General Biology 1 Unit 3: Cell Cycle PDF

Unit 3: Cell Cycle Lesson 3.2 Stages of Mitosis Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It!...

Unit 3: Cell Cycle Lesson 3.2 Stages of Mitosis Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It! 4 Mitosis 4 The Role of Mitosis in the Cell Cycle 5 G1 and G2 Phases’ Preparation for Mitosis 6 S Phase’s Preparation for Mitosis 6 Changes in Chromosome Number during Mitosis 7 The Chromosome 7 Haploid and Diploid Cells 7 The Phases of Mitosis 8 Prophase 9 Prometaphase 10 Metaphase 11 Anaphase 12 Telophase 12 Cytokinesis 13 Ploidy in Mitosis 14 Key Points 15 Check Your Understanding 16 Challenge Yourself 17 Photo Credits 17 Bibliography 17 Unit 3: Cell Cycle Lesson 3.2 Stages of Mitosis Introduction In the previous lesson, you have learned that the cell also undergoes its own life cycle. The cell cycle describes the different processes that the cell undergoes throughout its existence. Animals and plants reproduce in order to propagate their species. Cells undergo a very similar process in order to produce more cells that are similar to them. This process, known as mitosis, is an essential part of the cell cycle. Without it, our bodies will not be able to grow and develop properly. What is mitosis and why is it important? What are the stages of mitosis and how do these stages contribute to the division of the cell? 3.2. Stages of Mitosis 1 Unit 3: Cell Cycle DepEd Competency Learning Objectives Describe the stages of mitosis given In this lesson, you should be able to do the 2n=6 (STEM_BIO11/12- Id-f-7). following: Enumerate and describe the phases of mitosis. Differentiate between haploid and diploid chromosome counts. Warm Up Multiplication Is Also Division 15 minutes Mitosis, a type of cell division, consists of different stages and certain events happen in every single stage. This activity will give you an overview on the events in mitosis. Procedure 1. Find a partner. 2. Discuss the four figures given below. Brainstorm which part of the illustration represents the following: a. Chromosomes 1 and 2 b. Nuclear envelope c. Centrosomes d. Spindle fibers 3. In each figure, describe what happens on the four parts given in step 2. Organize your answer by completing the Table 3.2.1. 4. Answer the guide questions that follow. 3.2. Stages of Mitosis 2 Unit 3: Cell Cycle Figure A Figure B Figure C Figure D Observation Table Table 3.2.1. Description of Figure A, B, C, D Description Figure Chromosomes Nuclear envelope Centrosomes Spindle fibers A B C D 3.2. Stages of Mitosis 3 Unit 3: Cell Cycle Guide Questions 1. What is the final product of the process outlined in the activity? 2. Based on the centrosomes shown in the figures, what role do you think it plays in mitosis? 3. Based on this activity, how will you define mitosis? Learn about It! Mitosis The process of mitosis refers to how a cell, known as a parent cell, divides to produce new cells known as daughter cells. Recall that only somatic cells (also known as body cells) undergo mitosis. Sex cells, like sperm and egg cells, undergo a different process of the division known as meiosis. Take note that mitosis and meiosis both refer to the process of division that only individual cells undergo. Did You Know? There are many different processes that cells and entire organisms undergo that resemble mitosis. Mitosis is the division of somatic cells. Meiosis is the division of sex cells. Prokaryotes can also divide as a form of reproduction known as binary fission. Some animals can divide to form offspring in a process known as transverse fission. Some simple unicellular organisms, on the other hand, use mitosis or a similar process in order to reproduce. For example, in the 2001 film “Evolution”, the scientists observed an alien organism dividing to form two new organisms. They incorrectly termed this as mitosis, when in fact it is more similar to transverse fission. 3.2. Stages of Mitosis 4 Unit 3: Cell Cycle Why is mitosis important for cells? What are the different processes involved? The Role of Mitosis in the Cell Cycle The previous lesson discussed the cell cycle in much detail. It begins with interphase. Interphase has multiple phases namely: Gap 1 phase, S phase, and Gap 2 phase. The M phase follows interphase. It is in the M phase wherein the cell undergoes mitosis if it is a somatic cell or meiosis if it is a sex cell. The cell cycle A cell will only begin the M phase if the conditions are correct and favorable for cell division. These conditions are largely affected by how the cell is prepared for mitosis by the different stages of interphase. Recall that the cell cycle checkpoints determine if the cell is prepared for mitosis or not. If the checkpoint deems the cell prepared, then the process can continue. If not, then the cell will attempt to fix the errors or it may undergo apoptosis or programmed cell death. 3.2. Stages of Mitosis 5 Unit 3: Cell Cycle G1 and G2 Phases’ Preparation for Mitosis The Gap 1 and Gap 2 phases of interphase are highly similar in that they prepare the cell for division in multiple ways. These phases involve the growth of the cell and the preparation of materials for cell division. There are also cell cycle checkpoints that will check for proper growth after these phases. Proper growth of the cell before mitosis can occur. This is because cell contents will be partitioned later on to the daughter cells. If the parent cell does not grow properly or produce the necessary materials, then the division of cellular materials to be distributed to the daughter cells will not occur. S Phase’s Preparation for Mitosis The S phase, or the synthesis phase, involves the duplication of the cell’s DNA. Since mitosis aims to produce daughter cells that contain the proper amount of DNA, duplication of DNA is necessary beforehand. DNA replication occurs in the S phase. 3.2. Stages of Mitosis 6 Unit 3: Cell Cycle More specifically, the parent cell aims to produce a similar amount of chromosomes for the daughter cells in mitosis. Suppose that the parent cell does not double the DNA that it has before mitosis. If this is the case, then the parent cell will only be able to give the daughter cells half of the DNA that is required. Doubling the DNA beforehand, however, will result in the proper amount being given by the parent cell to both daughter cells. Changes in Chromosome Number during Mitosis The Chromosome Recall that a cell’s information is stored in DNA or deoxyribonucleic acid. DNA is in the form of chromatin in interphase and it condenses into chromosomes during mitosis. Fig. 3.2.1. shows the parts of a chromosome. Each chromosome is made up of two sister chromatids. The centromere connects the two halves of a chromosome, and the centromeres have accompanying protein complexes known as kinetochores. The short arm and the long arm are the short and long ends of the chromosome, respectively. It is in these arms that the genes are found. Each end of the chromosome is known as the telomere. These parts, along with others, play significant roles in mitosis. These will be discussed in more detail later on in this lesson. Haploid and Diploid Cells Since humans are diploid organisms, our cells have 2 pairs of each chromosome in our cells. The human genome has 23 pairs of chromosomes for a total of 46 chromosomes in each cell. 23 is the haploid number whereas 46 is the diploid number. 3.2. Stages of Mitosis 7 Unit 3: Cell Cycle Haploid is often represented as n. A numerical coefficient is often attached to n to denote the number of copies of chromosomes that a cell or an organism has. For example, diploid cells are represented as 2n since they have double the amount of chromosomes compared to haploid cells. Triploid cells are 3n, tetraploid cells are 4n, and so on. Therefore if a 2n cell has 6 chromosomes, its haploid would have 3 chromosomes. The number of sets of chromosomes that a cell or an organism has is known as ploidy. Different organisms have different levels of ploidy. Humans, as was mentioned earlier, normally have diploid cells. Water bears as shown in Fig. 3.2.2. are triploid. Salmons are tetraploid. Some species of frogs are dodecaploid with twelve sets of chromosomes. The Phases of Mitosis After the cell has grown and manufactured the necessary components, mitosis can begin. Mitosis aims to divide the parent cell into two identical daughter cells. These daughter cells should have the exact same amount of chromosomes as the parent cell. The daughter cell should also be able to receive the cytoplasm, organelles, and other components from the parent cell. In order for these to occur, the process of mitosis is made up of steps and subprocesses that allow the proper division of the many components that the daughter cells need. A cell proceeded to mitosis even though its genetic material was not duplicated in the S phase of interphase. What will happen to the daughter cells? 