Cell Cycle Cell Division GenBio |1 LEARNING OBJECTIVES: The learners will: Demonstrate an understanding of the cell cycle and cell division (i.e., mitosis and meiosis). Identify and differentiate the phases of the cell cycle and their control points describe and differentiate the stages of mitosis and meiosis given 2n=6 discuss and demonstrate crossing over and recombination in meiosis identify disorders and diseases that result from malfunctions in the cell during the cell cycle DISCUSSION Cell Division—involves the distribution of identical genetic material or DNA to two daughter cells. What is most remarkable is the fidelity with which the DNA is passed along, without dilution or error, from one generation to the next. Cell Division functions in reproduction, growth, and repair. Core Concepts: • • • • • • • • • • All organisms consist of cells and arise from preexisting cells. Mitosis is the process by which new cells are generated. Meiosis is the process by which gametes are generated for reproduction. The Cell Cycle represents all phases in the life of a cell. DNA replication (S phase) must precede mitosis so that all daughter cells receive the same complement of chromosomes as the parent cell. The gap phases separate mitosis from S phase. This is the time when molecular signals mediate the switch in cellular activity. Mitosis involves the separation of copied chromosomes into separate cells. Unregulated cell division can lead to cancer. Cell cycle checkpoints normally ensure that DNA replication and mitosis occur only when conditions are favorable and the process is working correctly. Mutations in genes that encode cell cycle proteins can lead to unregulated growth, resulting in tumor formation and ultimately invasion of cancerous cells to other organs. The Cell Cycle control system is driven by a built-in clock that can be adjusted by external stimuli (i.e., chemical messages). Checkpoint—a critical control point in the Cell Cycle where ‘stop’ and ‘go-ahead’ signals can regulate the cell cycle. • • Animal cells have built-in ‘stop’ signals that halt the cell cycles and checkpoints until overridden by ‘go-ahead’ signals. Three major checkpoints are found in the G1, G2, and M phases of the Cell Cycle. The G1 Checkpoint—the Restriction Point • • The G1 checkpoint ensures that the cell is large enough to divide and that enough nutrients are available to support the resulting daughter cells. If a cell receives a ‘go-ahead’ signal at the G1 checkpoint, it will usually continue with the Cell Cycle. This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines. Cell Cycle Cell Division GenBio |2 • • If the cell does not receive the ‘go-ahead’ signal, it will exit the Cell Cycle and switch to a non-dividing state called G0. Most cells in the human body are in the G0 phase. The G2 Checkpoint—ensures that DNA replication in S phase has been successfully completed. The Metaphase Checkpoint—ensures that all of the chromosomes are attached to the mitotic spindle by a kinetochore. Kinase—a protein which activates or deactivates another protein by phosphorylating them. Kinases give the ‘go-ahead’ signals at the G1 and G2 checkpoints. The kinases that drive these checkpoints must themselves be activated. • • • • • • The activating molecule is a cyclin, a protein that derives its name from its cyclically fluctuating concentration in the cell. Because of this requirement, these kinases are called cyclin-dependent kinases or CDKs. Cyclins accumulate during the G1, S, and G2 phases of the Cell Cycle. By the G2 checkpoint, enough cyclin is available to form MPF complexes (aggregations of CDK and cyclin) which initiate mitosis. MPF functions by phosphorylating key proteins in the mitotic sequence. Later in mitosis, MPF switches itself off by initiating a process which leads to the destruction of cyclin. CDK, the non-cyclin part of MPF, persists in the cell as an inactive form until it associates with new cyclin molecules synthesized during the interphase of the next round of the Cell Cycle. Stages of Mitosis and Meiosis A. Mitosis (apparent division)—is nuclear division; the process by which the nucleus divides to produce two new nuclei. Mitosis results in two daughter cells that are genetically identical to each other and to the parental cell from which they came. Cytokinesis—is the division of the cytoplasm. Both mitosis and cytokinesis last for around one to two hours. Prophase—is the preparatory stage, During prophase, centrioles move toward opposite sides of the nucleus. • The initially indistinct chromosomes begin to condense into visible threads. • Chromosomes