General Biology 1 - Cell Cycle (Unit 3) Study Guide PDF

Unit 3: Cell Cycle Lesson 3.1 Cell Cycle and Cell Divisions Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It! 4 The Genetic Material of...

Unit 3: Cell Cycle Lesson 3.1 Cell Cycle and Cell Divisions Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It! 4 The Genetic Material of Cells 4 Introduction to Cell Cycle 8 Stages of the Cell Cycle 9 Interphase 9 Mitosis or Meiosis 12 Cytokinesis 13 Cell Cycle Checkpoints 15 G1-to-S Checkpoint 16 G2-to-M Checkpoint 16 M Checkpoint 16 Key Points 17 Check Your Understanding 18 Challenge Yourself 19 Photo Credits 19 Bibliography 20 Unit 3: Cell Cycle Lesson 3.1 Cell Cycle and Cell Divisions Introduction You already know that your body is made up of cells. As a multicellular organism, these cells make up tissues, which will consequently make up organs and organ systems. Organisms, like plants and animals, rely on this complex organization for the proper functioning of their bodies. However, any sexually reproducing multicellular organism begins as a single-celled entity. This cell then relies on cellular events to produce more cells, the early stage of which is shown in the blastula above. Further growth and development proceed through the continuous reproduction and specialization of cells. The process repeats for as long as needed to ensure the maintenance and survival of an individual. Although not a perpetual process, organisms may still rely on the production of new cells on some occasions. Have you ever wondered how our bodies replenish cells? 3.1. Cell Cycle and Cell Divisions 1 Unit 3: Cell Cycle Learning Objectives DepEd Competency In this lesson, you should be able to do the Characterize the phases of the cell cycle following: and their control points Identify and describe the phases of the (STEM_BIO11/12- Id-f-6). cell cycle. Explain the significance of the cell cycle control points. Warm Up Pass the Ball 15 minutes The cell cycle has multiple steps. All of these steps are necessary to allow the resulting cells after division to function adequately because of the number of organelles and genetic that they will receive. Materials clay (one color) scissors yarn beads (three colors, 10 pieces each) Procedure 1. Form a group with four members. Assign each member to one of the following stages of the cell cycle. a. G1 phase b. S phase c. G2 phase d. M phase 2. To prepare for the activity, mold a piece of clay into a flattened circle of around 5 3.1. Cell Cycle and Cell Divisions 2 Unit 3: Cell Cycle inches in diameter. This represents the cell. Add the following to the clay circle. a. Obtain a piece of yarn around 3 inches long and add it to the circle. This represents DNA or the genetic material of the cell. b. Add three pieces of every bead color to the circle. This represents nutrients and other molecules present inside the cell. 3. Begin the activity by following the steps below. a. Give the cell model to the student assigned to the G1 phase. i. The assigned student must add clay to make the model larger. ii. Let that same student add beads to the cell model. b. Pass the model to the student assigned to the S phase. i. Let that student add an identical piece of yarn to the model. c. Pass the model to the student assigned to the G2 phase. i. The assigned student must add clay to make the model larger. ii. Let that same student add more beads to the cell model. d. Pass the model to the student assigned to the M phase. i. Let the assigned student split the model into two separate portions, with each receiving an equal quantity of yarn and beads. ii. Make sure that the two new models are roughly the same size as the one before the clay was added during the G1 phase. 4. For every step, observe and take note of what happens to the model. Complete Table 3.1.1. Also, write down what you think happens to the cell based on the changes in the model. 5. Answer the guide questions that follow. Data Table Table 3.1.1. Observations on the stages of the cell cycle Phase Observations in the Model What Happens to the Cell G1 S G2 M 3.1. Cell Cycle and Cell Divisions 3 Unit 3: Cell Cycle Guide Questions 1. What does each of the components in the model represent? 2. Why do you think the events happen in a stepwise manner? 3. Based on this activity, what do you think the cell cycle is for? Learn about It! The Genetic Material of Cells With very few exceptions, all of the cells in our body contain genetic material in the form of DNA or deoxyribonucleic acid, which is found in the nucleus of your cells. DNA is an extremely important molecule because it contains the information that codes for your traits and the processes that ensure survival. It does so because of its notable structure, which is made up of two helical strands of repeating units called nucleotides. These nucleotides form base pairs, the varying sequences of which code for the different traits. Most of your biological traits are actually coded for by DNA. Your eye color, eye shape, hair color, height, and many other heritable traits that are coded for by DNA. DNA, the primary genetic material of living organisms, stores genetic information in the sequences of its nucleotides. 3.1. Cell Cycle and Cell Divisions 4 Unit 3: Cell Cycle DNA has many forms by which it appears in the cell. The phase of the cell cycle usually dictates what form DNA will appear as. DNA is normally found in the form of chromatin while it is inside the intact nucleus of the cell. DNA, as a molecule, is very long. This is why it needs to be packaged into a form that can fit inside the cell. This is done through the basic structure known as the nucleosome. The nucleosome refers to the level of DNA packaging where a segment of DNA is wrapped around positively-charged proteins known as histones. Collectively, all of the DNA molecules of a cell constitute its genome. When the cell is in the phase of the cell cycle known as interphase, which will be discussed later on, then the DNA is usually found in the form of chromatin. Chromatin, the conformation of DNA that is observable during interphase, is a complex of DNA and histone proteins. Histone proteins, where the DNA molecule is wrapped around, helps organize the genetic material inside the nucleus. Chromatin will sometimes need to further condense into a highly coiled and compact structure. This highly condensed form of DNA is known as a chromosome (as shown in Fig. 3.1.1.). The condensation of the chromatin into the chromosome usually occurs during the cell cycle when the cell is about to divide, which will also be discussed later on. 3.1. Cell Cycle and Cell Divisions 5 Unit 3: Cell Cycle Diploid organisms have chromosomes that occur in pairs in each of their cells. Each pair of chromosomes are highly similar to each other but still bear a few differences. These pairs of chromosomes are known as homologous chromosomes. For example, humans have 46 chromosomes (such as in Fig. 3.1.2.), which is their diploid number. These chromosomes can be arranged into homologous pairs, for a total of 23 pairs. Fig. 3.1.2. A complete diploid set of human chromosomes comprises 23 homologous pairs. Each duplicated chromosome contains two identical members. Each member of the duplicated chromosome, known as a chromatid, contains the exact same genetic material as the other member. The two chromatids, having the exact same genetic material due to DNA replication, are then known as the sister chromatids. Each sister chromatid in a chromosome is linked to the other chromatid via the centromere. In summary, cells contain genetic information in the form of a double-helical molecule known as DNA. Segments of DNA are wrapped around proteins called histones, and the resulting complex is known as a nucleosome. Further coiling and condensation of the structure result in the formation of chromosomes, which are essential during cell division. 3.1. Cell Cycle and Cell Divisions 6 Unit 3: Cell Cycle The DNA inside cells has varying levels of organization—from the DNA wrapped around histone proteins forming nucleosomes, to highly condensed chromosomes. You are looking at a diploid cell which consists of four pairs of chromosomes. After DNA replication, how many chromatids are there? How many of these do you think are homologous? Tips Chromatin condenses into chromosomes. Each half of the chromosome is a chromatid. To help you remember this, imagine the chromatin to be a very thin string of DNA. Imagine the string being wound up into a thick rope. This thick rope is the chromosome. Now, imagine each half of that rope to be similar to the other half. These halves are the chromatids. 3.1. Cell Cycle and Cell Divisions 7 Unit 3: Cell Cycle Introduction to Cell Cycle Recall how different organisms that you know grow and develop. Plants produce seeds that sprout into seedlings. These seedlings grow, mature, and produce more seeds in order to produce more plants. Frogs (such as in Fig. 3.1.3.), on the other hand, lay eggs. These eggs hatch into tadpoles which metamorphose into froglets that eventually grow into adult frogs. These adult frogs will then lay eggs that will hatch into new tadpoles. You, a human, were born as an infant, grew into a child, are now a teenager, and will eventually become an adult. All species of organisms have their own life cycle that shows how they are born, develop, and mature. Fig. 3.1.3. Organisms go through their lives in a process known as the life cycle. The frog life cycle involves metamorphosis from a tadpole to an adult frog capable of reproducing to continue the life cycle. Much like entire organisms, individual cells also have their own life cycle. The life cycle of a cell is known as the cell cycle, and this describes how cells grow, develop, and reproduce. What are the different phases of the cell cycle and how does the cell make sure that these phases proceed smoothly? 3.1. Cell Cycle and Cell Divisions 8 Unit 3: Cell Cycle The cell cycle is a process that is essential to the growth and development of cells. In addition, it ultimately leads to the reproduction of cells. Similar to the life cycle of different organisms that occur in a stepwise fashion, different phases (as shown in Fig. 3.1.4.) also make up the cell cycle. Each of these phases will have specific events that contribute to the overall maintenance and survival of the cell. Fig. 3.1.4. The cell cycle involves different phases that constitute a series of preparations for a cell to divide to produce new cells. Stages of the Cell Cycle We mentioned earlier that the cell cycle is made up of multiple stages. Each of these stages will have specific events and occurrences that are crucial to help the cell function properly and be prepared for the division. There are three main stages of the cell cycle, namely, interphase, M phase, and cytokinesis. Interphase Most cells spend the majority of their time in the stage known as interphase. This stage is when the cell prepares itself for eventual cell division, which is the process of how cells produce more cells. During interphase, the major events that occur include the growth of the cell and the duplication of its genetic material. Interphase can be further subdivided into G1, S, and G2 phases. 3.1. Cell Cycle and Cell Divisions 9 Unit 3: Cell Cycle The phases of the cell cycle Gap 1 Phase The first stage of interphase is the G1 phase or gap 1 phase. G phases are considered gap phases because they intervene between DNA replication and cell division. G1 phase usually begins as soon as the previous cell cycle ends with the newly produced daughter cells. This phase involves the growth of the cell’s cytoplasm alongside the doubling of cellular organelles. Proteins and other molecules are produced for normal cell functioning. It is also during this stage that the rate of the synthesis of proteins is the highest. Other products that are produced include enzymes, nutrients, and energy molecules. A structure known as the centrosome will eventually be important during cell division. These centrosomes are composed of centrioles in animal cells. These are cytoskeletal elements that will be crucial in distributing the chromosomes into their respective daughter cells. During the G1 phase, the centrioles of the centrosomes move away to await and assist the events of mitosis. 3.1. Cell Cycle and Cell Divisions 10 Unit 3: Cell Cycle S Phase If all of the conditions of G1 checkpoint are satisfied, the S phase or the synthesis phase takes place. This stage is named so because it is when the cell synthesizes a copy of its DNA in a very notable process called DNA replication (shown in Fig. 3.1.5.). This refers to the event when an existing DNA produces another copy of itself. Through this process, the cell can effectively and almost accurately duplicate the genetic material that it has. Fig. 3.1.5 DNA replication duplicates the cell’s genetic material so that copies can be distributed to the daughter cells after cell division. 3.1. Cell Cycle and Cell Divisions 11 Unit 3: Cell Cycle The duplication of genetic material is very important in fulfilling the objectives of the cell cycle. Recall that mitosis aims to produce daughter cells that will contain the exact same copy of genetic material as the parent cells. This cannot occur if the parent cell only has one copy of the genetic material. For the daughter cells to have identical copies, the parent cell will need to duplicate their own genetic material. This is made possible by DNA replication with very minimal errors during the process. Gap 2 Phase After DNA is replicated in the S phase, the cell will then enter the G2 phase or gap 2 phase. This is the second gap period between the replication and cell division. Important processes that happen during this phase are continued growth and the production of materials that are necessary for cell division to occur. Proteins are also synthesized during this period, but not at the rate similar to that of the G1 phase. After the cell exits interphase, it should now have all the factors necessary for mitosis or meiosis to occur. The phases involved in mitosis and meiosis will be discussed in detail later in this unit. Mitosis or Meiosis Once the cell is sufficiently prepared to divide, the division of the nucleus (including the duplicated genetic material) will occur. This event in eukaryotic organisms involves either one of two processes, namely, mitosis and meiosis. These forms of cell division differ with the type of cell, examples of which are shown in Fig. 3.1.6. Sex cells or gametes undergo meiosis, whereas somatic cells or non-sex cells undergo mitosis. The goal of mitosis and meiosis is to divide the nucleus of the parent cell and distribute the genetic material to the resulting new cells. Mitosis and meiosis have several phases, and each of these phases has specific events that contribute to the proper partitioning of the genetic material. The phases of mitosis and meiosis will be further discussed later in this unit. 3.1. Cell Cycle and Cell Divisions 12 Unit 3: Cell Cycle Fig. 3.1.6. Somatic cells such as cheek cells (left) undergo mitosis, while sex cells such as sperm cells (right) are products of meiosis. Cytokinesis Once cell division concludes, then cytokinesis will occur as shown in Fig. 3.1.7.. Cytokinesis refers to the division of the cell’s cytoplasm including its components. Different organisms have different processes associated with cytokinesis. For example, in animal cells, cytokinesis begins with the formation of a cleavage furrow, which initiates the division of the cytoplasm to produce two new cells. Plants, by contrast, will form a new cell wall that will eventually divide the two cells. The cell wall material that is deposited during the cytokinesis in plants is known as the cell plate. Once the cells have fully divided, the products are known as daughter cells. Mitosis and meiosis have other important differences. One of these is the amount of genetic material present in the daughter cells. Mitosis produces cells with genetic material that is exactly similar in terms of amount and identity to that of the parent cell. Meiosis, on the other hand, produces daughter cells that have half the amount of genetic material, and this genetic material produced has some degree of variation compared to that of the parent cell. This type of division allows the reduction of the chromosome number to form the sex cells. For example, if a diploid cell with ten chromosomes undergoes meiosis, the resulting daughter cells will each consist of five chromosomes. The differences between mitosis and meiosis will be discussed in greater detail later in this unit. 3.1. Cell Cycle and Cell Divisions 13 Unit 3: Cell Cycle Fig. 3.1.7. Cytokinesis is marked by the formation of a cleavage furrow (left) in animal cells and a cell plate (right) in plant cells. Remember The process of mitosis and meiosis refers to the division of the nucleus and genetic material. Cytokinesis, on the other hand, refers to the division of the cytoplasm. Cytokinesis can only occur once the genetic material is properly distributed to the daughter cells. If you look at all of the stages of the cell cycle, you will notice a general trend. The cell needs to make sure that it is ready to divide, which it does so in interphase. Some of the preparations include duplicating the genetic material, acquiring nutrients, and doubling of the cell organelles. Once these preparations are done, the chromosomes are distributed to the daughter cell during mitosis. Finally, the cytoplasm and other components are divided during cytokinesis. The cell cycle, in general, is an alternation between the duplication of genetic material and the division of the cell. 3.1. Cell Cycle and Cell Divisions 14 Unit 3: Cell Cycle Cell Cycle Checkpoints Imagine the times that you make a project for school. What do you do when you make a mistake? Of course, you halt your progress in order to address the mistakes made. If the mistake is not too big, then you take your time to correct it. If the mistake becomes too big to fix, then you repeat your project all over again. This bears some similarities to how the cell cycle proceeds. When the cell detects errors or improper conditions, then it halts the progress of the cell cycle. It then tries to fix whatever errors are present. If the errors are repaired, then the cell cycle continues. If these errors are too serious, then the cell may destroy itself. This occurs through a process known as apoptosis, also known as programmed cell death. The process of self-destruction prevents errors from being passed on to the daughter cells. There are multiple checkpoints present throughout the entire cell cycle. These checkpoints are performed by specific protein molecules in the cell. Checkpoints include the G1-to-S checkpoint, G2-to-M checkpoint, and the metaphase checkpoint. Both the G1-to-S and G2-to-M checkpoints occur during interphase, while the metaphase checkpoint occurs during mitosis and meiosis. The cell cycle checkpoints 3.1. Cell Cycle and Cell Divisions 15 Unit 3: Cell Cycle G1-to-S Checkpoint By the end of the G1 phase, the cell cycle proceeds to the G1-to-S checkpoint, or simply the G1 checkpoint. This checkpoint makes sure that the cell is large enough with all the necessary energy reserves and doubled organelles and that there is no damage in the cell’s DNA before the cell cycle continues. If the G1-to-S checkpoint deems the cell to be unprepared, then the cell cannot proceed to the next phase. If a cell is not committed to further prepare for division, it may enter a stage known as the G0 phase. Likewise, a cell in which the conditions of the G1 phase are not met will also enter the G0 stage, thus it is also considered an extended G1 phase. The cell then may proceed with the cell cycle if the conditions become favorable for cell division to occur. Some cells, however, stay permanently at G0 such as cardiac and nerve cells. The premise of the resting state for the G0 phase is that their activities are