3.2. Stages of Mitosis 8 Unit 3: Cell Cycle Much like interphase, the process of mitosis is also divided into multiple phases. These phases are prophase, prometaphase, metaphase, anaphase, and telophase. Each phase performs a crucial role that contributes to the process of cell division. The stages of mitosis Prophase The first stage of mitosis (shown in Fig. 3.2.3.) is a phase known as prophase. In interphase, the cell’s genetic material is still in the form of chromatin. Once prophase begins, however, the chromatin will start to condense into compact structures known as the chromosomes. This process of condensation is important because DNA is normally a very long molecule. When it condenses, the resulting product is a small and compact chromosome that can easily be divided during mitosis. 3.2. Stages of Mitosis 9 Unit 3: Cell Cycle While the condensation of genetic material occurs, other events also take place in the cell during prophase. One such event is the shrinkage and eventual disappearance of the cell’s nucleolus. Fig. 3.2.3. A cell in prophase Another important occurrence is the formation of the mitotic spindle in the cytoplasm. The mitotic spindle is made up of proteins called microtubules and will be used for separating the chromatids later on in mitosis. This process of spindle formation occurs in the area of the cell known as the centrosomes, of which the cell has two. Each centrosome further contains a pair of centrioles and these help with the formation of the mitotic spindle. Prometaphase Prometaphase is shown in Fig. 3.2.4.. It begins after the nuclear envelope, or the membrane that surrounds the nucleus breaks down. The two centrosomes, along with the accompanying centrioles and developing spindles, will move toward opposite sides of the cell resulting in one centrosome on each side. Each side is known as a pole. The spindles then start growing to the middle of the cell. It is also during this phase that the chromosome’s kinetochores at the centromeres become apparent. This is important for the next phase of mitosis. 3.2. Stages of Mitosis 10 Unit 3: Cell Cycle Fig. 3.2.4. The centrosomes at the cell’s poles Metaphase During metaphase, the microtubules of the mitotic spindle attach and interact with the kinetochores of the chromosomes. This causes the chromosomes to align at the center of the cell in an area known as the metaphase plate (shown in Fig. 3.2.5.). Metaphase ends once the proper alignment of the chromosomes is achieved. This alignment is when the sister chromatids of each chromosome face the opposite poles of the cell. Fig. 3.2.5. A cell in metaphase 3.2. Stages of Mitosis 11 Unit 3: Cell Cycle Anaphase In anaphase, the mitotic spindle pulls and separates the sister chromatids apart. As shown in Fig. 3.2.6., the chromatids, now called daughter chromatids, are then pulled toward the opposite poles. The movement will stop once the daughter chromatids completely reach the opposite poles. Fig. 3.2.6. A cell in anaphase Telophase The final phase of mitosis which is known as telophase is shown in Fig. 3.2.7.. After the daughter chromatids reach the opposite poles at the end of anaphase, they will start to decondense. The form that the genetic material will take returns to the one similar to anaphase. The nucleolus will also start to reappear. The nuclear envelopes will also reform. Once this is complete, there will be two entire nuclei with one found on each pole. In animal cells, some of the events of the division of the cytoplasm will ensue even as telophase occurs. This process continues further in cytokinesis to become the cleavage furrow. In plants, on the other hand, golgi vesicles will start to accumulate to where the metaphase plate used to be. This accumulation will allow the vesicles to fuse and form the cell plate, which becomes the cell plate. Both the cleavage furrow of animals and cell plate of plants become the new “boundaries” of the daughter cells. 