Exercise 2: Mitosis and Meiosis PDF

UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory EXERCISE 2: MITOSIS AND MEIOSIS INTRODUCTION All new cells come from previously existing cells. New cells are formed by karyokinesis (the process in...

UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory EXERCISE 2: MITOSIS AND MEIOSIS INTRODUCTION All new cells come from previously existing cells. New cells are formed by karyokinesis (the process in cell division that involves replication of the cell’s nucleus) and cytokinesis (the process in cell division that involves division of the cytoplasm). There are two types of nuclear division: mitosis and meiosis. Mitosis, the cell division of non-reproductive cells, results in new somatic cells (2n) that are genetically identical to the parent cell. Mitotic cell division is involved in the formation of an adult organism from a fertilized egg, asexual reproduction, regeneration, and maintenance or repair of body parts. Meiosis results in the formation of gametes in plants, fungi, and animals. These cells have half the chromosome number of the parent cell (1n). Mitosis, a cell division, is best observed in cells that are growing at a rapid pace, such as in the whitefish blastula or onion root cell tips. Root tips contain a growth region called the apical meristem, where the highest percentage of cells are undergoing mitosis. The whitefish blastula is formed immediately after the egg is fertilized, a period of rapid growth and numerous cell divisions when mitosis can be observed. Mitosis consists of several stages, with an additional stage before and after mitosis. Interphase precedes mitosis. During interphase, the cell will have a distinct nucleus with one or more nucleoli that is filled with a network of threads of chromatin. DNA replication occurs during interphase. After duplication, the cell is ready to begin mitosis. Prophase is when the chromatin thickens until condensed into distinct chromosomes. The nuclear envelope dissolves and chromosomes are in the cytoplasm. The first signs of the microtubule-containing spindle also begin to appear in prophase. Next, the cell begins metaphase. During this phase, the centromere of each chromosome attaches to the spindle and is moved to the center of the cell. This level position is called the metaphase plate. The chromatids separate and pull to opposite poles during the start of anaphase. Once the two chromatids are separate, each is called a chromosome. The last stage of mitosis is telophase. At this time, a new nuclear envelope is formed and the chromosomes gradually uncoil, forming the fine chromatin network seen in interphase. Cytokinesis may occur by forming a cleavage furrow that will form two daughter cells when separated. 1 UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory Figure 1. Interphase and Mitosis Meiosis is more complex than mitosis and involves two nuclear divisions called Meiosis I and Meiosis II. These divisions result in the production of four haploid gametes and allow for genetic variation due to crossing over of genetic material. Prior to the process, interphase involves replication of the DNA. During prophase I, the first meiotic stage, homologous chromosomes move together to form a tetrad and synapsis begins. This is the stage in which crossing over occurs, resulting in the recombination of genes. In metaphase I, the tetrads move to the metaphase plate in the middle of the cell as in mitotic metaphase. Anaphase I brings the tetrads back to their original two-stranded form and moves them to opposite poles. During telophase I, the cell prepares for a second division. In prophase II, centrioles move to opposite ends of the chromosome group. In metaphase II, the chromosomes are positioned at the center of each daughter cell. Anaphase II involves the centromere and separation of chromatids. Telophase II occurs when the divided chromosomes separate into different cells, known as haploid cells. Figure 2. Meiosis 2 UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory Spermatogenesis and Oogenesis Meiosis, the process by which gametes are formed, can also be called gametogenesis (= “creation of gametes”). The specific type of meiosis that forms sperm is called spermatogenesis, and the formation of egg cells, or ova, is called oogenesis. The most important thing to remember about both processes is that they occur through meiosis, but there are a few specific distinctions between them. Spermatogenesis The male testes have tiny tubules containing diploid cells called spermatogonia that mature to become sperm. The basic function of spermatogenesis is to transform each diploid spermatogonium into four haploid sperm cells. This quadrupling is accomplished through meiotic cell division as detailed in the last section. During interphase, before Meiosis I, each spermatogonium’s 46 single chromosomes are replicated to form 46 pairs of sister chromatids. Sister chromatids exchange genetic material through synapsis before the first meiotic division. In Meiosis II, the two daughter cells go through a second division to yield four cells containing a unique set of 23 single chromosomes that, ultimately, mature into four sperm cells. Starting at puberty, a male will produce literally millions of sperm every single day for the rest of his life. Figure 3. Spermatogenesis Oogenesis Similar to spermatogenesis, oogenesis involves the formation of haploid cells from an original diploid cell, the primary oocyte, through meiosis. The female ovaries contain primary oocytes. There are two major differences between the male and female production of gametes. First, oogenesis only leads to the production of one ovum, or egg cell, from each primary oocyte (this is in contrast to the four sperm that are generated from every spermatogonium). Of the four daughter cells that are produced when the primary oocyte divides, three are much smaller than the fourth. These three smaller cells, called polar bodies, eventually disintegrate to leave only the last ovum as the final product of oogenesis. The production of one egg cell via oogenesis normally occurs only once a month, from puberty to menopause. 3 UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory Figure 4. Oogenesis This exercise describes the events of the cell cycle and compares mitosis and meiosis. This will enable you to understand the differences in the processes and outcomes of each mode of cell division. OBJECTIVES At the end of this exercise, you should be able to: 1. identify the stages of mitosis and meiosis; 2. explain the differences and similarities between mitosis and meiosis; and 3. locate the stages of mitosis in onion root tips. MATERIALS slides/pictomicrographs (i.e. Mitosis, Meiosis, Onion root tip) Illustrations Bond papers Pencil ACTIVITY 1. Mitosis A. Mitosis in Plants Study the prepared slides/pictomicrographs/diagrams of various stages of cell division and locate cells in all the five major stages of division. Draw and label all stages of mitosis. For convenience, mitosis has been classified into four stages: prophase, metaphase, anaphase and telophase. The period of the cell cycle between divisions (G1, S, G2) was previously thought to be a resting stage, and is referred to as interphase. It is now clear that this period is characterized by a high level of metabolic activity in which the cell undergoes specialization or performs its normal functions. Regulatory events control whether or not the DNA will be copied, a process that commits the cell to undergoing mitosis again. It should be kept in mind that the process is dynamic and continuous and each stage actually passes imperceptibly into the next. 4 UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory 1. Interphase: Nuclei in interphase show little definable structure except the prominent, darkly stained spherical nucleoli. The chromosomes are uncondensed and give the nuclei a relatively homogeneous appearance. During the S (synthesis) phase of the cell cycle, the chromosomes undergo replication to produce two sister chromatids attached to each other at a single point called the centromere. 2. Prophase: The first microscopically visible evidence of division is when the chromosomes condense such that individual chromosomes become distinguishable. As prophase progresses, the chromosomes become shorter, thicker, and more distinct. Coiling becomes more pronounced and eventually, the coils take on a regular, smooth appearance. As the chromosomes become shorter and thicker, the nucleoli progressively disappear. 3. Metaphase: At the end of prophase, the nuclear membrane disappears and a spindle composed of microtubules develops from the two poles toward the equator. Some of the spindle fibers extend from pole to pole while others become attached to the chromosomes. Spindle fibers become attached to the centromere from each pole of the spindle. The chromosomes migrate to the middle of the spindle, equidistant from the poles. There the chromosomes become aligned in a single plane. 4. Anaphase: Anaphase begins with the poleward movement of sister chromatids. The centromere may be in the middle of the chromosome or at any position between the middle and the ends. Thus, chromosomes in anaphase may be V-J- or rod-shaped as they trail behind the centromere. As soon as the chromatids separate in anaphase, they represent separate chromosomes. 5. Telophase: The chromosomes are reorganized into a nuclear structure with a membrane and the nucleoli reappear. The chromosomes uncoil, become thinner and more thread- like, and gradually return to their interphase state. 5 UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory Figure 5. Appearance of different phases of mitosis in onion root tip. 6 UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory B. Mitotic Index in the Onion Root Tip The mitotic index is defined as the proportion of cells that are in the process of dividing. The onion root tip is an area of active growth and, therefore, one would expect that mitosis is occurring at a rapid rate. What the mitotic index will do is tell us how active this tissue is. Using a prepared slide/pictomicrograph of an onion root tip, count 100 cells and indicate which stage of mitosis each of these cells is in. Enter your data in the following table: Table 1. Calculation of Mitotic Index What is the mitotic index of your sample? _________________________ Why is the mitotic index less than 100%? __________________________________________ What does this tell you about how long it takes mitosis to occur? ______________________ ___________________________________________________________________________ 7 UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory C. Cytokinesis in Plants In most organisms, cell division consists of two processes; namely, nuclear division (karyokinesis) and cytoplasmic division (cytokinesis). In plants, cytoplasmic division starts before nuclear division is complete and the two processes finish simultaneously. In a few instances, however, they are separated by a considerable period. Re-observe the later stages of nuclear division and look for evidence of cytoplasmic division. The first indication is the appearance of a continuous fluid film or cell plate at the equator. Pectic substances and other compounds are then deposited and the cell plate becomes a rigid layer, the middle lamella, separating the two protoplasts. Cellulose and other materials are produced on either side of the middle lamella and the new cell walls are formed. Record and draw your observations. Activity 2. Meiosis A. Study the prepared slides/pictomicrographs of the various stages of meiosis. As with mitosis, meiosis has been divided into several stages. You should understand the sequence of events necessary to form a haploid gamete and not become overwhelmed with the exact stages that are described below. Draw and label all stages of meiosis. MEIOSIS -PART I: 1. Interphase - Nuclei in interphase show little definable structure except the prominent, darkly stained spherical nucleoli. The chromosomes are uncondensed and give the nuclei a relatively homogeneous appearance. During the S (synthesis) phase of the cell cycle, the chromosomes undergo replication to produce two sister chromatids attached to each other at a single point called the centromere. This is the same as for cells entering mitosis. 2. Prophase I - The highlight of this stage is the unique pairing (synapsis) of homologous chromosomes to form a tetrad. This pairing permits an exchange of DNA between homologous chromosomes resulting in crossing over, or recombination. In the best preparations, it is possible to identify the four chromatids involved in forming the tetrad. 3. Metaphase I - Tetrads are lined up on the equatorial plate and will often appear as loops or circles. 4. Anaphase I - Homologous chromosomes move to opposite poles. Movement is a direct result of the attachment of the spindle fibers to the centromere. 5. Telophase I - Each of the two new nuclei that start to re-form here have only one-half the number of chromosomes as the original cell, but each is still duplicated. MEIOSIS - PART II: 1. Interphase II - The nuclei will uncoil and often there is some delay prior to the start of the second part of meiosis. IMPORTANT: NO replication of genetic material occurs in the second phase of meiosis! 2. Prophase II - Chromosomes start to contract and are visible again. 3. Metaphase II - The duplicated chromosomes line up on the equatorial plate. 4. Anaphase II - Centromeres separate which permits the duplicated chromatids to move toward opposite poles 8 UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory 5. Telophase II - the nuclei start to re-form. Each contains a haploid number of chromosomes (see photograph h). The actual production of a functional egg or sperm often requires some morphological change after the completion of meiosis. It can be seen, for example, that sperm are not the immediate product of Meiosis II, but are formed only after the cytoplasm is transformed into a long extension to form the tail. B. Comparison of Mitosis and Meiosis Using the prepared slides or Figure 6 (on the next page), compare the processes of mitosis and meiosis. Similarities 1. 2. 3. 4. 5. Differences 1. 2. 3. 4. 5. 9 UP Tacloban College-DNSM Bio 140.1: Elementary Genetics Laboratory Figure 6. Mitosis and Meiosis 10 GUIDE QUESTIONS 1. Why is mitosis essential for eukaryotes? 2. How do the daughter cells formed by mitosis compare genetically to each other? 3. How would you expect the mitotic index of a cancer tissue to compare with that of the onion root tip? 4. What structure(s) is(are) responsible for movement of chromosomes during the mitotic process? 5. What would occur if a cell completed karyokinesis but not cytokinesis? 6. Why is meiosis essential for sexually-reproducing organisms? 7. How would a tubal ligation or vasectomy influence meiosis in individuals who elected to have one of these operations? 8. Identify what is accomplished in Meiosis I? Meiosis II? 9. Synapsis of homologous chromosomes is perhaps the most important event in meiosis. What is it and what does it do? 10. When does crossing-over occur and how does it influence the gametes formed? EVALUATION CRITERIA Drawing and label - 35 pts Tables & Comparison - 23 pts Answers to Guide Questions - 12 pts TOTAL - 70 pts REFERENCES Newman. 2015. Biology 110 Laboratory Manual. Retrieved from: http://lycofs01.lycoming.edu/~newman/Bio110IntroBio/Lab06-Mitosis-Meiosis-Mendel.pdf Mitosis and Meiosis. (n.d.). Retrieved from: https://www.southalabama.edu/biology/notes/121%20lab/_baks/BLY121_lab08_Mitosis.pdf.0001.25e3.bak Mitosis and Meiosis. (n.d.). Retrieved from: https://bio.libretexts.org/Ancillary_Materials/Laboratory_Experiments/General_Biology_Labs/BIOL_110 7%3A_Principles_of_Biology_I_Lab_Manual_(Burran_and_DesRochers)/Lab_09%3A_Mitosis_and_Meio sis