GRADE: Eighth

CONTENT STANDARD: Life Science

CONTENT TOPIC: Reproduction and Heredity

CONCEPT: The cell is the basic unit of living things.

CONTENT OBJECTIVE: 8E1.00 To understand the mechanics of cellular division

INSTRUCTIONAL OBJECTIVES: The learner will

1.01 recognize the importance of cell division to organisms.
1.02 compare cell division in prokaryotic and eucaryotic cells.
1.03 identify the stages of mitosis.
1.04 recognize meiosis in reproductive cells.

I. Importance of cell division to all organisms
A. reproduction in unicellular organisms
B. regeneration
C. healing and repairing process
D. growth
II. Types of cell division
A. Fission in prokaryotic cells
B. Mitosis in eucaryotic cells
1. Stages of mitosis
a. prophase
b. metaphase
c. anaphase
d. telophase
C. Meiosis in reproductive cells

TN COMPONENT OF SCIENCE: Habits of Mind

TN GOAL:

To enable students to demonstrate ways of thinking and acting inherent on the practice of science; and to exhibit an awareness of the historical and cultural contributions to the enterprise of science.

3.2 ASSUMPTIONS - The recognition and the criticism of the validity of an argument through presentation of data and differentiation between fact and assumption in the preparation of an explanation for a natural phenomenon are vital parts of the scientific process.

3.2a Science is based upon suppositions derived from observations of natural phenomena.

BENCHMARK: New ideas in science sometimes spring from unexpected findings, and they usually lead to new investigations.

3.2b Predictions are based on previous knowledge.

BENCHMARK: Knowledge and creative insight are usually required to recognize the meaning of the unexpected.

3.2d The validity of an investigation cannot be accepted unless the complete investigation can be independently duplicated.

BENCHMARK: Accurate record keeping and replication allow students to test the validity of experiments.

TIME REQUIRED:

Approximately 2-3 instructional periods

Chromosomes, eucaryotic cells, fission, meiosis, mitosis, prokaryotic cells, regeneration



This classroom connector addresses instructional objective 2.01.

Every multicellular organism begins life as a single cell. By the time you were born you were made of many trillions of cells. Where do these cells come from? (According to the cell theory, every cell comes from another cell.) Human cells are not the only kind of cells that make more of its own kind. All organisms, both multicellular and unicellular, make new cells by a similar process of cell division. Each cell is able to form new cells just like itself.

In unicellular organism, cell division is the way these organisms reproduce. The cell divides producing a duplicate of itself.

In multicellular organisms, cell division has several functions. One of these functions is regeneration. In regeneration, parts of organisms that have been damaged grow back or are replaced. For example, a starfish can grow a new arm if one is cut off. Do you know of any other animals that can replace lost parts? (Some lizards can grow new tails, some species of worms can grow back a lost part, salamanders can grow new legs, crayfish can grow new claws, etc.) A free-living flatworm called a planarian has an amazing ability of regeneration.

Another function of cell division is the healing of wounds which in a way is a type of regeneration. Whenever you have a cut or scrape on your skin, you can see the healing take place over a period of time. While the wound heals, new cells are made to replace the destroyed and damaged cells. The new cells are formed by cell division.

A fourth important result of cell division is the growing process. After a cell divides, both new cells grow and divide to form more new cells. Some cells can divide every few hours. (Using chalkboard or over head projector show how number of cells can increase.)

GRAPH INSERT

As the number of cells increase in an organism so does its size.

1. Have students determine the number of cells formed from a single cell in 24 hours if division occurs every two hours. (2048 cells in 24 hrs)

2. Use either planarian or tubifex worm to demonstrate regeneration.

Materials: several tubifex worms (which may be obtained in aquarium store or biological supply company); clean single edge razor blade; several petri dishes or other small glass or plastic dish; water; microscope (optional)

Procedure:

1. Cut each worm as shown and separate the sections into individual dishes.

2. Add a small amount of water to each dish.

3. Maintain at room temperature for three weeks.

4. Replace water that evaporates.

Observations: Regeneration of worms takes approximately two to three weeks.

KEY QUESTION: Where does regeneration occur in your body? (Skin or bone)

REGENERATION IN PLANARIA

Materials: planarian may be obtained from a biological supply company; several petri dishes; single edge razor blade; microscope; microscope slide; water; tooth pick; Epsom salts

Procedure:

1. Put a planarian in a culture dish and add a few crystals of Epsom salts. In a few minutes the worm will be anesthetized.

2. Transfer the planarian to a drop of water on a glass slide with a toothpick.

3. Cut the planarians in several different ways according to the diagrams.

4. Transfer each piece to a separate dish.

5. Add fresh water and put the dishes in a dark place.

6. Observe the animals daily, changing water as you do so.

7. Record any changes by description or drawing.

KEY QUESTION: Which reactions regenerated most quickly?

