HOW CELLS REPRODUCE
I. As we know living cells must reproduce themselves to maintain life.
A. In unicellular organisms, cell division acts to increase the number of individuals present,
while in multicellular organisms, cell division acts to replace dead or damaged cells.
B. Reproduction-
C. In cell division, each newly made cell must contain the genetic information that is needed
to run the activities of the cell.
1. Each new cell must receive a copy of the parental cell’s DNA.
2. The new cells must also inherit the organelles and polymers needed for the cell to
function properly.
D. Living cells can reproduce themselves by dividing. All living cells divide by one of two
possible processes. The types of cell division include:
II. THE CELL CYCLE
A. Steps in The Cell Cycle
1. The G-1 Step (Gap 1)
a. What happens in the G-1 Step?
2. The S Step (Synthesis)
a. In this step, the cell’s DNA replicates or doubles itself. This produces two
identical copies of DNA. Why is this important?
b. Each double helix is referred to as a chromatid. Pairs of chromatids, known as
sister chromatids are held together by a centromere.
3. The G-2 Step (Gap 2)
a. In this step, a cell’s organelles double themselves. Why is this important?
b. The enzymes that are needed for cell division to occur are also produced in
this step.
c. Interphase-
4. Mitosis-cell division, including division of the chromosomes.
III. MITOSIS
A. General Features of Mitosis:
1. Mitosis occurs in somatic cells
2. In mitosis, one cell divides to produce two genetically identical cells.
3. Mitosis is a quick process that proceeds through four distinct phases.
B. The Phases of Mitosis
1. Prophase-Events/Steps
a. Chromatin condenses to form distinct homologous chromosomes. At this point
the chromosomes become visible in the cell.
b. 2 pairs of centrioles appear on the outside of the cell’s nucleus. Centrioles are
hollow tubes that are composed of microtubules.
c. Next, the centrioles begin moving towards opposite ends of the cell. As they
migrate, the centrioles begin producing the Spindle Apparatus (Fibers).
1. The homologous chromosomes will attach to the spindle fibers as
mitosis continues.
d. Eventually, the spindle fibers will fill the entire cell.
e. Once the centrioles reach opposite ends of the cell, asters develop.
1) What are asters and what is their function?
f. At the end of prophase, the nuclear membrane disappears. The homologous
chromosomes are then free to attach to the spindle fibers.
2. Metaphase-Events/Steps
a. During this phase, the chromosomes line up in the center of the dividing cell.
The chromosomes line up at a region known as the metaphase plate.
b. In this phase, the cell is preparing to pull the chromosomes apart and then
divide to produce two new cells.
3. Anaphase-Events/Steps
a. In this phase, the spindle fibers shorten and begin to pull on the homologous
chromosomes.
b. Eventually, the chromosomes are pulled apart. Furthermore, they are pulled
towards opposite ends of the cell.
c. At the same time of chromosome separation, the cell is elongating.
d. Anaphase is complete when the chromosomes have reached opposite ends of
the dividing cell.
4. Telophase-Events/Steps
a. By this phase, the chromosomes have reached opposite ends of the dividing
cell.
b. Cytokinesis occurs-this is division of the cytoplasm. Why is this important?
c. New nuclear membranes from in the center of the two new cells.
d. At this point, the chromosomes lose their identity and develop into chromatin
again.
e. Now, the two new cells begin the G-1 Step of the Cell Cycle.
C. Mitosis is similar in plant and animal cells; however, there are some differences between
cell division in plants and animals. Some of these differences include:
D. What regulates and controls Mitosis?
E. Cells must stop mitosis at some point. Failure to stop cell division, at least for a brief period of
time, can lead to the development of cancer. What is cancer?
1. What stops mitosis in living cells?
2. Oncogenes3. Carcinogenesis4. Mutations lead to the formation of many forms of cancer.
F. Variations on Mitosis:
1. Binary Fission-
2. Budding-
G. Generally, mitosis is associated with cells that reproduce asexually (without the formation of
gametes). Meiosis, a second type of cell division, is associated with sexual reproduction.
IV. STEM CELLS-are found in most animals. These cells retain the ability to divide and differentiate
throughout the lifetime of the animal.
A. These cells may be used to clone individuals, however, their major use may come in the
health care field.
B. Researchers are studying stem cells to determine if they can be used to generate new,
healthy tissue. If successful, this could be used to treat: