MITOSIS AND MEIOSIS: HOW CELLS DIVIDE
• Program Objectives
After viewing this program, students should be able to:
• Describe clearly the differences between mitosis and meiosis; explain what function each serves in the organism.
• Understand the role of mitosis in the normal life cycle of an organism.
• Describe what happens during each stage of mitosis: interphase, prophase, metaphase, anaphase and telophase.
• Explain how mitosis insures that all cells in an organism will carry the same genetic information.
• Name some of the ways in which mitosis leads to reproduction
• Define clone.
• Explain what cancer is and speculate on what might cause it.
• Describe what happens, step-by-step, during meiosis.
• Identify when and how meiosis results in a reduction of the number of chromosomes; explain why this is necessary.
• Explain how DNA replicates.
• Explain why chromosomes occur in homologous pairs.
• Describe how meiosis and sexual reproduction introduce variation.
• Summary of Content
Dramatic photos show a frog egg developing from a single cell to two cells to a multicelled blastula, opening this investigation of the process of mitosis. As the frog egg continues developing into a tadpole and finally a frog, we see that mitosis does two things for the organism: it is the process by which almost all new cells are produced, and it is the mechanism for transmitting genetic instructions (for growth and development) from the original zygote to all other cells in the organism.
To see how this happens, we turn to a detailed study of the process of mitosis. Each step is illustrated by photomicrographs and by clear illustrations. Although mitosis is a continuous, ongoing process, scientists have divided it into several distinct stages.
Toward the end of interphase, the genetic material in the nucleus replicates. Illustrations show how the process of DNA replication takes place. Once replicated, the chromosomes coil up and become visible. This marks the beginning of prophase. Diagrams show clearly how the two chromatids are still attached at the centromere to form each chromosome. During prophase, the nuclear membrane fades from view and the centrioles move to opposite ends of the cell. A network of fibers called the spindle stretches between them. In metaphase, the chromosomes line up at the cell’s equator and the centromeres become attached to spindle fibers. Anaphase is the stage during which the actual separation of nuclear material takes place. The centromere replicates and the two chromatids move along the spindle to opposite end of the cell. The final stage is called telophase. As cytokinesis takes place, the nuclear membrane reappears, and mitosis nears completion.
A final section of the program looks at the ways in which mitosis in plants is different from mitosis in animals. Although the processes are similar in most respects, plant cells lack centrioles and have rigid cell wall outside the cell membrane. These factors cause some differences in mitosis, which are illustrated with both diagrams and photomicrographs.
Now that we have taken a close look at just how mitosis works, we can turn to an investigation of its significance for organisms.
Striking photomicrographs of dividing paramecia show that for very simple, one-celled organisms, mitosis is a process of reproduction. We soon see, however, that mitosis has many other functions in more complex organisms. A series of photographs of chicken embryo development helps to develop the concept of differentiation. Through the discussion of differentiation, we come to see that all cells in one organism carry the genetic information for the whole organism—even though any single cell expresses only a part of that information. This is one of the functions of mitosis: to insure that every new cell contains the same genetic information as all other cells.
The significance of this concept becomes more apparent as the program moves on to discuss vegetative propagation. Mitosis can be a method of asexual reproduction for plants. Under the right conditions, a single cell of a mature plant can be used to grow an entire new plant. Because the original cell contains all the genetic information, the developing new plant will differentiate to include all parts. Can this process work in animals? In humans? The mechanisms of cloning are explained, and some experiments, both successful and unsuccessful, are described.
Mitosis serves other functions in the organism. We take a look at its role in tissue repair and maintenance. In humans, mitosis produces new cells to heal cuts; in salamanders, mitosis can produce an entire new tail or leg. This is regeneration. It involves more than just replacing cells—it also involves differentiation.
What controls the process of mitosis, triggering cells to divide when new cells are needed, and allowing regeneration in some organisms but not in others? We don’t yet have the answer to that question, but we do know one thing: the mechanism doesn’t always work perfectly. Sometimes cells begin uncontrolled division. This is what we call cancer. Photos, diagrams and a discussion of this abnormal mitosis conclude Part Two.
• Part Three
Single-celled organisms, like the paramecium shown in the opening of Part Three, reproduce through mitosis. Each offspring is genetically identical to the parent organism. Higher plants and animals reproduce in quite a different way. Through sexual reproduction the genetic traits of two parents are combined: the offspring is not identical to either parent. In order for this to happen, a process of cell division called meiosis takes place in each parent.
An in-depth study of meiosis begins with a discussion of the role of chromosomes in inherited traits. The program explains that it takes a homologous pair of chromosomes to determine a trait and that each parent contributes one chromosome to each homologous pair in the offspring. Before meiosis begins, the chromosomes in the nucleus replicate. As in mitosis, this results in a double chromosome with two chromatids joined at an unreplicated segment called the centromere.
Meiosis is a two-step process. The initial primary sex cell divides twice, resulting in four cells. Each resulting cell has half as many chromosomes as did the original. Each division proceeds in stages—prophase, metaphase, anaphase and telophase. The program discusses each stage in detail and demonstrates how meiosis proceeds, with clear diagrams and with photomicrographs.
During prophase I, homologous pairs are drawn together—even if they are widely separated in the cell—and from tetrads. This is called synapsis. During metaphase I, the tetrads line up at the cell’s equator. The spindle fibers become attached to the chromosomes at the centromeres. In anaphase I the homologous pairs separate and the chromosomes are drawn toward opposite ends of the cell. By the end of telophase I, two new cells have formed. Each has only half as many chromosomes as did the primary sex cell.
Each cell now divides again in four stages: prophase II, metaphase II, anaphase II, telophase II. The chromatids of each chromosome separate during this division in a process similar to mitosis. The four resulting cells, if male, will mature into sperm.
Meiosis in the female is similar in almost all respects. During the first division, however, the cytoplasm is divided unequally. One large cell and a smaller “polar body” result. Each of these splits again in the second division. Again, the larger cell divides unevenly. The result is one large cell, which will mature into an egg, and three polar bodies, which will die.
At the end of Part Three, we see how the union of the male and female reproductive cells will yield an entirely new combination of genetic traits.
• Part Four
Students often find it difficult to keep track of the differences between meisosis and mitosis and to understand what is significant about each; Part Four comes to their aid with a direct point-by-point comparison of the two processes.
Mitosis and meiosis differ in their end products. Mitosis results in two cells, each genetically identical to the original. Meiosis, on the other hand, results in four cells, none of which is identical to the original. Each new cell has only half as many chromosomes as did the original. This fact gives meiosis its name: it’s a Greek word meaning “reduction.”
A second difference is in the process itself. Mitosis is a single division; meiosis is a sequence of two divisions.
Mitosis can—and does—take place in almost every type of cell. It is responsible for growth and development, repair and maintenance, and asexual reproduction. Meiosis occurs only in specialized cells located in particular parts of an organism. Its only function is to produce gametes for sexual reproduction.
The cells resulting from mitosis are viable and can survive in the same environment as did the original cell. The cells produced by meiosis, on the other hand, insures that the offspring will never be identical to either parent. Meiosis produces variation within a species.
Understanding how and why meiosis produces variation is central to grasping the significance of meiosis and the difference between meiosis and mitosis. The program offers a detailed look at this process by carefully tracing the path of a pair of chromosomes during meiosis. We see how the genes controlling traits are randomly sorted into the four gametes that result from meiosis. This, in itself, can produce a great deal of variation. But another factor, crossing-over, multiplies by many times the possible gene combinations that can result from meiosis. Illustrations show how chromosomes can overlap during synapsis and how chromosome segments can be exchanged between homologous chromosomes.
• Questions for Discussion and Review
1. What happens during interphase? What kinds of activities take place that are not related to mitosis?
2. Explain the process of DNA replication. How does mitosis insure that all cells in one organism have the same genetic information?
3. What are the differences between plant and animal mitosis?
• Part Two
1. In what kinds of organisms does mitosis serve as the method of reproduction?
2. What are some of the functions mitosis serves in the organism?
3. How does vegetative propagation demonstrate that every cell contains the genetic blueprint for the entire organism?
4. What is the difference between identical and fraternal twins? Which is a result of mitosis? How are identical and fraternal twins produced?
5. How is a clone produced? What would a clone of you be like—if such a thing were possible?
• Part Three
1. How many chromosomes does it take to control a single trait? Where do the two chromosomes come from?
2. Why is it necessary for the reproductive cell, the gamete, to contain only half the normal number of chromosomes?
3. How and when is the number of chromosomes reduced during meiosis?
4. Describe the stages of each meiotic division.
5. How are male and female meiosis different? What function does this serve?
• Part Four
1. What are the main differences between mitosis and meiosis?