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Mitosis and Meiosis

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Teacher's Guide

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MITOSIS AND MEIOSIS

Running Time: 23 minutes INTRODUCTION The ability to reproduce is perhaps the most unique process that characterizes living things. Given that all living organisms eventually die, it is essential that life be transmitted to future generations. The incredible diversity of life that surrounds us stands as testimony to the success of the biological reproductive processes that have continued unabated for the three-and-one-half billion years that life has existed on earth. Reproduction, when viewed at its most elementary level, assures that the DNA of one living cell will be passed on in an unaltered form to its two daughter cells. This is what happens when one-celled organisms reproduce, when new cells are formed to replace dead cells in adult multicellular organisms, and during embryological development: DNA instructions are passed on so that the new cells can function normally and reproduce properly. In order to reproduce, all cells that make up the bodies of organisms follow the same sequential steps: DNA replication, mitosis, cell division, and cell growth. These processes characterize the life cycle of the cell. The intricate steps followed by the cell in duplicating its DNA, sorting it into chromosomes, and then separating the chromosomes into two equal and identical groups, forms the basis of the first part of this program. More biologically advanced organisms have two sexes and are capable of combining in their offspring the DNA from the female parent with the DNA from the male parent--a process which results in more diverse and hardy individuals than is possible in the asexual reproductive processes that involve just one parent. From a purely biological point of view, the complex bodies of sexually reproductive organisms can be seen as fantastically intricate containers that have developed primarily to protect the germ cells that produce the sperm and eggs. 1

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The cell reproductive process that makes sexual reproduction possible is called meiosis or reduction division. The steps involved in duplicating the DNA and properly sorting it out that occurs during meiosis is the subject of the second half of this program. PROGRAM SUMMARY This video program is designed for grades 10-12, but could be very useful in introducing cell reproduction to students in grades 7-9 as well. The program consists of two parts... The first part, lasting about 15 minutes, is devoted to the subjects of the interphase state, DNA replication, chromosome structure, and offers a detailed look at each stage of mitosis. A short, optional video quiz follows the first part of the program. The second part is about 8 minutes in length. It looks at the diploid and haploid states, at fertilization, and at the various stages of meiosis. A short, optional video quiz also follows this portion of the program. Throughout this entire program, microscopic images are interwoven with animated sequences to allow the processes of mitosis and meiosis to be more easily understood. STUDENT OBJECTIVES After viewing this program and participating in the learning activities, students should be able to... · Describe the subcellular events that occur during Interphase, Prophase, Metaphase, Anaphase, and Telophase. · Describe the subcellular events that occur during each major stage of meiosis. · Contrast Mitosis and Meiosis and evaluate the importance of these two processes to living organisms. 2

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· Describe the life cycle of a typical cell. · Define and use in context vocabulary words appropriate to their academic level. SUGGESTED LESSON GUIDE TEACHER PREPARATION 1. Read this guide and preview the video before showing it to your class. Although this lesson combines both video parts as one learning session, you may choose to show the video in two sessions or to stop the video at the end of the first part and review before going on to the second part. There is a short interactive video quiz at the end of each part of the video. These video quizzes are also provided in the form of blackline master 6. 2. Review the blackline masters and duplicate those you choose to use. 3. Make arrangements to have a VCR and television set available for use at the time and place selected for viewing. 4. Arrange to have microscopes available for student use. 5. Purchase, or obtain from school collections, the following microscope slides: allium (onion) root tip mitosis, spermatogenesis in rat testis, Drosophila salivary gland chromosome preparation. Optional materials: whitefish blastula mitosis and fertilized living eggs of snails, frogs, or toads. BLACKLINE MASTERS Blackline Masters 1 & 2, Vocabulary List, are words used in the video and their definitions. This list can be distributed to the class before the video presentation so students can better understand the terms presented. This list should be retained by students for reference. The list has also been included in this Teacher's Guide on pages 7-10. 3

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Blackline Master 3 is a diagram of The Stages of Mitosis. Blackline Masters 4 and 4a are diagrams of The Stages of Meiosis. Blackline Master 5, Crossword Puzzle, gives students an opportunity to use some of the terms introduced in the video. The answers to the puzzle can be found on page 21 of this guide. Blackline Master 6 is a copy of the questions posed in the Interactive Video Quiz for both parts of the video. You can either distribute the quiz before the viewing of the video or after, whichever approach you've chosen to take. Answers to the quiz are provided in this Teacher's Guide on page 16 for Part 1 and page 20 for Part 2. INTRODUCING THE PROGRAM Introduce this program by describing the mitotic processes involved in cell reproduction. Describe the role of mitosis in the development of an embryo, in replacing worn out cells, and in cancer where the rate of cell reproduction is rapid and uncontrolled. Contrast mitosis to meiosis and explain the importance of sexual reproduction in creating biologically vigorous individuals. Describe how meiosis makes it possible for sperm and egg to join together so that the fertilized egg will have the same, diploid, number of chromosomes present in the somatic cells of each parent. Before starting the video, distribute the blackline masters you've chosen to use during the video presentation, e.g. Blackline Masters 1 & 2, Vocabulary List, and Blackline Master 6, Interactive Video Quiz. If you've chosen to use the Interactive Video Quiz, you should inform the students that they will be expected to answer the questions which will appear on the screen following each part of the video program. 4

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VIDEO PRESENTATION Viewing time: Part 1, 15 minutes; Part 2, 8 minutes FOLLOW-UP ACTIVITIES DISCUSSION The script of the video presentation has been provided on pages 10-21 for your reference in leading the discussion. 1. Discuss the "life cycle" of a typical cell. 2. Besides replicating its DNA, what other activities might be expected to occur during interphase in different types of cells--for example, white blood cells, nerve cells, and cardiac muscle cells? 3. What reasons might account for the fact that some cells, such as bacterial cells, cancer cells, embryonic cells, and disease fighting cells, reproduce each day while others, such as nerve cells, hardly ever reproduce? 4. Discuss the biological significance of the haploid state, the diploid state, and the polyploid state. PROJECTS 1. Microscopic Examination of Plant Mitosis Have students examine stained and prepared slides of the root tip cells of an onion. These preparations provide a simple, inexpensive biological system for observing mitotic stages in a clear and unambiguous way. Have students locate each mitotic stage and record the number of interphase, prophase, metaphase, anaphase, and telophase cells present in the field of view at medium magnification. Tally numbers for the entire class and convert these numbers into percentages of total cells found in each stage-these numbers will be proportional to the time spent in each stage of the life cycle of these cells. Have students locate newly-divided cells that are just growing into full-sized cells and are in the interphase stage. Discuss cell growth as part of interphase. 5

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2. Microscopic Examination of Animal Mitosis (Optional) Have students locate each mitotic stage in slides prepared from whitefish blastulas. (You will find that the mitotic stages in these preparations are much less clear than in the onion samples--but they are useful in that they allow students to visually compare dividing plant and animal cells.) If you have been able to obtain recently fertilized snail, frog, or toad eggs, observe them under a dissecting microscope as they divide. Try to locate 2 cell, 4 cell, 8 cell, 16 cell and the more advanced stages of embryological development. Raise these embryos to adulthood in an aquarium and observe the daily changes that occur during their development. Discuss the differentiation of cells in the context of embryological development, or assign this subject as a topic for library research. 3. Microscopic Examination of Chromosomes and Genes Have students observe the giant chromosomes found in specially prepared slides of the salivary glands of the fruit fly Drosophila melanogaster. Because the DNA of these cells has replicated 9-10 times without subsequent cell division, a large number of DNA strands will be present side by side in a single chromosome. It is because of this that these chromosomes are very thick and, hence, extremely useful for laboratory observation. Each band of light or dark stain on a chromosome may indicate the location of an individual gene. Chromosome "Puffing" in the regions of specific bands is believed to indicate gene activity (messenger RNA synthesis, inactivation of protein repressors, etc.). In the context of the fruit fly chromosomes, discuss the "Human Genome Project." Describe the basic research techniques employed in mapping the exact location of all human genes found on the 23 different human chromosomes, or assign this subject as a topic for library research. 6

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4. Microscopic Examination of Cells Undergoing Meiosis Have students observe the meiotic processes of spermatogenesis in prepared slides obtained from the testes of a rat. Although it is difficult to identify specific meiotic stages, it is possible to identify germ cells in the early, middle, and later meiotic stages simply by noting their proximity to the sperm found in the centers of the tubules (peripheral cells will be in early meiotic stages and those toward the center will be in the later meiotic stages). Compare the process of spermatogenesis to the complementary female meiotic process called oogenesis by which haploid eggs are produced, or assign this subject as a topic for library research. VOCABULARY LIST Anaphase: The mitotic stage that follows metaphase; duplicated chromosomes separate at the centromere and migrate toward the mitotic centers. Asters: Microtubules and fibers that radiate out from the centrioles. Asexual Reproduction: Reproduction involving only one parent. Blastula: A hollow ball of cells formed during the early stages of embryological development. Whitefish blastula cells are used to demonstrate animal mitosis. Centromere: The part of a chromosome where the chromatids are joined together. Centriole: In animal cells, a cytoplasmic organelle that organizes the mitotic spindle fibers during cell reproduction. Chromatid: One of the two strands that make up chromosomes seen in prophase and metaphase that have duplicated their DNA during interphase. During anaphase, chromatids separate to form daughter chromosomes. 7

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Chromatin: The coils of DNA and protein that condense to form chromosomes. Chromatin can be thought of as chromosomes with no distinct shape. Chromosome: Distinct wormlike structures formed from chromatin during cell reproduction. Crossing Over: An exchange of chromosomal material between homologous pairs that occurs during Prophase One of meiosis. Cytokinesis: Cytoplasmic division that follows division of the nucleus. Diploid: Having two of each chromosome. Humans have 23 different chromosomes, yet in each body cell, these chromosomes occur in twos called homologous pairs. For this reason, each body cell possesses a diploid number of 46 chromosomes. DNA Replication: The process of doubling the DNA that occurs before mitosis. Germ Cells: The only cells that can undergo meiosis-found in the ovaries of females and the testes of males. Haploid: The actual number of different types of chromosomes a cell possesses. Homologous Pairs: In diploid cells, a pair of identical chromosomes is called an homologous pair. Interphase: The phase of a cell's life cycle between the reproductive stages of mitosis. DNA replication occurs during interphase. Most cells spend about 95% of their life cycles in interphase. Meiosis: The process that germ cells undergo by which the number of diploid chromosomes is reduced by half. Sperm and egg cells are created by meiosis.

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Metaphase: The stage of mitosis where duplicated chromosomes line up along the center of the mitotic spindle. Microtubules: Tiny tubes that make up most of a cell's "cytoskeleton." Spindle fibers are made up of microtubules. Mitosis: The duplication and division of the chromosomes and nucleus during cell reproduction. Mitotic Centers: The centers of mitotic activity of a cell-toward which separated chromosomes migrate. Oogenesis: The meiotic process that results in the formation of eggs in a female. Ova: Another word for eggs. Ovum: One egg. Polyploid: Having more than a diploid number of chromosomes. Prophase: The first stage of mitosis when chromosomes form from chromatin and the nuclear membrane is absorbed into the cell. Reduction Division: Cell division such as occurs in meiosis that results in the production of cells with half the number of chromosomes found in the original parent cells; cell reproduction without DNA replication. Sexual Reproduction: Reproduction requiring two parents. Somatic Cells: Body cells. Cells other than germ cells. Spermatogenesis: The meiotic process that results in the formation of sperm cells in males.

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Spindle Fibers: Microtubules visible during cell division that are involved in separating the chromosomes into two separate, yet identical groups. Synapsis: The pairing of homologous chromosomes during meiosis. Synapsis does not occur during mitosis. Telophase: The last stage of mitosis when the chromosomes return to the form called chromatin and the nuclear membrane reforms. Telophase usually happens simultaneously with cytokinesis. MITOSIS AND MEIOSIS SCRIPT OF VIDEO PRESENTATION Nearly every organism that is made up of many cells begins life as the single cell of a fertilized egg. That single cell divides over and over again until eventually an embryo is formed that is made up of trillions of cells of many different types. Over time, the embryo develops into a baby, and even at a very young age, some of the baby's cells begin to wear out. In fact, in a typical human being, every second of every day witnesses the death of about 50 million cells. Therefore, new cells must be constantly produced to replace old, dead and damaged cells. For cells to reproduce themselves, whether in a developing embryo or in a fully grown adult organism, certain definite steps must be followed to assure that the new cells will contain exactly the same genetic materials, or genes, that were originally present in the parent cell, and the essential process underlying the reproduction of cells is called mitosis. Mitosis is defined as "the duplication and division of the nucleus of a cell and its chromosomes during cell reproduction." 10

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Scientists recognize four distinct stages of mitosis: first is Prophase, second is Metaphase, third is Anaphase, and fourth is Telophase. An average of about six percent of a cell's total lifespan is spent in these four stages of mitosis, while the other 94% of its life is spent in a stage that is not considered to be a part of mitosis called interphase. INTERPHASE: The Resting Stage Because the events that occur in interphase allow mitosis to take place, let us begin our examination of mitosis by taking a closer look at interphase cells. Interphase is defined as "the period of a cell's life cycle between one mitosis and the next mitosis; the period of cell growth." As can be seen in this microscopic image on an onion root, some cells are dividing, but the nuclei of most of the cells are round and intact...these are interphase cells. Average human cells, such as these that form connective tissue, spend about 19 hours in interphase and only between 50 to 90 minutes in mitosis and cell division. However, there is considerable variation among different types of cells as to how much of their life cycles are spent in interphase and in mitosis. For example, this human nerve cell very rarely reproduces; instead, it can remain in interphase for up to sixty years. Because nerve cells rarely reproduce, even to replace dead or damaged cells, nerve damage, such as results from strokes or spinal cord injuries, is usually very serious. On the opposite end of the scale from nerve cells are cancer cells. These purple stained cancer cells, which stand out clearly against a background of red blood cells, were taken from a leukemia victim. 11

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Cancer cells like these do not remain in the interphase state very long; instead, mitosis and cell division continues at a furious pace. In fact, cancer can be thought of as cells that never rest, that divide over and over again, never remaining very long in interphase. Even though it is often thought of as a resting state between cell divisions, a lot is actually happening inside the nucleus throughout much of this stage of a cell's life cycle. During interphase, deoxyribonucleic acid, or DNA, the enormous molecule that forms the cell's genes and that holds all of the cell's operating instructions, duplicates itself. Deep inside the nucleus of interphase cells, the two DNA strands unwind as new copies of the DNA are created. Scientists refer to this process of DNA duplication as replication because exact replicas of the DNA are produced. The replication of a cell's DNA makes it possible for two complete sets of biological instructions to be sent into the new daughter cells. During mitosis, DNA is found within chromosomes; but chromosomes don't actually exist during interphase, instead, DNA and protein form the grainy threads we see here called chromatin. The presence of chromatin is one indication a cell is in interphase, for only with the onset of mitosis does the chromatin mold itself into the distinct shape of the chromosomes seen here. Scientists still do not understand the exact roles that chromosomes play within the cell, but it seems likely that chromosomes are the best way to package DNA, and 12

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consequently genes, for shipment into the newly-forming cells. The chromosomes seen here, taken from the salivary gland of a fruit fly, provide a convenient material for scientific study because they are ten times longer and one hundred times thicker than normal chromosomes. Working with these cells, scientists have discovered that the dark and light bands we see here are the actual location of genes on the chromosomes. And their studies have also shown that when a particular gene on one of these chromosomes becomes active, the spot on the chromosome where that gene is located takes on a strange, puffed-out appearance. During mitosis, the appearance, movement, separation and disappearance of chromosomes is of critical importance to the cell reproductive process. In fact, by recognizing what is happening to the chromosomes, we can identify each of the four stages of mitosis. Now let us follow the events inside of a cell as it starts to undergo the intricate process of mitosis. PROPHASE: The First Stage of Mitosis The earliest sign that a cell is leaving interphase and is entering the first stage of mitosis, called prophase, is that the chromatin begins to form itself into the definite shapes of separate chromosomes; and at the same time, the nuclear membrane that separates the nucleus from the cytoplasm begins to be absorbed into the cell. As these things happen, the tiny nucleolus, found in the nucleus, also disappears. Thus, prophase can be defined as "the stage of mitosis when the chromosomes first appear and the nuclear membrane and nucleolus disappear from view." As prophase progresses, the chromosomes become more distinct; and now that the chromosomes can be seen, a 13

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rather odd fact becomes apparent: that chromosomes are always found in groups of two called homologous chromosomes. This means there are two copies of each chromosome present in every cell of the body with the exception of certain sex cells. In our example, there is one pair of tall chromosomes and one pair of short chromosomes. This state, where double chromosomes are present, is called the diploid condition of a cell. The diploid condition of a cell means that there will be two complete sets of DNA instructions present inside its nucleus, so that if something is wrong with one set of instructions, the other set will still be able to provide the cell with the information it needs to function properly. Under close examination, we find that in early prophase each individual chromosome--whether tall or short--has a rather thickish appearance. This is because the DNA of each chromosome has replicated itself during interphase. On looking even more closely, we see that each chromosome actually consists of two parts called sister chromatids that contain the duplicated DNA; and at first, the sister chromatids are stuck very close together all along their lengths. It can now be seen that during prophase, and part of interphase, there are actually four complete sets of DNA instructions present because the DNA of each homologous pair is doubled at this point. The sister chromatids are held together at a specific region called the centromere. Joined together in this way, the chromosomes begin to arrange themselves so they can separate into the newly-forming cells; and as they do so, the sister chromatids become much more obvious. Besides the changes in chromatin and chromosomes, other important events are also occurring inside the cell when prophase begins. In animal cells, a structure called a 14

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centriole or centrosome divides into two daughter centrioles that migrate to opposite ends of the cell. Between the centrioles, a delicate arrangement of microtubules, called the spindle, is formed. The microtubules that make up the spindle are called spindle fibers. Spindle fibers are critical to cell reproduction because they help arrange the chromosomes and later in mitosis separate them into two equal groups. In normal cells, all the complicated events that define prophase take between 30 and 60 minutes to complete, finally ending when the next stage of mitosis, called metaphase, is reached. METAPHASE Metaphase, the second stage of mitosis, is defined as "the stage of mitosis when all the chromosomes are lined up along the center or equator of the cell." Throughout this short 5- to 10-minute stage, the chromosomes are attached to the spindle fibers, and the centromeres that bind the sister chromatids together split apart.

ANAPHASE The splitting of the centromeres signals the start of the third stage of mitosis called anaphase. Anaphase is defined as "the stage of mitosis when the sister chromatids separate and move toward opposite poles of the cell," and when this happens, they are no longer called sister chromatids--they are now called "daughter" chromosomes. This movement happens fairly rapidly, in about five minutes, with the result that the spindle fibers disappear from view and a full "diploid" set of chromosomes is now found at each end of the cell. 15

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TELOPHASE AND CYTOKINESIS The fourth and final stage of mitosis is called telophase. Telophase is defined as "the stage of mitosis when the new daughter chromosomes change back into the threads of chromatin and new nuclear membranes begin to form." Also during telophase, new nucleoli appear in each newlyforming nucleus. As the final stage of mitosis concludes, the cytoplasm divides in half as cell membranes close up around the two new daughter cells. This final process of cell reproduction is called cytokinesis. Cytokinesis is defined simply as "the division of the cytoplasm during mitosis." Between ten and fifteen minutes are required to finish both telophase and cytokinesis. When the entire cell reproductive process is completed, the two new cells are returned to the interphase state, and each possesses the identical genes of the parent cell. These two new "half-size" cells will then go on to grow larger until each achieves the full size of the original parent cell. End of Part 1 Video Instructional Quiz, Part 1 1. The state of mitosis __________________. A. metaphase seen here is called

2. Chromatin shortens and thickens to form these distinct structures called _________________. A. chromosomes 16

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3. DNA is replicated during the stage between cell divisions called ___________________. A. interphase 4. True or False: The division of the cytoplasm following mitosis is called cytokinesis. A. True 5. The first stage of mitosis, the stage when the nuclear membrane begins to be reabsorbed into the cell, is called _____________________. A. prophase Part 2 MEIOSIS In the first part of this program we learned how body cells reproduce themselves by mitosis so that the new cells contain the same diploid, or double, number of chromosomes found in the parent cell. Now we will learn how certain diploid germ cells found only in the ovaries and testis undergo the process similar to mitosis called meiosis that results in the production of sex cells that have only one half the number of chromosomes found in the parent cells. In the case of human beings, the diploid body cells all have 46 chromosomes, whereas the sex cells, the sperm and egg, have only 23 chromosomes. This condition where paired chromosomes are absent is called the haploid condition of a cell. When two haploid cells, sperm and egg, join together at fertilization, a new diploid cell is formed that will then undergo mitosis over and over again, resulting in the creation of a brand new individual. Meiosis is similar to mitosis in many ways, but in other ways, it is quite different. Now let us take a closer look at some of the details of this fascinating process. 17