already outside the confines of the normal cell cycle. These G0 cells are not resting per se because they are still metabolically active. G2-to-M Checkpoint The M phase, which involves either mitosis or meiosis, will proceed after the G2 phase ends. However, the G2-to-M checkpoint (also known as the G2 checkpoint) makes sure that the cell is prepared before mitosis or meiosis. Some of the factors checked during this checkpoint include the presence of DNA damage, whether the chromosomes are properly replicated, or if environmental conditions are favorable for cell division to take place. If the DNA is properly replicated, no damage is present, and if environmental conditions are optimal, then the cell proceeds to either mitosis or meiosis. M Checkpoint The metaphase checkpoint is found in metaphase, during mitosis. It is in metaphase where the spindles are finally attached to the sister chromatids, which will then be separated during anaphase. In order to ensure a proper and equal separation of these sister chromatids, the metaphase checkpoint will first check if the spindles are properly attached. The cell proceeds to anaphase if all spindles are properly attached. Otherwise, mitosis pauses to remedy the error. 3.1. Cell Cycle and Cell Divisions 16 Unit 3: Cell Cycle Did You Know? There are different genes and processes that help regulate cell cycle checkpoints. These processes that regulate the cell cycle checkpoints can malfunction (or genes can go altered), which can result in the uncontrollable division of cells. This leads to the formation of tumors, which are characteristic of cancer. Key Points ___________________________________________________________________________________________ The life cycle of a cell is known as the cell cycle, and it describes how cells grow, develop, and reproduce. With very few exceptions, all of the cells in your body contain genetic material in the form of DNA or deoxyribonucleic acid, which is usually found in the nucleus of your cells. There are three main stages in the cell cycle, namely, interphase, M phase, and cytokinesis. Interphase is the stage when the cell prepares itself for eventual cell division, which can either be mitosis or meiosis. It is also the stage when the cell is metabolically active and performs its specialized function. Cytokinesis refers to the division of the cell’s cytoplasm and other components. This process significantly differs between animal cells and plant cells, primarily because of the presence of cell walls in the latter. 3.1. Cell Cycle and Cell Divisions 17 Unit 3: Cell Cycle The cell cycle involves a series of events that lead to the production of cells. ___________________________________________________________________________________________ Check Your Understanding A. Identify the cell cycle phase or checkpoint being referred to in the following statements. 1. This is when DNA replication occurs. 2. This is the first stage of the interphase. 3. The growth of the cell happens in this phase (2 possible answers). 4. This refers to the division that occurs in somatic cells. 5. This refers to the division that occurs to produce sex cells. 6. This checks if the chromosomes are properly duplicated. 7. This refers to the division of the cytoplasm. 8. This is when the cleavage furrow or cell plate appears. 9. This is when the cell duplicates its genetic material. 10. This is the first interphase checkpoint. 3.1. Cell Cycle and Cell Divisions 18 Unit 3: Cell Cycle B. The first two words are separated by a colon. These two words are related to each other. Analyze this relationship in order to fill in the missing word that will complete the second pair. 1. mitosis: somatic cells meiosis: __________ 2. gap 1: G1 to S checkpoint mitosis: __________ 3. diploid: 46 chromosomes haploid: __________ 4. mitosis: nucleus cytokinesis: __________ 5. plants: cell plate animals: __________ 6. interphase: chromatin mitosis: __________ 7. condensed: chromosome uncoiled: __________ 8. animals and plants: life cycle cell: __________ 9. chromosomes: homologous pairs chromatids: __________ 10. haploid: one set diploid: _________ Challenge Yourself Answer the following questions. 1. Differentiate between the amount of genetic material present in the daughter cells of mitosis and meiosis. 2. Explain why cell cycle checkpoints are necessary. 3. How does apoptosis play a role in the cell cycle? 4. What is the importance of interphase in relation to cell division? 5. Why is DNA synthesis in S phase necessary? Photo Credits Polytene_chromosomes_(26_2_97)_Salivary_glands_of_nonbiting_midges_larvae_(Chironomi dae) by Doc. RNDr. Josef Reischig, CSc. is licensed under CC-BY SA 3.0 via Wikimedia Commons. 3.1. Cell Cycle and Cell Divisions 19 Unit 3: Cell Cycle Human Cheek Cells by Joseph Elsbernd is licensed under CC BY-SA 2.0 via Wikimedia Commons. Sperm Stained by Bobjgalindo, is licensed under CC BY-SA 4.0 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. 3.1. Cell Cycle and Cell Divisions 20