Chromosomes / Cell Mitosis
In order for growth to occur, the cells must divide. One cell divides into two, the two divides into four and the four divides into 8 and so on.
Q. How do cells divide?
Activity: Cell division
In preparation for cell division, the thread-like DNA coils and thickens to form what is called a chromosome.
Each living thing has a precise number of chromosomes in the cell.
Just prior to cell division, the chromosomes line up in the center of the cell. Other cell organelles, the two centrioles, produce spindles (thread-like in appearance). Some of these spindles attach themselves to the central part on each chromosome. These spindles later start to retract in opposite directions, causing the chromosomes to split into two equal halves.
Each chromosome now becomes two half chromosomes called chromatids. These chromatids migrate to opposite poles of the cell with the help of the spindles. The cell fluid and all the other things inside each cell separate along with these chromatids as they pool to opposite poles (or ends) of the cell.
The cell ultimately divides into two, each cell carrying a half sister chromosome of the other, a process called mitosis.
Onion Root Tip Experiment - part 1
1. Set the bottom part of an onion in water. Leave it there for about 4-5 days, until the roots begin to grow.
2. When the roots are about 2 cm long, with active growth, cut several millimeters of some of the the roots off.
3. Place the roots on a microscope slide, cut them lengthwise.
4. Press another slide down on the roots to mash them, and then remove the top slide.
5. Add some nuclear stain such as toluidine blue.
6. To stop the cell division and to quicken the stain, warm the slide over a flame for a few seconds, without allowing the liquid to boil.
7. Add a few more drops of stain and let the sample sit for a few minutes.
8. After a few minutes, blot the stain with a paper towel.
9. Add a few drops of water, and put a coverslip on it.
10. Observe with a microscope. Look for the various stages of cell division.
|Activity: Onion Root Tip
Experiment - part 2 - Flashcards
Each student is given several flash cards. Identify the stage of mitosis of each cell, and make a count of the number of cells in each stage. Compile your data in the table below.
From this, determine the percentage of cells in each stage of cell division. The more cells in any particular stage of cell division, the longer the relative duration of that particular stage.
List the stages in order of their relative duration, ranging from the longest duration to the shortest duration.
Why do children resemble their parents?
This is explained in a process known as inheritance. It explains how we can obtain traits from our parents and pass them on to our children.
This process involves a cell division called meiosis, which is slightly different from mitosis. When meiosis is ready to start; similar chromosomes (called homologous) that were inherited from each of the two parents line up in pairs inside the cell. (Remember that chromosomes are made up of coiled DNA; each unit of DNA is a gene / trait). These chromosome pairs exchange some of their genes through a process called crossing-over. At the end of crossing-over the chromosomes pairs are now different from the original chromosomes that were there at the beginning of meiosis. These pairs of chromosomes are later separated from each other with the help of the spindles (as in mitosis) and migrate to opposite poles of the cell. Immediately division of the cell into two daughter cells occurs. Each daughter cell now carries half the number of the chromosomes of the original the cell. This is how the sperm and egg cells are produced. Each manís sperm carries the chromosomes with traits of both manís parents similarly each womanís egg carries chromosomes of the womanís parents. Sperm and egg cells always have half the number of chromosomes when compared to other cells. When these two cells (egg and sperm) unite again during fertilization, the full number of chromosomes of a cell is restored in the cell. The children who are the product of this union have cells that carry traits of parents and grand parents from both sides of the families.
Activity suitable for elementary students
A. DNA structure
At the conclusion of this exercise activity, students will be able to describe the alpha helical structure of a eukaryotic cell DNA found in the nucleus of a cell.
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Content provided by Patricia Ahanotu, Catherine Carter, Essie Smith, and Pamela Gore, Georgia Perimeter College
Page created by Pamela J.W. Gore
Georgia Perimeter College,
Page created August 9, 2007