We have learned that cell division is an important process in all organisms. Take a sheet of paper and write down the four important functions of cell division in organisms. (reproduction, regeneration, healing, and growing) Let students compare answers and then summarize what happens in each one.



This classroom connector addresses instructional objectives 2.02, 2.03, and 2.04.

We have learned that cell division is the way a cell reproduces. How does this occur? (A cell divides to form two new identical cells.) Biologists know that there are different ways that cells reproduce. This is determined by the way the nuclear material is arranged in the cells.

One kind of cell has its nuclear material spread throughout the cytoplasm in the cell. These kind of cells are called prokaryotic cells. These cells are found only in the unicellular organisms in the kingdom Monera. Bacteria and blue-green algae are examples of these kind of cells.

The prokaryotic cells reproduce simply by dividing in half. This process is called fission. Before fission begins, the nuclear material doubles. During fission the cytoplasm and nuclear material of the cell divides in half to form two new cells of the same size. Each new cell has the same amount as the original cell. In some bacterial cells, fission can occur as often as ever 20 minutes under the right conditions.

Most other unicellular organisms such as paramecium can reproduce by fission also.

The other kind of cell has a membrane separating the nucleus from the rest of the cytoplasm. These cells are called eucaryotic cells. Animals and plants are made of eucaryotic cells.

The dividing of a eucaryotic cell is referred to as a cell division called mitosis. Cell division can be broken down into four stages. The division is a continuous process, but in order to study mitosis we will look at some major things which happen in each stage. We will not note every change that occurs in each stage, but you should be aware that there are other activities involved.

Pass out drawing of cells for students to label during discussion.

The first stage is called prophase. During this stage, the chromosomes in the nucleus appears as rod-shaped strands. Chromosomes are made of protein and a chemical called DNA. Each living thing has a certain number of chromosomes in all its cells. The number of chromosomes in our cells is 46. A dog has 78, a cat has 32, a rose has 14. This is a very important number for each organism. During prophase the chromosomes not only appear, but they double in number. Also during this time, the nuclear membrane disappears.

The second stage of mitosis is called metaphase. During this stage, the chromosomes line up in the center of the cell.

The third stage is called anaphase. The chromosomes divide in half and move to each side of the cell.

After this the final stage called telophase occurs. A nuclear membrane forms around each set of chromosomes. The cell membrane forms around each nucleus and then the two cells split apart. The two new cells are identical - each has the same number of chromosomes that the cell had before it divided.

The new cells will grow and perform the activities that keep it alive. This time is often called interphase since the cell is not dividing.

Mitosis occurs in all cells of an organism's body except in the reproductive cells where another type of cell division occurs. This type of cell division is called MEIOSIS. In meiosis each cell ends up with exactly one-half the number of chromosomes that it originally had. Human cells have 46 chromosomes so if meiosis occurs, the new cells will have how many chromosomes? (23 - 12 of 46.) If a frog has 26 chromosomes, how many chromosomes will be in the reproductive cells? (13 - 1/2 of 26.)

The process of meiosis is similar to mitosis; however, in meiosis there are two divisions of cells instead of one. The first division of cells is mitosis with the number of chromosomes staying the same.

In the second division, each of the two new cells divide again, but the number of chromosomes does not change during this division. This results in the new cells having one-half the number of chromosomes. Meiosis is often called reduction division because the new cells have half the normal number of chromosomes.

DIAGRAM OF MEIOSIS

(1. Students can view prepared slides of prokaryotic and eucaryotic cells. They will make drawings of each and describe the difference between each.

2. Students label drawing of stages of mitosis.

3. Students examine prepared slides of onion root tip under microscope. Locate the cells in the different stages of mitosis.)

We have studied three ways that cell division occur. Which kind of cell division occurs in prokaryotic cells? (Fission) The cell simply divides in half. What kind of cell division occurs in eucaryotic cells? (Mitosis) What happens to the number of chromosomes during mitosis? (The two new cells have the same number as the original cell.) A third kind of cell division is called meiosis. It occurs only in what kind of cells? (Reproductive) What happens to the number of chromosomes during meiosis? (Reduced by 1/2)

(Have students find number of chromosomes in other organisms.)

chromosomes - rod-shaped bodies that appear in cell during mitosis

eucaryotic cells - a cell that has a well defined cell nucleus

fission - a kind of asexual reproduction that takes place when cells divide

meiosis - cell division in which sex cells having half the number of chromosomes are produced

mitosis - process in which a cell divides to form two identical cells

prokaryotic cells - a cell that has its nuclear material spread throughout the cell

regeneration - replacing a lost parts

This is the time this file has been accessed since 04/04/98.

The University of Tennessee at Martin is not responsible for the information or views expressed here.

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