3.2. Stages of Mitosis 12 Unit 3: Cell Cycle Fig. 3.2.7. A cell in telophase Tips When trying to differentiate between the appearance of cells in the different phases of mitosis, remember that the first letter of the phase shares the first letter of what the cell looks like in that phase. In Prophase, the cell’s genetic material resembles Polka dots or strings. In Metaphase, the chromosomes Migrate to the Middle of the cell. In Anaphase, the chromosomes move Away from each other. In Telophase, the parent cell starts to form Two new cells. Cytokinesis After telophase, cytokinesis will divide the cell into two. Cytokinesis is not a phase of mitosis but is closely related to it. Recall from the previous lesson that each of the two daughter cells will inherit a nucleus, similar genetic material, organelles, and more. 3.2. Stages of Mitosis 13 Unit 3: Cell Cycle Ploidy in Mitosis In humans, the number of chromosomes in somatic cells is 46 (two sets of chromosome) while sex cells have 23 (one set of chromosome). Fusion (fertilization) of egg and sperm cells, each having 23 chromosomes, restore the number of an individual which is 46. Therefore, if a cell before the S phase has 3 pairs of chromosomes for a total of 6 (or 2n=6), there is a total of six chromatids. The six chromatids become 12 after DNA replication in the S phase. Do not be confused with chromatids and chromosomes. Refer to Fig. 3.2.8. to visualize the number of chromosomes and chromatids before mitosis, after replication, and after mitotic division. This is the reason why the daughter cells have the same number of chromosomes as the parent cell when it undergoes mitotic division. Fig. 3.2.8. The number of chromosomes before and after mitosis 3.2. Stages of Mitosis 14 Unit 3: Cell Cycle Key Points ___________________________________________________________________________________________ DNA is in the form of chromatin in interphase and it condenses into chromosomes during mitosis. These phases are prophase, prometaphase, metaphase, anaphase, and telophase. Each phase performs a crucial role that contributes to the process of cell division. ○ Chromosome condensation happens in prophase. Spindle formation also begins. ○ Prometaphase begins after the completion of the breakdown of the nuclear envelope. ○ Metaphase involves the alignment of the chromosomes in the metaphase plate. ○ The sister chromatids move to the opposite poles of the cell in anaphase. ○ Telophase ends when two nuclei are formed in the cell. The stages of mitosis ___________________________________________________________________________________________ 3.2. Stages of Mitosis 15 Unit 3: Cell Cycle Check Your Understanding A. Identify the phase of mitosis that the following events belong to. 1. This begins after the nuclear envelope breaks down. 2. The nucleolus reappears. 3. The nucleolus disappears. 4. The chromosomes align in the middle of the cell. 5. The sister chromatids move to opposite poles. 6. Two nuclei are formed. 7. The mitotic spindle starts to form. 8. The spindles start to grow to the middle of the cell. 9. This ends after the chromatids face the opposite poles of the cell. 10. The daughter chromatids decondense. B. Sequence the following events based on when they happen in the cell, starting from the interphase. Write A for the earliest event and J for the latest. 1. G2 phase 2. G1 phase 3. Metaphase 4. Anaphase 5. Cytokinesis 6. Telophase 7. Metaphase checkpoint 8. G1 checkpoint 9. G2 checkpoint 10. Prophase 3.2. Stages of Mitosis 16 Unit 3: Cell Cycle Challenge Yourself Answer the following questions. 1. Why is the S phase necessary before mitosis? 2. What role does the mitotic spindle play in mitosis? 3. Why does the genetic material need to condense in prophase? 4. What is the reason for the chromosomes aligning at the metaphase plate? 5. What will happen if the daughter cells do not receive the necessary chromosomes from the parent cell? Photo Credits Waterbear by Bob Goldstein and Vicky Madden, is licensed under CC BY-SA 3.0 via Wikimedia Commons. Mitosis in yellow onion cells by Pixelmaniac pictures, is licensed under CC BY-SA 3.0 via Wikimedia Commons. Bibliography Boyer, Rodney F. Concepts in Biochemistry. Hoboken, NJ: Wiley, 2006. Hickman, Cleveland P. Integrated Principles of Zoology. New York, NY: McGraw-Hill, 2011. Miller, Stephen A., and John P. Harley. Zoology. New York, NY: McGraw-Hill, 2010. Russell, Peter J. Biology: the Dynamic Science. Student Ed. Belmont, CA: Thomson/Brooks/Cole, 2008. Starr, Cecie, Ralph Taggart, Christine A. Evers, and Lisa Starr. Biology: the Unity and Diversity of Life. Boston, MA: Cengage, 2019. 3.2. Stages of Mitosis 17