GB1-Q1-Chapter-2-Cell-Division PDF

The Cell: CHAPTER Modification, Cycle, Division, and Disorders Q1 GENERAL BIOLOGY 1 2 D I S C U S S I O N...

The Cell: CHAPTER Modification, Cycle, Division, and Disorders Q1 GENERAL BIOLOGY 1 2 D I S C U S S I O N :T H E C E L L C ell, in biology, the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed. A single cell is often a complete organism in itself, such as a bacterium or yeast. Other cells acquire specialized functions as they mature. These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms, such as humans and other animals. Although cells are much larger than atoms, they are still very small. The smallest known cells are a group of tiny bacteria called mycoplasmas; some of these https://www.discovermagazine.com/planet- single-celled organisms are spheres as small as 0.2 μm in earth/the-human-body-under-the- diameter (1μm = about 0.000039 inch), with a total mass microscope of 10−14 gram—equal to that of 8,000,000,000 hydrogen Can you now clearly distinguish atoms. Cells of humans typically have a mass 400,000 eukaryotic cells from prokaryotic cells? times larger than the mass of a single mycoplasma Let us learn more about eukaryotic bacterium, but even human cells are only about 20 μm cells and identify the different types of across. It would require a sheet of about 10,000 human tissues and cells of plants and cells to cover the head of a pin, and each human organism animals. Can you recall what makes a is composed of more than 30,000,000,000,000 cells. plant cell different from an animal cell? (https://www.britannica.com/science/cell-biology) A plant cell contains a rigid cell wall, plastids, vacuole, fixed regular shape, and stores excess glucose as starch. An animal cell INTRODUCTION: stores excess glucose as glycogen, have centrioles, THE DIFFERENT TYPES OF CELLS and generally have an amorphous shape. A prokaryotic cell does not have a true nucleus. DNA is in an unbound region called the nucleoid. It does not have membrane bound organelles too. Single – celled organisms under Domain Bacteria and Domain Archaea have prokaryotic cells. They are called prokaryotes. A eukaryotic cell is a type of cell with membrane-enclosed nucleus and organelles. Organisms under Domain Eukarya (protists, fungi, plants, and animals) have eukaryotic cells. They are called eukaryotes.  tissue repair, organ transplantation, and for the treatment of disease. Like animals, plants are multicellular eukaryotes which are composed of organs, tissues, and cells with highly specialized functions. There are two types of plant tissues: meristematic tissue and permanent (or non-meristematic) tissue. Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main tissue types: vascular, dermal, and ground tissue. Cancer Cells Unlike all of the other cells listed, cancer cells work to destroy the body. Cancer results from the development of abnormal cell properties that cause cells to divide uncontrollably and spread to other locations. Cancer cell development can originate from mutations stemming from exposure to chemicals, radiation, and ultraviolet light. Cancer can also have genetic origins such as chromosome replication errors and cancer-causing viruses of the DNA. Cancer cells can spread rapidly because they develop decreased sensitivity to antigrowth signals and proliferate quickly in the absence of stop commands. They also lose the ability to undergo apoptosis or programmed cell death, making them even more formidable. Meristem Meristem, region of cells capable of division and growth in plants. Meristems are classified by their location in the plant as apical (located at root and shoot tips), lateral (in the vascular and cork cambia), and intercalary (at internodes, or stem regions between the places at which leaves attach. There are three primary meristems: the protoderm, which will become the epidermis; the ground meristem, which will form the ground tissues comprising parenchyma, collenchyma, and sclerenchyma cells; and the procambium, which will become the vascular tissues (xylem and phloem). Unlike most animals, plants continue to grow throughout their entire life span because of the unlimited division of meristematic regions. 2.1 THE CELL MODIFICATION Living organisms can be unicellular, such as bacteria and protists, or they can be multicellular, Stem Cells like plants, animals, and fungi. Unicellular Stem cells are unique in that they originate organisms, like bacteria, can perform all life as unspecialized cells and can develop into functions within one single cell. They can transport specialized cells that can be used to build specific molecules, metabolize nutrients, and reproduce organs or tissues. Stem cells can divide and within this one cell. On the other hand, multicellular replicate many times in order to replenish and repair organisms need many different types of cells to tissue. In the field of stem cell research, scientists carry out the same life processes. Each of these take advantage of the renewal properties of these special types of cells has a different structure that structures by utilizing them to generate cells for helps it perform a specific function.  Cell Modification (specialization or antenna that receives sensory information for the differentiation) is a process that occurs after cell cells and processes these signals from the division where the newly formed cells are surrounding fluids. For example, the cilia present in structurally modified so that they can perform the kidney bend forcefully as the urine passes. This their function efficiently and effectively. sends signals to the cells that the urine is flowing. Multicellular organisms begin as just one 2. FLAGELLA single cell—a fertilized egg. Growing from one Flagellum, plural flagella, is a whip - like single cell to trillions of specialized cells that structure that acts primarily as an organelle of perform different functions is a process that locomotion in the cells of many living organisms. happens with the regulation of DNA and RNA. Flagellar motion causes water currents necessary Deoxyribonucleic Acid, or DNA, controls the way for respiration and circulation in sponges and cell’s function. It also determines what type of coelenterates. Most motile bacteria move by means specialized cells will be made. RNA translates and of flagella. transcribes the DNA code into proteins (the structures of a cell), it also plays a role in cell 3. VILLI differentiation. Villus, plural villi, are