first become visible during early prophase as long, thin, and intertwined filaments but by late prophase, chromosomes are more compacted and can be clearly discerned as much shorter and rod-like structures. • As the chromosomes become more distinct, the nucleoli also become more distinct. By the end of prophase, the nucleoli become less distinct, often disappearing altogether. This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines. Cell Cycle Cell Division GenBio |3 Metaphase—is when chromosomes become arranged so that their centromeres become aligned in one place, halfway between the two spindle poles. The long axes of the chromosomes are 90 degrees to the spindle axis. The plane of alignment is called the metaphase plate. Anaphase—is initiated by the separation of sister chromatids at their junction point at the centromere. The daughter chromosomes then move toward the poles. Telophase—is when daughter chromosomes complete their migration to the poles. The two sets of progeny chromosomes are assembled into two-groups at opposite ends of the cell. The chromosomes uncoil and assume their extended form during interphase. A nuclear membrane then forms around each chromosome group and the spindle microtubules disappear. Soon, the nucleolus reforms. B. Meiosis—reduces the amount of genetic information. While mitosis in diploid cells produces daughter cells with a full diploid complement, meiosis produces haploid gametes or spores with only one set of chromosomes. During sexual reproduction, gametes combine in fertilization to reconstitute the diploid complement found in parental cells. The process involves two successive divisions of a diploid nucleus. First Meiotic Division The first meiotic division results in reducing the number of chromosomes (reduction division). In most cases, the division is accompanied by cytokinesis. Prophase I—has been subdivided into five substages: leptonema, zygonema, pachynema, diplonema, and diakinesis. • • • • • Leptonema—Replicated chromosomes have coiled and are already visible. The number of chromosomes present is the same as the number in the diploid cell. Zygonema—Homologue chromosomes begin to pair and twist around each other in a highly specific manner. The pairing is called synapsis. And because the pair consists of four chromatids it is referred to as bivalent tetrad. Pachynema—Chromosomes become much shorter and thicker. A form of physical exchange between homologues takes place at specific regions. The process of physical exchange of a chromosome region is called crossing-over. Through the mechanism of crossing-over, the parts of the homologous chromosomes are recombined (genetic recombination). Diplonema—The two pairs of sister chromatids begin to separate from each other. It is at this point where crossing-over is shown to have taken place. The area of contact between two non-sister chromatids, called chiasma, become evident. Diakinesis—The four chromatids of each tetrad are even more condensed and the chiasma often terminalize or move down the chromatids to the ends. This delays the separation of homologous chromosomes. In addition, the nucleoli disappear, and the nuclear membrane begins to break down. This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines. Cell Cycle Cell Division GenBio |4 Metaphase I—The spindle apparatus is completely formed and the microtubules are attached to the centromere regions of the homologues. The synapsed tetrads are found aligned at the metaphase plate (the equatorial plane of the cell) instead of only replicated chromosomes. Anaphase I—Chromosomes in each tetrad separate and migrate toward the opposite poles. The sister chromatids (dyads) remain attached at their respective centromere regions. Telophase I—The dyads complete their migration to the poles. New nuclear membranes may form. In most species, cytokinesis follows, producing two daughter cells. Each has a nucleus containing only one set of chromosomes (haploid level) in a replicated form. Second Meiotic Division The events in the second meiotic division are quite similar to mitotic division. The difference lies, however, in the number of chromosomes that each daughter cell receives. While the original chromosome number is maintained in mitosis, the number is reduced to half in meiosis. Prophase II—The dyads contract. Metaphase II—The centromeres are directed to the equatorial plate and then divide. Anaphase II—The sister chromatids (monads) move away from each other and migrate to the opposite poles of the spindle fiber. Telophase II—The monads are at the poles, forming two groups of chromosomes. A nuclear membrane