life cycle of a cell can be divided into TWO general periods:
Interphase and Mitosis.
1. The period of active growth and metabolism is known as interphase.
The cell spends most of is “life” in interphase.
2. Mitosis is a brief, but very important, period of cell division. Mitosis
is a form of asexual reproduction.
B. Interphase- During this time, the cell is “busy” going about its work:
1. The cell increases in size
2. Makes new organelles
3. Produces ATP (energy)
4. Makes food (if photosynthetic)
5. Stores food
6. At the end of interphase, just before cell division, DNA is copied in a
process known as DNA Replication. Now, two sets of DNA exist:
one for each new daughter cell.
C. Mitosis- is cell division. It has four distinct stages:
1. Prophase- the 1st stage;
a. During this first mitotic stage, the nucleolus fades and
chromatin (replicated DNA and associated proteins)
condenses into chromosomes. (chromosome = colored
b. Each replicated chromosome comprises two chromatids, both with
the same genetic information
c. Chromatids are held together by centromeres
d. The nuclear membrane and nucleolus disintegrates.
e. Centrioles (made of microtubules) move to opposite ends of the
f. Spindle fibers form to separate sister chromatids
2. Metaphase- 2nd stage of mitosis
a. Doubled chromosomes become attached to the spindle
fibers at the centromere
b. Chromosomes are aligned on the midline or equator of the
c. Each chromatid is attached to a fiber going in opposite directions
to assure equal distribution of DNA (p. 168)
3. Anaphase- the 3rd stage of mitois
a. Centromeres split and sister chromatids go in
opposite directions, one to each newly forming cell
b. Each chromatid now becomes an individual
chromosome (p. 169)
4. Telophase- the 4th stage of mitosis
a. Chromatids reach opposite poles
b. Chromosomes revert back to threads of chromatin (they
c. Spindle fibers break down
d. Nuclear envelope forms around new DNA
e. Plasma membrane separates, forming 2 new cells. Cytoplasm
pinches and divides.
f. In plants, a new cell wall is made called a cell plate, separating
5. Cytokinesis- Once two nuclei are formed, mitosis is over.
However, two new cells are not formed until the cytoplasm of
the cell divides. This process is called cytokenesis.
6. The Bottom Line: The RESULT OF MITOSIS is two new cells,
each identical to the parent cell! In animals, this kind of cell division
occurs in body cells (somatic cells) like epidermis, blood, bone, etc. In
plants, growth in root tips and shoot tips occurs through mitosis.
See more about mitosis, including the pictures (above) and
animations, at The Biology Project:
The Cell Cycle
Interphase is made up of three phases:
Each chromosome has one chromatid.
The cell grows in size.
Synthesis of organelles occurs.
This is when DNA synthesis occurs.
Each chromosome has two chromatids.
The synthesis of enzymes and other proteins in preparation for mitosis
occurs during this period.
Cells that permanently leave the cycle
Some cells remain permanently in G1. Examples of some in your body that do this are: skeletal
muscle, nerve cells
Control of the cell cycle
Mammalian cells typically divide only about 50 times.
This limit is set by the presence of repeated sequences of DNA at the tips of the chromosomes.
In young cells, the sequence TTAGGG is repeated hundreds or thousands of times but each time
the cell divides, it loses 50 to 200 of these repeats. Cells that have divided many times have
fewer of these repeats left.
When the telomere is reduced to a certain size, the cell will no longer divide.