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PROPHASE ONE The earliest part of the first stage of meiosis, called prophase one, begins just as it does in mitosis: chromosomes form from chromatin, and these chromosomes contain DNA replicated during the preceding interphase. As prophase one progresses, two things happen in the germ cells that didn't occur during prophase in mitosis: first, the identical, or homologous, chromosomes form pairs with one another, so that the tall chromosomes find their mates and the short chromosomes do likewise. This pairing of homologous chromosomes during the first prophase of meiosis is defined as synapsis. The second difference between prophase one of meiosis and the one we saw in mitosis is that parts of one paired chromosome can actually be exchanged with parts of its opposite pair. This process, called crossing over, is defined as the exchange of chromosomal material during synapsis and occurs only during the first prophase of meiosis. METAPHASE ONE During the next stage of meiosis called metaphase one, the centromeres of the homologous chromosomes line up on the equator of the cell. In contrast, during mitosis, the centromeres of the homologous chromosomes align themselves in a completely independent fashion. ANAPHASE ONE During the third stage of meiosis called anaphase one, the homologous chromosomes, each made up of two sister chromatids, move apart, centromeres intact, one toward each mitotic center. In contrast, during mitosis the centromeres separate during anaphase and the sister chromotids move apart as new 18

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"daughter" chromosomes. TELOPHASE ONE During the fourth stage of meiosis called telophase one, each new cell contains just one of each homologous chromosome, and each of these chromosomes still consists of two "sister" chromatids. PROPHASE TWO The fifth stage of meiosis, called prophase two, follows right on the heels of telophase one without an interphase stage in between them, as happens in mitosis. This means the DNA of the chromosomes in the prophase two cells has not been replicated--and this explains why the chromosomes of prophase two cells have not sprouted any new sister chromatids. From now on out, meiosis follows the same stages as mitosis. METAPHASE TWO During the sixth stage of meiosis, called metaphase two, the chromosomes line up on the equator of the cell, and now the centromeres split apart. ANAPHASE TWO And during the seventh stage of meiosis, called anaphase two, the centromeres separate for the first time, and now the sister chromatids become daughter chromosomes as they move toward opposite poles of the cell. TELOPHASE TWO Finally, during the last stage of meiosis, called telophase two, new nuclear membranes form. And as the chromosomes change back into chromatin, we can see that four new sex cells have been produced from the single original 19

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diploid germ cell. But since the DNA was only replicated once throughout a series of two cytoplasmic divisions, the final number of chromosomes in the sex cells is only half the number present in the diploid parent cell, in other words, the sex cells are haploid. Now when two haploid sex cells, sperm and egg, join together, the fertilized egg will contain the same diploid number of chromosomes found in any body cell. Meiosis is very important. It enables all organisms that reproduce sexually, that is require both male and female parents, to do so without continually multiplying the amount of DNA in the nucleus. For without meiosis, the human diploid number of 46 chromosomes would reach 368 in just three generations, thus creating a very messy state inside the third generation cells. Meiosis also provides an effective mechanism whereby the DNA of the two parents may be re-sorted and re-combined in new ways, and for this reason, no two human beings are exactly the same. Meiosis assures that each new generation will be unique and will always have new possibilities. The result is that each generation will always have its own great athletes, scientists, artists, musicians and inventors. And, as strange as it seems, meiosis also provides a physical bridge between successive generations, as parents pass along a tiny amount of their own substance--their own DNA--to their children. Video Instructional Quiz, Part 2 1. As a result of meiosis, sex cells are formed that have ____________the number of chromosomes of body cells. A. half 20

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2. True or False: Pairs of identical chromosomes are also called homologous chromosomes. A. True 3. True or False: Normal body cells are diploid because they possess one of each different chromosome. A. False 4. True or False: During meiosis, the DNA is replicated twice. A. False 5. True or False: In plant and animal cells, meiosis occurs only in germ cells. A. True BLACKLILNE MASTER 5 CROSSWORD PUZZLE ANSWERS

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