small finger-like The differences in structure and function projections located in the walls of the small between the cells mean that they are specialized intestine. Their function is to increase the surface cells. Cell specialization occurs through the process area in order to maximize the absorption of digested called gene expression. Gene expression is the food. Each villus has a muscle strand which allows process by which the instructions in our DNA are the villi to contract and expand and a lacteal which converted into a functional product, a protein. It is absorbs fatty acids and glycerol and transports it likened to a switch, being a regulated process, away from the villi. Microvilli are microscopic cells allows the specific combination of genes that are that project from the villus which further increases turned on or off, expressed or repressed. surface area for faster absorption. Microvilli have Expressed genes within the genome dictates how a protein pumps that move monosaccharides and cell function. This is to say that not all genes are amino acids into the bloodstream and contains expressed during differentiation. Cell specialization mitochondria so that large amounts of DNA can be immediately starts after fertilization which will made for the active transport of nutrients. eventually result to the formation of the different organs. 4. LEUKOCYTE Leukocyte cells work to keep the human Cell differentiation is an important process in the body free of infection. These cells find and destroy development of complex organisms. It results to microbes within the human body, responding to and the formation of various tissues and organs treating infection. Because these cells must move thus, making an organism fully functional. to the site of infection, they are highly mobile and even capable of pushing through capillary walls The process of cellular differentiation leads cells to when necessary to reach sites of infection. assume their final morphology(form) and Leukocytes are highly flexible, capable of shifting physiology(function) throughout development and shape as necessary as they move throughout the adulthood. body. They have nuclei and do not contain hemoglobin. Leukocytes are classified as Common Examples of Modified Cells and Tissues granulocytes (with granules) and agranulocytes in Animals (without granules). 1. CILIA 5. RED BLOOD CELLS A cilium, or cilia (plural), are small hair-like Red blood cells carry oxygen around the protuberances on the outside of eukaryotic cells. body. They are well suited to this function because They are primarily responsible for locomotion, either they contain hemoglobin which binds with oxygen of the cell itself or of fluids on the cell surface. Cilia molecules. RBCs don't have a nucleus, allowing are classified as motile and non – motile. Motile cilia more space to carry oxygen. In addition, they are are located on the epithelial cells of several internal flat disc in shape (biconcave) which gives them a organs such as lungs, trachea, and digestive large surface area, and the best chance of system. They are also found on protozoans such as absorbing as much oxygen as they can. paramecium and help them in locomotion. Cilia are only found in eukaryotic cells. The important 6. NERVE CELLS functions performed by cilia involve locomotion and Nerve cells transmit electrical signals. They sensory functions. Multiple cilia wave in a rhythmic are adapted to this function because they are thin motion that keeps the internal passageways free and can be more than 1 meter long. This means from mucus or any foreign agent. The non-motile they can carry messages up and down the body cilia can be found in the dendritic knob of the over large distances. Nerve cells have branched olfactory neuron. A few non-motile cilia act as an connections at each end. These join to other nerve cells, allowing them to pass messages around the body. They have a fatty (myelin) sheath that surrounds them. The fatty sheath increases the speed at which the message can travel. 7. MUSCLE CELLS A muscle cell is generally elongated and elastic containing mitochondria in large number. The elongated and elastic feature helps muscle tissues to contract and relax. Contraction and relaxation of muscle tissues help in movement. The large number of mitochondria is very important in tissue respiration in the muscle cells. Common Examples of Modified Cells and Tissues 1. Cell specialization, also known as cell differentiation or cell in Plants modification, is the process by which generic cells change into specific cells meant to do certain tasks within the body. 8. ROOT HAIRS 2. Muscle cells, blood cells, villi, root hair cells, cilia and flagella Root hair cells are modified epidermal cells are common examples of modified cells with specialized of the roots which has a long and narrow protrusion. function. It has a large vacuole with lots of mitochondria in 3. Specialized cells have special features that allow them to the cytoplasm. Unlike any typical plant cells, root perform their functions effectively. hair cells have no chloroplasts. They function mainly to absorb water and mineral salts by 4. Cell specialization occurs through the process called gene osmosis and active transport. The hair-like structure expression. helps to increase the surface area of the root hair 5. Gene expression allows the specific combination of genes cell, thus helps the root hair cell to absorb more that are turned on or off, expressed or repressed. Expressed water and mineral salts. Furthermore, it helps to genes within the genome dictates how a cell function. penetrate in between soil particles in search of water and mineral salts. 6. The differences in structure and function between the cells mean that they are specialized cells. 9. GUARD CELLS 7. Deoxyribonucleic Acid, or DNA, controls the way cell’s Guard cells are cells surrounding a stoma. function. It also determines what type of specialized cells will They help to regulate the rate of transpiration – be made. evaporation of water in plants – by opening and 8. Stem cells are cells that can become any type of specialized closing the stomata. They are specialized in such a cell in the body. way that the cell wall in the inner side of the guard cells is thicker than the outer side. This feature helps the guard cells to bend outward when they become turgid (swollen). This results in the opening of the stoma. If the guard cells become flaccid What is cell modification? (sagging), the guard cells will bend inward resulting Cell specialization or modification occurs after cell in the closing of the stoma. division wherein newly formed cells are structurally modified so that they can perform their function 10. CUTICLE efficiently and effectively. The plant cuticle is an extracellular hydrophobic layer that covers the aerial epidermis of all land plants, providing protection against desiccation and external environmental stresses. It contains cutin, a waxy, water-repellent substance. It coats, waterproofs, and protects the above-ground parts of plants and helps prevent water loss, abrasions, infections, and damage from toxins. 