forms around each set of chromosomes and cytokinesis follows. The chromosomes uncoil and extend. Cytokinesis—The telophase stage of mitosis is accompanied by cytokinesis. The two nuclei are compartmentalized into separate daughter cells and complete the mitotic cell division process. In animal cells, cytokinesis occurs by the formation of a constriction in the middle of the cell until two daughter cells are formed. The constriction is often called cleavage, or cell furrow. However, in most plant cells this constriction is not evident. Instead, a new cell membrane and cell wall are assembled between the two nuclei to form a cell plate. Each side of the cell plate is coated with a cell wall that eventually forms the two progeny cells. Meiosis Mitosis 1. Requires two nuclear divisions 1. Requires one nuclear division 2. Chromosomes synapse and cross over 2. Chromosomes do not synapse nor cross over 3. Centromeres survive Anaphase I 3. Centromeres dissolve in mitotic anaphase 4. Halves chromosome number 4. Preserves chromosome number 5. Produces four daughter nuclei 5. Produces two daughter nuclei 6. Produces daughter cells genetically different 6. Produces daughter cells genetically identical to from parent and each other parent and to each other This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines. Cell Cycle Cell Division GenBio |5 7. Used only for sexual reproduction 7. Used for asexual reproduction and growth Table 1: Comparison of Mitosis and Meiosis Meiosis I compared to Mitosis Meiosis II compared to Mitosis Meiosis I Mitosis Meiosis II Mitosis Prophase I Prophase Prophase II Prophase Pairing of homologous chromosomes Metaphase I No pairing of chromosomes Metaphase No pairing of chromosomes Metaphase II No pairing of chromosomes Metaphase Diploid number of duplicated chromosomes at metaphase plate Anaphase Sister chromatids separate becoming daughter chromosomes that move to the poles Telophase Bivalents at metaphase Duplicated plate chromosomes at metaphase plate Anaphase I Anaphase Haploid number of duplicated chromosomes at metaphase plate Anaphase II Homologues of each bivalent separate and duplicated chromosomes move to poles Sister chromatids separate, becoming daughter chromosomes that move to the poles Sister chromatids separate, becoming daughter chromosomes that move to the poles Telophase I Telophase Telophase II Two haploid daughter Two diploid cells not identical to daughter cells, the parent cell identical to the parent cell Table 2: Meiosis compared to Mitosis Four haploid Two diploid daughter cells not daughter cells, genetically identical identical to the parent cell Disorders and diseases that result from the malfunction of the cell during the cell cycle • incorrect DNA copy (e.g., cancer) • chromosomes are attached to string-like spindles and begin to move to the middle of the cell (e.g., Down Syndrome, Alzheimer’s, and Leukemia) Other chromosome abnormalities: • • arise from errors in meiosis, usually meiosis I; occur more often during egg formation (90% of the time) than during sperm formation; This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines. Cell Cycle Cell Division GenBio |6 • • become more frequent as a woman ages. Aneuploidy—is the gain or loss of whole chromosomes. It is the most common chromosome abnormality. It is caused by non-disjunction, the failure of chromosomes to correctly separate: • homologues during meiosis I or • sister chromatids during meiosis II SUMMARY GLOSSARY anaphase third stage of mitosis (and meiosis), during which sister chromatids separate into two new nuclear regions of a dividing cell cell cycle life cycle of a single cell, from its birth until its division into two new daughter cells centromere region of attachment for two sister chromatids This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines. Cell Cycle Cell Division GenBio |7 centrosome cellular structure that organizes microtubules during cell division checkpoint progress point in the cell cycle during which certain conditions must be met in order for the cell to proceed to a subsequence phase cleavage furrow contractile ring that forms around a cell during cytokinesis that pinches the cell into two halves cyclin one of a group of proteins that function in the progression of the cell cycle cyclin-dependent kinase (CDK) one of a group of enzymes associated with cyclins that help them perform their functions cytokinesis final stage in cell division, where the cytoplasm divides to form two separate daughter cells diploid condition marked by the presence of a double complement of genetic material (two sets of