|Life Science||Reproduction and Heredity 8E1.00||Habits of Mind||Assumptions 3.2 abd|
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
TN COMPONENT OF SCIENCE: Habits of Mind
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.
This classroom connector addresses instructional objective 2.01.
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.)
As the number of cells increase in an organism so does its size.
2. Use either planarian or tubifex worm to demonstrate regeneration.
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)
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
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?
This classroom connector addresses instructional objectives 2.02, 2.03, and 2.04.
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.
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.)
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
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