11. XYLEM AND PHLOEM Xylem transports water and soluble mineral nutrients from roots to various parts of the plant. It is responsible for replacing water lost through transpiration and photosynthesis. Phloem translocate sugars made by photosynthetic areas of Apical modification plants to storage organs like roots, tubers or bulbs. It is a cell modification found on the apical surface of the cell. a. Cilia and Flagella ⎯ Cilia are usually short, hair-like structures that move in waves. ⎯ Flagella are long whip-like structures. ⎯ Formed from microtubules. Figure 12. Pseudopods. Source: https://www.quora.com/What-purpose-does-a- pseudopod-serve-to-anamoeba d. Extracellular matrix (ECM) ⎯ Compound secreted by the cell on its apical surface. Figure 9. Cilia and flagella. ⎯ Cell wall in the extracellular structure in plant Source: https://byjus.com/biology/difference-between-cilia- cells that distinguishes them from animal cell. and-flagella/ ⎯ Glycoprotein is the main ingredient of ECM in b. Villi and Microvilli animal cells. ⎯ Villi are finger-like projections that arise from ⎯ They cover external surface, line up internal epithelial layer in some organs. They help to organs, take up nutrients, export wastes, and increase surface area allowing for faster and interact with the external environment. Figure 13. Extracellular matrix acts like glue to bind the cells more efficient adsorption. together in the tissue and provides mechanical strength. ⎯ Microvilli are smaller projections that arise from the cell’s surface that also increase surface area allowing faster and more efficient adsorption. Source: https://www.regentys.com/research/ Basal Modification Figure 11. Structure of microvilli. ⎯ Cell modification found on the basal surface Source: https://microbenotes.com/microvilli-structure-and- functions/ of the cell desmosomes/hemidesmosomes. ⎯ Anchoring junction on the basal surface of These projections increase the surface area of the the cell Rivet-like links between small intestine for the absorption of nutrients, and cytoskeleton and extracellular matrix as a higher surface area = higher rate of components such as the basal lamina that transportation processes such as diffusion, they underlie epithelia. Primarily composed of thus increase the rate of absorption. keratin, integrin, and cadherin. c. Pseudopods ⎯ Temporary, irregular lobes formed by amoebas and some other eukaryotic cells. ⎯ Bulge outward to move the cell or engulf prey. ⎯ From the Greek word pseudes and podos, meaning “false” and “feet”. Figure 14. Cell modification found on the basal surface of the cell hemidesmosomes. Source: https://commons.wikimedia.org/wiki/File:402_Types_of_ Cell_Junctions_new.jpg Lateral modification A cell junction that provides contact between neighboring cells or between the cell and extracellular matrix. a. Tight Junction ⎯ Acts as barriers that regulate the movement of the water and solutes between epithelial layers. ⎯ Prevent leakage of ECF Figure 17. Gap junctions allow small molecules to flow between neighboring cells. Source: https://commons.wikimedia.org/wiki/File:402_Types_of_ Cell_Junctions_new.jpg 2.2 THE CELL CYCLE All organisms reproduce for one reason – to ensure the survival of their species. Reproduction makes use of the process of cell division. Figure 15. Tight junctions join two together to form a leak-proof sheet. Cell division is important for two reasons: Source: https://commons.wikimedia.org/wiki/File:402_Types_of_ To be able to produce offspring. Cell_Junctions_new.jpg To generate new cells that will replace worn out or damaged cells. b. Adhering Junction ⎯ Anchoring junction on the lateral surface There are two types of cell division, namely of the cell. mitosis which happens in body cells or somatic cells ⎯ Very similar to the anchoring junction of and meiosis which involves the gametes or sex the basal surface of the cell. cells. ⎯ Fasten cells to one another. In order to better understand cell division, you need to learn first the cell cycle. This cycle involves distinct and regular phases of growth, DNA duplication, and cell division that are needed to allow growth and repair. The cell cycle is divided into two main stages: 1. Interphase – non-dividing stage (G1, S, and G2 phases, G0) 2. Cell division – dividing stage (mitosis for somatic cells and meiosis for sex cells Figure 16. Adhesion junctions act like screws together with cytoskeletal fiber to form a strong sheet. Source: https://commons.wikimedia.org/wiki/File:402_Types_of_ Cell_Junctions_new.jpg c. Gap Junction ⎯ Also known as communicating junctions ⎯ Closable channel that connects the cytoplasm of adjoining animal cells. ⎯ Presence of connexon that allow direct exchange of chemical between the cytoplasm of the cells. Image credit: "The cell cycle: Figure 1" by OpenStax College, Biology (CC BY 3.0). Source: https://www.khanacademy.org/science/ap-biology/cell- communication-and-cell-cycle/cell-cycle/a/cell-cycle-phases 2.2.1 STAGES OF CELL CYCLE “Checkpoints” or control points are moments when STAGE 1: INTERPHASE the cell can “check” its internal conditions and Interphase is the growth period in the cell cycle “decide” whether to progress to the next phase or characterized by cell preparation by replication of its remain. It is similar to what happens during a police genetic information and all its organelles. operation checkpoint. When you have met the requirements asked by the police officer in-charge, you can go pass the checkpoint. The main activities done during cell checkpoint are summarized below. STAGE 2: CELLULAR DIVISION: MITOSIS AND MEIOSIS (Note: This will be discussed in the next pages) Some cells undergo the cell cycle only once or they stop dividing and enter the stage known as the gap zero or G0. In this stage, cells are unlikely Cell division is a very important process in all living to divide but still continue to perform normal organisms. During the division of a cell, DNA replication functions. and cell growth also take place. All these processes, Such cells, like neuron cells and heart example cell division, DNA replication, and cell growth, muscle cells that are highly differentiated or hence, must take place in a coordinated way to ensure correct division and formation of progeny cells containing specialized and that the body cannot easily replace, intact genomes. are said to be permanently in G0. The sequence of events by which a cell duplicates Immune cells that are needed at a later time, its genome, synthesizes the other constituents of the cell such as lymphocytes, remain in G0 for many years and eventually divides into two daughter cells is termed until such time that the body needs to recognize an cell cycle. Although cell growth (in terms of cytoplasmic invader. Only when an invader binds to the increase) is a continuous process, DNA synthesis occurs lymphocyte’s receptor that the lymphocyte starts to only during one specific stage in the cell cycle. The divide rapidly to help get rid of the infection. replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell 2.2.2 CELL CYCLE CHECKPOINTS division. In order to prevent mutations/chromosomal aberrations and ensure major events occur at correct times, several cell cycle checkpoints are present at various times in the cycle preventing cells 2.3 CELL DIVISION: MITOSIS from proceeding to the next stage unless all criteria M is the phase of the cell cycle in which the had been met. microtubular apparatus assembles, binds to the chromosomes, and moves the sister chromatids apart. Called mitosis, this process is the essential step in the separation of the two daughter genomes. Although mitosis is a continuous process, it is traditionally subdivided into four stages: prophase, metaphase, anaphase, and telophase. Fig. Stages of cell cycle and their respective control points. Source: http://courses.washington.edu/bot113/summer/WebReadings/ PdfReadings/TABL E_COMPARING_MITOSIS_AND.pdf What is Mitosis? STAGES OF MITOSIS Mitosis, a process of cell duplication, or Prophase is marked by the initiation of reproduction, during which one cell gives rise to two condensation of chromosomal genetically identical daughter cells. The newly material. The chromosomal material formed daughter cells are genetically identical to the becomes untangled during the parent cell and to each other. Strictly applied, the process of chromatin condensation. term mitosis is used to describe the duplication and The centriole, which had undergone distribution of chromosomes, the structures that duplication during S phase of carry the genetic information. During the mitosis interphase, now begins to move process, the cell’s nucleus along with the towards opposite poles of the cell. chromosome is divided to form two new daughter The completion of prophase can thus cell nuclei. The daughter nuclei inherit the same be marked by the following number of chromosomes as that of the parent PROPHASE characteristic events: nucleus. ▪ Chromosomal material condenses to form compact Mitotic 2.3.1 THE ROLE OF MITOSIS chromosomes. Chromosomes are Mitosis is important to multicellular seen to be composed of two organisms because it provides new cells for growth chromatids attached together at and for replacement of worn-out cells, such as skin the centromere. cells. Many single-celled organisms rely on mitosis ▪ Initiation of the assembly of mitotic as their primary means of asexual reproduction. spindle, the microtubules, the proteinaceous components of the cell The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis; hence the chromosomes are spread through the cytoplasm of the cell. The key features of metaphase are: METAPHASE ▪ Spindle fibers attach to kinetochores of chromosomes. ▪ Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibers to both poles. At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as Mitosis helps in the splitting of chromosomes during chromosomes of the future daughter ANAPHASE cell division and generates two new daughter cells. nuclei, begin their migration towards Therefore, the chromosomes form from the parent the two opposite poles. Key events: chromosomes by copying the exact DNA. ▪ Centromeres split, and chromatids Therefore, the daughter cells formed as genetically separate. uniform and identical to the parent as well as to ▪ Chromatids move to opposite each other. Thus, mitosis helps in preserving and poles. maintaining the genetic stability of a particular The chromosomes that have reached population. their respective poles decondense and lose their individuality. The Cytokinesis - The eukaryotic cell has partitioned its individual chromosomes can no replicated genome into two nuclei positioned at longer be seen, and chromatin opposite ends of the cell. While mitosis was going material tends to collect in a mass in on, the cytoplasmic organelles, including the two poles. This is the stage which mitochondria and chloroplasts (if present), were TELOPHASE shows the following key events: reassorted to areas that will separate and become ▪ Chromosomes cluster at opposite the daughter cells. The phase of the cell cycle when spindle poles and their identity is the cell divides is called cytokinesis. It generally lost as discrete elements. involves the cleavage of the cell into roughly equal ▪ Nuclear envelope assembles halves. around the chromosome clusters. (Nucleolus, Golgi complex and ER reform)  The mitosis cell cycle includes several phases that result in two new diploid daughter cells. Each phase is highlighted here and shown by light microscopy with fluorescence. Click on the image to learn more about each phase. (Image from OpenStax College with modified work by Mariana Ruiz Villareal, Roy van Heesheen, and the Wadsworth Center.) Source: https://askabiologist.as u.edu/cell-division Mitosis is a 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. Somatic cells (nonreproductive cells or body cells) such as skin, and all the cells in our body except gametes contain the normal number of chromosomes which is diploid (2n) undergo this type of cell division. The cells that make up the plant’s roots, stems, and leaves, also undergo mitosis. Mitosis is broken down into four stages: prophase, metaphase, anaphase, and telophase. Cytokinesis completes the mitotic phase of the cell cycle which sometimes overlaps telophase. Cleavage furrow is formed in animal cells and cell plate for plant cells after Figure 2. Stages of Mitosis | Source: https://owlcation.com cytokinesis producing two daughter cells. A detailed diagram of the stages of mitosis visible, the centrosomes start to form, etc. So, what is shown in Figure 2. Study the diagram closely and happens after interphase? take note of the changes in the following structures: nuclear envelope, nucleoli, location of the During prophase, (remember “pro” means centrosomes and chromosomes, and thickness beginning) the centrioles move to the opposite sides of the chromosomes as the cell proceeds from of the nucleus, early mitotic spindle (in animal cells one stage to another. It is evident that during it starts from the centrosome – a microtubule- interphase, the chromosomes are less condensed, organizing center) starts to form (polymerize), the the nuclear envelope is intact, the nucleoli are chromatin become visible and condensed into  chromosomes, the nucleoli disappear, and the two divisions result in four daughter cells (rather nuclear envelope starts to beak. than the two daughter cells of mitosis), each with only half as many chromosomes as the parent In prometaphase, the nuclear envelope cell—one set, rather than two. Meiosis reduces the fragments, the chromosomes become more amount of genetic information resulting to its condensed, some microtubules attach to the importance in sexual reproduction and genetic kinetochore of either side of the sister chromatids. diversity among sexually reproducing organism. During metaphase, (remember “meta” means middle) the centrosomes are now at the opposite The overview of meiosis shows for a single pair of poles of the cell, the mitotic spindle is complete, the homologous chromosomes in a diploid cell, that chromosomes are now aligned at the metaphase both members of the pair are duplicated, and the plate and each sister chromatids’ kinetochore is copies sorted into four haploid daughter cells. attached with the microtubules coming from Recall that sister chromatids are two copies of one opposite poles. chromosome, closely associated all along their lengths; this association is called sister chromatid. In anaphase, (remember “away”) the cohesins Together, the sister chromatids make up one (protein) holding together the sister chromatids of duplicated chromosome. In contrast, the two each chromosome are cleaved by an enzyme called chromosomes of a homologous pair are individual separase. The sister chromatids separate and start chromosomes that were inherited from different moving to opposite poles as the microtubules parents. shorten. This is the shortest phase of mitosis as telophase eventually follows. During telophase, (remember “two”) two daughter nuclei and nucleoli are formed, the chromosomes become less condensed, and the remaining microtubule has depolymerized. Cytokinesis completes the mitosis by the formation of the cleavage furrow in animal cells and cell plate in plant cells producing two daughter cells identical to the parent cell. Mitosis ensures that there is an exact copy of genetic information is being passed through the new daughter cells. a. Centromere - the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached. b. Chromatin - a complex of DNA and protein that condenses during cell division. c. Cytokinesis - division of the cytoplasm. d. Kinetochores - are protein complexes associated with centromeres. e. Metaphase plate - an imaginary structure at the midway point between the spindle’s two poles. f. Mitotic spindle - a structure made of microtubules that controls chromosome movement during mitosis. g. Sister chromatids - joined copies of the original chromosome. 2.4 CELL DIVISION: MEIOSIS Many of the steps of meiosis closely resemble corresponding steps in mitosis. Meiosis, like mitosis, is preceded by the duplication of chromosomes. However, this single duplication is Fig. Overview of meiosis: how meiosis reduces chromosome number. After the chromosomes duplicate in interphase, the diploid cell divides followed by not one, but two consecutive cell twice, yielding four haploid daughter cells. divisions called meiosis I and meiosis II. These Source: https://socratic.org/questions/58b351bb7c0149440494bfed 2.4.1 IMPORTANCE OF MEIOSIS Through the process of meiosis, rapid generation of new genetic combinations happen to sex cells during their development. There are three mechanisms that contribute to this genetic variation: independent assortment, crossing-over, and random fertilization. Independent Assortment Humans have 23 pairs of homologous chromosomes. In fact, these 23 chromosomes that you receive from your parents are a matter of chance. The random distribution of homologous chromosomes during meiosis is called independent assortment. In metaphase I, maternal and paternal chromosomes lined up at the equator of the cell, but eventually, these are pulled apart randomly at opposite poles in anaphase I. Each of the 23 pairs segregates or separates independently. Each daughter cell gets one chromosome from each homologous pair. Independent assortment is shown in the figure below with just four pairs of homologous chromosomes for simplified illustration. With four pairs of homologous chromosomes, you may come up with 24 or 16 possible combinations. Thus, 223 (about eight million) with different gene combinations can be produced from one original cell by this mechanism alone for humans. Fig. Meiosis I and Meiosis II. Describes in detail the stages of the two divisions of meiosis for an animal cell whose diploid number is 6 (2n = 6) Fig. The alignment of chromosomes in MEIOSIS I: Separates homologous chromosomes the middle of the metaphase plate is MEIOSIS II: Separates sister chromatids Source: https://apbiologyctd.wordpress.com/genetics/meiosis/ random and can result in astounding possibilities in genetic variability. Crossing-Over Another factor that contributes to genetic variation is crossing-over. This occur during prophase I of meiosis, where chromosomes line up in the process called synapsis, while sections of their DNA are exchanged. DNA exchange during crossover adds more recombination probabilities to the independent assortment of chromosomes that occur later in meiosis. The number of genetic combinations in the gametes is practically unlimited. In addition, because the zygote that forms a new individual is created by the random fusion of two gametes, fertilization squares the number of possible outcomes (223 x 223 = 64 trillion).  STAGES OF MEIOSIS substages: prophase I (the longest almost 90% MEIOSIS I MEIOSIS II of meiosis), metaphase I, anaphase I, and PROPHASE I PROPHASE I telophase I. Prophase of the first meiotic Meiosis II is initiated division is typically longer immediately after Prophase I is subdivided into five and more complex when cytokinesis, usually before compared to prophase of the chromosomes have fully substages: leptotene, zygotene, pachytene, mitosis. It has been further elongated. In contrast to diplotene, and diakinesis. During leptotene, subdivided into the following meiosis I, meiosis II replicated chromosomes are visible and the cell five phases based on resembles a normal mitosis. contains diploid number of chromosomes. In chromosomal behavior: zygotene, synapsis happens when the replicated 1. Leptotene The nuclear membrane 2. Zygotene disappears by the end of maternal chromosomes pair up with the replicated 3. Pachytene prophase II. The paternal chromosome (chromosome 1 from the 4. Diplotene and chromosomes again become mother to father’s chromosome number 1). During 5. Diakinesis compact. pachytene, crossing over occurs when the METAPHASE I METAPHASE II homologue exchanges genetic materials in the The bivalent chromosomes At this stage the align on the equatorial plate. chromosomes align at the chromatids’ chiasmata. This process results in the The microtubules from the equator and the microtubules difference of genetic materials of the sister opposite poles of the spindle from opposite poles of the chromatids which leads to variation among attach to the pair of spindle get attached to the daughter cells (gametes). In diplotene, the crossing homologous. kinetochores of sister over has completely taken place and tetrads begin ANAPHASE I ANAPHASE II The homologous It begins with the to separate. During diakinesis, the tetrads become chromosomes separate, while simultaneous splitting of the more condensed. sister chromatids remain centromere of each associated at their chromosome (which was centromeres. holding the sister chromatids together), allowing them to move toward opposite poles of the cell. TELOPHASE I TELOPHASE II The nuclear membrane and Meiosis ends with telophase II nucleolus reappear, in which the two groups of cytokinesis follows, and this chromosomes once again get is called as diad of cells. enclosed by a nuclear Fig. Crossing over of Chromosomes Although in many cases the envelope; cytokinesis follows Source: Reece et. al, 2014, Campbell Biology chromosomes do undergo resulting in the formation of some dispersion, they do not haploid daughter cells. reach the extremely extended In metaphase I, the tetrads (sister and non- state of the interphase sister chromatids) line up at the metaphase plate, nucleus. the microtubules attach to the kinetochore of each sister chromatids. Meiosis is a type of cell division that produces In anaphase I, the homologous gametes (egg cell or sperm cell) or spores which chromosomes separate, and the sister chromatids contain ½ the normal number of chromosomes move to opposite poles. called the “haploid” number (the symbol is n). When telophase I begins, each half of the During meiosis diploid cells (2n) are reduced to cell has a complete haploid set of duplicated haploid cells (n). During sexual reproduction, chromosomes. Each chromosome is composed of gametes combine in fertilization to reconstitute the two sister chromatids; one or both chromatids diploid complement found in parental cells. If include regions of non-sister chromatid DNA. meiosis did not occur the chromosome number in Cytokinesis occurs through the formation of each new generation would double and the cleavage furrow (in animal cells) and cell plate offspring would die since polyploidy (having more (plants) producing 2 haploid daughter cells. than 2 sets of chromosomes) is not favorable for Cytokinesis ends Meiosis I producing two animals although an advantage to some plants. daughter cells each containing replicated Meiosis in males is called spermatogenesis that chromosomes. Hence, each cell will undergo produces sperm while in females it is called another round of division called Meiosis II. oogenesis that produces ova. Before meiosis occurs, the cell undergoes interphase like in The events in the second meiotic division are quite mitosis, ensuring cell growth, replication of DNA, like mitotic division. During prophase II, each cellular organelles, and the maturity of the cell. The daughter cells still contain 2 sets of chromosomes process involves two successive divisions of a which start to condense. In metaphase II, the sister diploid nucleus. chromatids lined up at the metaphase plate with microtubules attached to its kinetochore. In Meiosis I is considered as the reductional anaphase II, the sister chromatids separate and stage because it reduces the number of start migrating to opposite poles. In telophase II, the chromosomes by half. This comprises four chromosomes are now at each pole and  constrictions start to form in the cytoplasm to form 4 variability allows the organism to survive through daughter cells. Cytokinesis ends meiosis II natural selection. Hence, sexual reproduction producing 4 haploid daughter cells containing allows the species to perpetuate over the asexually unduplicated chromosomes (reduced to half) reproduced organisms. unlike in mitosis that the original chromosome number is maintained. The daughter cells produced in meiosis are not genetically identical to its parent 2.5 DISORDERS RESULTING due to crossing over of homologous chromosomes. During gamete formation the different FROM CELL CYCLE chromosomes assort independently from one MALFUNCTION another in metaphase I. Moreover, the chromosomes separate randomly during anaphase. The key to understanding the different disorders and diseases because of the malfunction of cells a. Chiasma (Pl. Chiasmata) – a segment of the lies on our knowledge of the cell cycle. If you can chromosomes where crossing over occurs between still recall in the previous discussion, we have homologous non-sister chromatids. tackled that cell cycle has different phases and each part has its own checkpoint to monitor the activities b. Crossing over – the exchange of genetic material of the cell. Failure to regulate cell activities may between non-sister chromatids during prophase I of result to various disease and disorder. Some of meiosis. these are mentioned on the next page. c. Homologous chromosomes – a pair of Malfunctions occur during the cell cycle despite the chromosomes of the same length, centromere presence of cell cycle checkpoints. Mitotic errors position, and staining pattern that possess genes result in incorrect DNA copy (ex. cancer). In for the same characters at corresponding loci. One some cases, chromosomes are attached to homologous chromosome is inherited from the string-like spindles and begin to move to the organism’s father, the other from the mother. middle of the cell (ex. Down syndrome, leukemia). Meiotic errors arise during meiosis I d. Synapsis - the pairing and physical connection of and II and occur more often during egg cell duplicated homologous chromosomes during formation (90%) than during sperm formation and prophase I of meiosis. become more frequent as woman ages. Most of the abnormalities are due to non-disjunction or the e. Tetrad - four chromosomes sister and non-sister failure of homologous chromosomes, or sister chromatids. chromatids to separate during meiosis. One of the reasons for this is the absence or failure of the spindle fibers to form during prophase. Fertilization SIGNIFICANCE OF MITOSIS AND MEIOSIS of the defective gametes with a normal one (see Have you ever wondered how you grow and Figure below) results in offspring with either extra or develop from the time you were born? One of the lacking a specific chromosome a condition called reasons for our growth is mitosis. It allows the aneuploidy. Remember…. an abnormal sexually reproducing organism to grow and chromosome number (means an abnormal amount develop from a single cell to a sexually mature of DNA) is damaging to the offspring. organism. Mitosis also allows our body cells to repair damaged tissues. For example, when you accidentally cut your finger, it eventually heals after a few days. In some species such as plants and bacteria, mitosis allows asexual reproduction. One example is binary fission in bacteria and tissue Individuals with lacking a certain culture or cloning in plants. Tissue culture allows chromosome (2n-1) are having condition mass production of plants such as orchid, monosomy (ex. Down syndrome, Turner macapuno, banana, etc. for big plantations which syndrome), while the individuals with an extra are advantageous compared to other process such chromosome (2n+1) are having a condition, as sexual reproduction because it takes time. trisomy (ex. Klinefelter syndrome and Triple X). The characteristics of the common disorders are Meiosis is significant in the production of gametes shown in the table. Furthermore, few of the in both plants and animals or spores in sporophyte karyotypes (the number and visual appearance of plants. It reduces the number of chromosomes the chromosomes in the cell nuclei of an organism sets by half and introduces genetic variability or species) are shown for reference. Mitosis will among the gametes or spores. Genetic variation subsequently transmit the anomaly to all embryonic occurs due to crossing over and independent cells. assortment of chromosomes during meiosis I. This  Nondisjunction can also occur during The abnormal cells may remain at the mitosis. If this error occurs in the early embryonic original site if they have too few genetic and cellular development of the organism, then it will give a changes to survive at another site. In that case, the significant effect on the organism because it will be tumor is called a benign tumor. Most benign passed along by mitosis to many cells. Aside from tumors do not cause serious problems and can be aneuploid condition, some organisms have more removed through surgery. than two complete chromosome sets in all somatic In contrast, a malignant tumor includes cells. The general term for this chromosomal cells whose genetic and cellular changes enable alteration is polyploidy; the specific terms triploid them to spread to new tissues and impair the (3n) and tetraploid (4n) indicate three or four functions of one or more organs. This process is chromosomal sets, respectively. This condition is called metastasis. Malignant tumors are usually common among plants because it is lethal to treated through radiation, chemotherapy, and other animals to have this kind of condition. The table on medical procedures. the next page presents some of the disorders Chronic myelogenous leukemia (CML) is resulting from nondisjunction of chromosomes with a cancer of the blood which results from the its corresponding characteristics. translocation (exchange of segments) of chromosome 22 and 9 during mitosis of cells that would become white blood cells which activate a gene that leads to uncontrolled cell cycle progression. The key to understanding the different disorders Fig. Karyotypes A. Down’s syndrome B. Turner syndrome and diseases as a result of the malfunction of cells Source: www.alilamedicalimages.org lies on our knowledge of the cell cycle. If you can still recall in the previous discussion, we have 2.5.1 Common Disorders Resulting from Cell tackled that cell cycle has different phases and each Cycle Malfunction part has its own checkpoint in order to monitor the Disorders Characteristics activities of the cell. Failure to regulate cell activities Poor muscle tone, short neck, may result to various disease and disorder. 1.Down syndrome or flattened facial profile and Trisomy 21 nose, small head, ears, and A. CANCER mouth, with mental retardation One of the most common disorders we know Smallmouth and jaw, short today but without cure yet is cancer. Cancer refers 2. Edward syndrome neck, shield chest, clenched to a group of diseases characterized by or Trisomy 18 hands, back part of the skull is uncontrolled and abnormal cell division. It occurs prominent when there is a disruption in the cell cycle. Instead Poor breast development, no of stopping and starting at appropriate points, 3.Turner syndrome or menstruation, undeveloped cancerous cells divide continuously until a Monosomy 23 (X) ovaries, short stature, disorganized solid mass of cells called tumor is constriction of the aorta formed. 4.Klinefelter Developed breast, wide hips, Tumors can be categorized as benign or syndrome or Trisomy poor beard growth, small malignant. Benign tumors are cancer cells that 23 (XXY) testicular size, sterile male remain clustered together, which may be harmless Slightly taller than average, at or not and can probably be cured when removed out 5.Triple X risk of learning disabilities, of the body. Malignant tumors are cancer cells that fertile has break away or metastasized. This cancer cells Aside from the nondisjunction of are transported to the bloodstream of the lymphatic chromosomes, a malfunction of the signaling system to the other parts and form more tumors. system of the cell cycle like the mutation of genes that encode cell cycle proteins can lead What causes cancerous cells? to unregulated growth. One example is cancer. It ⎯ Cancer is caused mainly by changes or is an abnormal cell growth when the cell does not mutations to the DNA within cells. respond to cell cycle control checkpoints. Hence, the cell grows continuously forming What are some of the risk factors contributing to a mass of cells called a tumor. These cells keep on cancer? dividing because they lack appropriate cell death. ⎯ Lifestyle factors (e.g.: smoking, high-fat diet, Normal cells usually undergo apoptosis working with toxic chemicals) (programmed cell death) but in the case of tumors, ⎯ Family history, inheritance, and genetics (e.g., they can override the cell cycle checkpoints. inheritance of breast cancer)  ⎯ Some genetic disorder. the number of chromosomes and any ⎯ Exposure to certain viruses (e.g., cervical abnormalities. cancer which is caused by human papilloma virus) Numerical abnormality also called aneuploidy, a ⎯ Environmental exposures (e.g., exposure to condition which occurs when an individual has a pesticides and fertilizers, radiations, and missing chromosome from a pair (monosomy) or carcinogens). has more than two chromosomes of a pair (trisomy, tetrasomy, etc.). Why are tumors dangerous inside the body? ⎯ Generally, cancer cells do not perform the 2.5.2 EXAMPLES OF CHROMOSOMAL specialized functions of the normal cells in the ABNORMALITIES UNDER THIS CATEGORY body. INCLUDE THE FOLLOWING: ⎯ Example, if the cancer cells are in the brain, they do not perform their supposed function Down Syndrome (Trisomy 21) which is to transmit electrical signals for ✓ The most common response. Moreover, if they continue to grow disorder of trisomy is and form tumors, it can cramp the brain in the Down syndrome, wherein limited skull. This might affect the other parts the 21st chromosome of the brain and their functions because has three instead of two cancer cells also compete for nutrients and chromosomes. blood supply with other healthy cells. If left ✓ Most cases of Down unchecked, it may hinder the proper syndrome are not due to functioning of the body. inheritance but on random mistakes during How is cancer treated? formation of reproductive ⎯ Chemotherapy – uses certain drugs to kill cells of the parents. actively dividing cells. This procedure is ✓ Physical manifestations: systemic, which means that drugs are Short neck, with excess introduced throughout the body orally (taken skin at back of the neck. by mouth) or intravenously (injection) Flattened facial profile and nose. Small head, ⎯ Surgery – involves removal of the cancerous ears, and mouth. Upward slanting eyes. body part. Turner Syndrome (45, XO) ⎯ Radiation therapy – involves the exposure of ✓ A condition that affects X-rays to kill cancer cells and shrink the tumor only female as a result of size. one of the X chromosomes (sex B. GENETIC DISORDERS chromosome) is missing A change in the number or structure of or partially missing. chromosomes can dramatically change the traits of ✓ Physical manifestations: an organism and can cause serious problems. Webbed neck, short Abnormal chromosomes most often happen as a stature, swollen hands result of an error during cell division. Chromosome and feet. Some have abnormalities often happen due to one or more of skeletal abnormalities, these: kidney problems, and/or congenital heart ⎯ Errors during dividing of sex cells (meiosis) defect. ⎯ Errors during dividing of other cells (mitosis) ⎯ Exposure to substances that can cause birth Klinefelter Syndrome defects (teratogens) (47, XXY) ✓ A condition resulting from two or more X chromosomes in males. ✓ Manifestations are typically more severe if three or more X chromosomes are present as in (48, XXXY) Fig. Normal human karyotype. Male karyotype (left) and or (49, XXXXY). female karyotype (right). ✓ Physical manifestations: Primary features are Karyotyping is the process by which photographs infertility and small of chromosomes are taken in order to determine the poorly functioning chromosome complement of an individual, including testicles. Sometimes  includes weaker muscle, greater height, poor d. Inversion – a section of the chromosome coordination, less body hair, breast growth and becomes changed by rotation at 180 less interest in sex. degrees. Trisomy X Syndrome (47, Cri-du-chat Syndrome (5p XXX) minus syndrome) ✓ Characterized by the ✓ A genetic condition presence of extra X caused by the deletion of chromosome in each genetic material on the cell of a female small arm (p arm) of ✓ Physical chromosome 5 manifestations: Often ✓ Physical manifestations: taller than normal, mentally retarded, has affected individuals abnormal development of have usually mild glottis and larynx symptoms to none at resulting from a crying all. Occasionally there sound that sound like the are learning difficulties, delayed speech, meowing of a cat. decreased muscle tone, seizures, or kidney problems. A change (even a very slight change) in the number or structures of chromosomes can drastically change the Patau Syndrome traits of an organism and can cause serious disorders, (Trisomy 13) diseases, or abnormalities. ✓ Caused by having an additional copy of chromosome 13 in some or all the body’s cells. ✓ Physical manifestations: Clenched hands, cleft lip or palate, extra fingers, or toes (polydactyly), hernias, kidney, wrist or scalp problems, low-set ears, small head, undescended testis. Edward Syndrome (Trisomy 18) ✓ Caused by having additional copy of chromosome 18 ✓ Physical manifestations: Cleft palate, Clenched fists, defects of lungs, kidneys and stomach, deformed feet, heart defects, low-set ears, severe developmental delays, chest deformity, slowed growth, small head, small jaw. Structural abnormalities occur when the chromosome’s structure is altered, which can take several forms such as: a. Deletion – a portion of a chromosome is missing or deleted; b. Duplication – segment of a chromosome is repeated twice; c. Translocation – transfer of a section of one chromosome to non-homologous chromosome; Source: https://apbiologyctd.wordpress.com/genetics/meiosis/