chromosomes, one set inherited from each of two parents) G0 phase phase of the cell cycle, usually entered from the G1 phase; characterized by long or permanent periods where the cell does not move forward into the DNA synthesis phase G1 phase first phase of the cell cycle, after a new cell is born G2 phase third phase of the cell cycle, after the DNA synthesis phase homologous describes two copies of the same chromosome (not identical), one inherited from each parent interphase entire life cycle of a cell, excluding mitosis kinetochore region of a centromere where microtubules attach to a pair of sister chromatids metaphase second stage of mitosis (and meiosis), characterized by the linear alignment of sister chromatids in the center of the cell This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines. Cell Cycle Cell Division GenBio |8 metaphase plate linear alignment of sister chromatids in the center of the cell, which takes place during metaphase mitosis division of genetic material, during which the cell nucleus breaks down and two new, fully functional, nuclei are formed mitotic phase phase of the cell cycle in which a cell undergoes mitosis mitotic spindle network of microtubules, originating from centrioles, that arranges and pulls apart chromosomes during mitosis prophase first stage of mitosis (and meiosis), characterized by breakdown of the nuclear envelope and condensing of the chromatin to form chromosomes S phase stage of the cell cycle during which DNA replication occurs sister chromatid one of a pair of identical chromosomes, formed during DNA replication somatic cell all cells of the body excluding gamete cells telophase final stage of mitosis (and meiosis), preceding cytokinesis, characterized by the formation of two new daughter nuclei EVALUATION Multiple Choices: 1. The cytoplasmic division in plants must be facilitated by a mechanism called a. cleavage furrow formation b. spindle equator formation c. spindle plate formation d. cell plate formation e. a & d 2. What is a major feature of prophase in mitosis? a. Chromosomes condense b. Crossing-over between homologous chromosomes c. Nuclear envelope breaks up d. a & c This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines. Cell Cycle Cell Division GenBio |9 e. All of the above 3. In which stage in interphase does the DNA replication take place? a. G1 b. G2 c. S d. G1 & S e. G1 & G2 4. When all chromosomes line up at the spindle equator, it is referred as the stage(s) of a. Metaphase b. Anaphase c. Anaphase I d. Anaphase II e. None of the above 5. The cells that make up our body, and usually have a diploid chromosome number, are known as a. Sex cells b. Germ cells c. Diploid cells d. Prokaryotic cells e. Somatic cells 6. Meiosis only occurs in a. Somatic cells b. Germ cells c. Animal cells d. Plant cells e. Prokaryotic cells 7. Sister chromatids break apart in which of the following stages? a. Metaphase b. Anaphase c. Anaphase I d. Anaphase II e. a & d 8. A different molecular form of the same gene is known as a/an a. phenotype b. genotype c. allele d. homologous pair e. sister chromatids pair 9.Crossing- over can be found in the stage of a. Prophase I b. Prophase II c. Interphase d. Anaphase I e. Anaphase II This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines. Cell Cycle Cell Division GenBio | 10 10. Which of the following organelles is/are important during the stage of metaphase? a. Mitochondrion b. Ribosome c. Centrioles d. Golgi Apparatus e. All of the above 11. Which of the following process will result in a reduction of the chromosome number by half? a. Mitosis b. Meiosis c. Cytoplasmic division d. DNA replication e. Fertilization 12. The gamete usually has a ___________ chromosome number. a. haploid b. diploid c. triploid d. tetraploid e. polyploidy 13. Which of the following best describe the term “crossing over”? a. An exchange of information between two sister chromatids b. A molecular interaction between two sister chromatids c. A molecular interaction between two non-sister chromatids d. A separation of two sister chromatids e. None of the above 14. There are DNA replications during the stage(s) of a. interphase prior to mitosis b. interphase prior to meiosis I c. interphase between meiosis I and meiosis II d. a & b e. All of the above 15. Sexual reproduction requires all of the following except a. meiosis b. mitosis c. gamete formation d. fertilization e. None of the above This is intellectual property of Divine Grace Montessori and High School (DGMHS) System. Any unauthorized reproduction in any form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines.