MODULE ITHE CELLULAR BASIS OF LIFE

Authors:Eunice BasarioJohn Patrick Del Rosario

October 13, 2015II. INTRODUCTIONPre-test1. What part of the cell is responsible for breaking down and digesting things?a. Ribosomesb. Lysosomesc. Endoplasmic reticulumd. Vacuole2. What part of the cell makes proteins?a. Ribosomesb. Mitochondriac. Lysosomesd. Vacuole3. Who is the scientist who first discovered the cell?a. Theodore Schwannb. Rudolf Virchowc. Robert Hooked. Anton Van Leeuwenhoek4. What part of the cell serves as the fluid medium where organelles are suspended?a. Karyoplasmb. Cytoplasmc. Lysosomed. Vacuole5. What class of cell lacks a nucleus and other membrane-bounded cellular substructures?a. Prokaryotesb. Eukaryotesc. Nerve cellsd. Sensory cells6. What part of cell controls its overall activity?a. Nucleusb. Nucleolusc. Endoplasmic reticulumd. Mitochondria7. What is the study of cells named?a. Cytokinesisb. Histologyc. Cytologyd. Celography8. What part of the cell serves to process, package, and export proteinsa. Mitochondriab. Endoplasmic reticulumc. Nucleolusd. Golgi apparatus9. What process of cell division sex cells are formed?a. Meiotic cycleb. Mitotic cyclec. Metaphased. Anaphase10. In what stage of mitotic cycle do chromosomes are distributed at the equator?a. Anaphaseb. Metaphasec. Prophased. Telophase

A. Discussion ProperThe concept that all living material is made up of cell is a central unifying one in modern Biology. Both living and non-living things are composed of molecules made from chemical elements such as Carbon, Hydrogen, Oxygen, and Nitrogen. The organization of these molecules into cells is one feature that distinguishes living things from all other matter.1.1 HISTORY OF THE CELLIn the course of microscopic studies of cork, Robert Hooke (1665), noticed that the material under study was made up of many little boxes. Ten years later, Anton van Leeuwenhoek, using a hand-held lens, described several tiny microscopic creatures later found to be Bacteria and Protozoa. The cell theory of life was develop over a period of many years. The initial statement was formulated by Lorenz Oken in 1805. Theodore Schwann stated the theory essentially in its present form. Subsequent research led Rudolf Virchow to state that every cell forms from a pre-existing cells. The current theory is a set of six statements;1) all living material is made up of cells,2) all cells are derived from pre-existing cellsmost cells arise by cell division, but in sexual organisms they may be formed by the fusion of sperm and egg, 3) a cell is the most elementary unit of life4) every cell is bounded by a plasma membrane5) all cells have strong biochemical similarities6) most cells are small, about 0.001 cm in length

1.2 CELL STRUCTURE AND COMPOSITIONCells contain a variety of internal structures called organelles. An organelle is a cell component that performs a specific function in that cell. Microscopic studies shown that a cell has three fundamental parts namely, cytoplasm, cell membrane and nucleus. The cytoplasm consists of a thick, semifluid aggregate of chemical compounds called cytosol. It serves as the reservoir for the entry and exit of materials in the cell.The cell membrane or plasma membrane, also known as plasmalemma, serves as the outer boundary of the cells. It consists of double layer of fats or lipid with scattered proteins. It performs three major functions: a) separates the contents of the cell from external environment, b) regulates the passage of materials into and out of the cell, and c) allows communication with other cells. Figure 1. The cell and its organellesFigure 2. The plasma membrane

The nucleus is generally an oval-shaped or spherical shaped structure. It regulates and coordinates all the activities of the cell. The nucleus contains the cells chromosomes (human, 46, fruit fly 6, fern 1260) which are normally uncoiled to form a chromatinic network, which contain both linear DNA and proteins, known as histones. These proteins coil up (dehydrate) at the start of nuclear division, when the chromosomes first become visible. Whilst most cells have a single nucleus some cells (macrophages, phloem companion cells) have more than one and fungi have many nuclei in their cytoplasm they are coenocytic (= common cytoplasm throughout)Figure 3. The nucleus

The nucleus is surrounded by a double membrane called the nuclear envelope, which has many nuclear pores through which mRNA, and proteins can pass. These dimples make it look like a golf ball. Most nuclei contain at least one nucleolus (plural, nucleoli). The nucleoli are where ribosomes are synthesised.Mitochondria are found scattered throughout the cytosol, and are relatively large organelles (second only to the nucleus and chloroplasts).Figure 4. The powerhouse of the cell

Mitochondria have their own DNA, and new mitochondria arise only when existing ones grow and divide. They are thus semi-autonomous organelles. Mitochondria are the sites of aerobic respiration, in which energy from organic compounds is transferred to ATP. ATP is the molecule that most cells use as their main energy currency. Mitochondria are more numerous in cells that have a high energy requirement - our muscle cells contain a large number of mitochondria, as do liver, heart and sperm cells.Ribosomes are the site of protein synthesis in a cell. Unlike most other organelles, ribosomes are not surrounded by a membrane.They are the most common organelles in almost all cells. Some are free in the cytoplasm (Prokaryotes); others line the membranes of rough endoplasmic reticulum (rough ER).The Endoplasmic Reticulum is a system of membranous tubules and sacs. The primary function of the ER is to act as an internal transport system, allowing molecules to move from one part of the cell to another. The quantity of ER inside a cell fluctuates, depending on the cell's activity. Cells with a lot include secretory cells and liver cells.Figure 5. The ribosomes

The rough ER is studded with 80s ribosomes and is the site of protein synthesis. It is an extension of the outer membrane of the nuclear envelope, so allowing mRNA to be transported swiftlyFigure 6. Endoplasmic Reticulum

to the 80s ribosomes, where they are translated in protein synthesisThe smooth ER is where polypeptides are converted into functional proteins and where proteins are prepared for secretion. It is also the site of lipid and steroid synthesis, and is associated with the Golgi apparatus. Smooth ER has no 80s ribosomes and is also involved in the regulation of calcium levels in muscle cells, and the breakdown of toxins by liver cells. Both types of ER transport materials throughout the cell.The Golgi apparatus is the processing, packaging and secreting organelle of the cell, so it is much more common in glandular cells. The Golgi apparatus is a system of membranes, made of flattened sac-like structures called cisternae. It works closely with the smooth er, to modify proteins for export by the cell.Lysosomes are small spherical organelles that enclose hydrolytic enzymes within a single membrane. They are the site of protein digestion thus allowing enzymes to be re-cycled when they are no longer required. They are also the site of food digestion in the cell, and of bacterial digestion in phagocytes. Lysosomes are formed from pieces of the Golgi apparatus that break off.Figure 7. Golgi Bodies

In animal cells, which have no cell wall, an internal framework called the cytoskeleton maintains the shape of the cell, and helps the cell to move. The cytoskeleton consists of two structures:a) microfilaments (contractile). They are made of actin, and are common in motile cells.b) microtubules (rigid, hollow tubes made of tubulin).

Microtubules have three functions:a) To maintain the shape of the cell.b) To serve as tracks for organelles to move along within the cell.c) They form the centriole.

The centriole consists of two bundles of microtubules at right-angles to each other. Each bundle contains 9 tubes in a very characteristic arrangement. At the start of mitosis and meiosis, the centriole divides, and one half moves to each end of the cell, forming the spindle. The spindle fibres are later shortened to pull the chromosomes apart.Figure 8. Centrioles

Most of the organelles and other parts of the cell are common to all Eukaryotic cells. Cells from different organisms have an even greater difference in structure.Plant cells have three additional structures not found in animal cells:

Cellulose cell walls

Chloroplasts (and other plastids)

A central vacuole

Figure 9. An animal cell (left) and a plant cell (right)

One of the most important features of all plants is presence of a cellulose cell wall. The cell wall is freely permeable (porous), and so has no direct effect on the movement of molecules into or out of the cell. The rigidity of their cell walls helps both to support and protect the plant.The most prominent structure in plant cells is the large vacuole. The vacuole is a large membrane-bound sac that fills up much of most plant cells. The vacuole serves as a storage area, and may contain stored organic molecules as well as inorganic ions.A characteristic feature of plant cells is the presence of plastids that make or store food. The most common of these are chloroplasts the site of photosynthesis. Each chloroplast encloses a system of flattened, membranous sacs called thylakoids, which contain chlorophyll. The thylakoids are arranged in stacks called grana. Figure 10. Chlorophyll

1.3 MULTICELLULAR ORGANIZATIONIn a unicellular organism, one cell carries out all of the functions of life. In contrast, most cells in a multicellular organism are specialized to perform one or a few functions more efficiently. Because of cell specialization, the cells of multicellular organisms depend on other cells in the organism for their survival. In most Multicellular Organisms, we find the following organization:a. Cellular Level: The smallest unit of life capable of carrying out all the functions of living things.b. Tissue Level: A group of cells that performs a specific function in an organism.c. Organ Level: Several different types of tissue that function together for a specific purpose. d. Organ System Level: Several organs working together to perform a function. The different organ systems in a multicellular organism interact to carry out the processes of life

1.4 CELL CLASSESTwo classes of cells exist: the prokaryotes and the eukaryotes. Eukaryotes are organisms whose cells normally contain a nucleus while prokaryotes are organisms whose cells lack a nucleus and have no membrane-bound organelles.

CharacteristicsProkaryotesEukaryotes

Organismsbacteria and cyanobacteriaprotists, fungi, plants and animals

Nuclear Membraneabsent presents

DNAloop of DNA in the cytoplasmthin, very long DNA organized into chromosomes in the nucleus

RNA and proteinboth synthesized in the same compartmentRNA synthesized in the nucleus; protein synthesized in the cytoplasm

Cytoplasmno cytoskeleton; very few organelles present cytoskeleton present; many organelles present

Cellular Organizationmainly unicellularmainly multicellular with differentiation of cells

Cell sizegenerally 1 to 10 m in linear dimensiongenerally 10 to 100 m in linear dimension

Table 1. Comparison of Eukaryotes and Prokaryotes

1.5 CELL DIVISIONCell division is a very important process in all living organisms. It depends on two complementary events-the replication of DNA molecules that make up the basic genetic material of all cells and the orderly separation of the products of this replication. In prokaryotes, where only a single unit of DNA exists, these two events are intimately coupled with an inward growth of the cell membrane. In eukaryotes, the process is more complex. Here, the DNA is combined with histone protein and is separated into two or more discrete chromosomes that are enclosed in a distinct nuclear membrane. The Mitotic CycleMitosis is part of the overall cell cycle that includes a long phase called interphase which may be subdivided into three stages---G1, S, and G2 ---on the basis of the synthetic activities occurring within them. The synthesis of DNA occurs only during the S phase, when it coincides with the synthesis of the histone protein. As a result, of these coupled syntheses, each chromosomes now consists of two sister chromosomes, called chromatids, that are identical in their morphological and genetic organization and that are joined at the kinetochore. Chromatids become visible when mitosis sets in; the remainder of the mitotic cycle involves their separation into two offspring nuclei. Mitosis depends on four essential stages of the mitotic cycle. Figure 11. Diagrammatic presentation of Mitotic Cycle

I. Prophase Changes in the internal configuration of the nucleoprotein component of each chromatid cause a cycle of coiling to be initiated in which the chromosomes become shorter and thicker. Toward the end of the prophase, the microtubules forming the spinder proliferate in the cytoplasm. The end of prophase is signalled by the disruption of the nuclear membrane.II. Metaphase The manner in which chromosomes are distributed at the equator following a reorientation mechanisms depends on the relative sizes of the members of the chromosome set, as well as on the size of the cell itself. Figure 12. Prophase and Metaphase Stage of Mitotic Cycle

III. Anaphase The connection between sister chromatids is broken when the kinetochore divides and the component chromatids have separated completely. All sister kinetochores begin their movement toward opposite poles simultaneouslyIV. Telophase A new nuclear membrane begins to form at the surface of each of the two separated sets of chromosomes. At the same time, the chromosomes themselves uncoil and return to an extended (and diffuse) interphase state. Cytokinesis - The completion of cell division requires the cytoplasm be divided following division of the nucleus. Figure 13. Anaphase and Telophase Stage of Mitotic Cycle

Significance of MitosisMitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants and in some social insects haploid cells also divide by mitosis. It is very essential to understand the significance of this division in the life of an organism. Mitosis results in the production of diploid daughter cells with identical genetic complement usually. The growth of multi-cellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio. A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced. Meiotic CycleThe production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialized diploid cells. This specialized kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. The key features of meiosis are as follows: Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication. Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase. Meiosis involves pairing of homologous chromosomes and recombination between them. Four haploid cells are formed at the end of meiosis II.Meiotic events can be grouped under the following phases:a. MEIOSIS I1. Prophase I. First stage. Leptotene (literally thin thread), the chromosomes become visible but initially remain uncoiled. Localized areas of increased coiling, called chromomeres, then form.

Second Stage. Zygotene (yolked thread), in a process called synapsis, the chromosomes shorten and homologous chromosomes associate or meet.

Third Stage. Pachytene (thick thread), is a long period in which the bivalent chromosomes shorten and appear to be rodlike. Crossing over takes place where there is an exchange of chromosomal segments.

Fourth Stage. Diplotene (double thread), is characterized by partial separation of the chromosomes into for separate chromatids, but are still connected together in a portion called chiasmata.

Fifth Stage. Diakinesis, the chromosomes contract further, thereby increasing the tightness of the coiling.

2. Metaphase I. The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.

3. Anaphase I. The chiasmata complete their terminalisation, freeing the sister kinetochore pairs to move poleward.

4. Telophase I. The nuclear membrane reforms, nuclei reappear, and cytokinesis occurs, forming two daughter cells. Figure 13. Prophase I and Metaphase I Stage of Meiotic Cycle

b. MEIOSIS II1. Prophase II: Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact.

2. Metaphase II: At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.

3. Anaphase II: It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell.

4. Telophase II: Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells.

Figure 14. Second phase of Meitotic Division

Significance of MeiosisMeiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.

III. SUGGESTED ACTIVITIESActivity #1: Cell Show and Tell Students are assigned a single part of the cell, like the mitochondria and are tasked to bring in object from home that represents the structure. Any item will work, but students must present the object and explain how it represents the cell structure. Activity #2: Cell Charades This is a fun game to play at the end of the cell unit, students must act out the part of the cell structure while their team-mates try to guess the structure. Each team have only at least three tries to guess. An alternative to the game format is to ask students to come up with hand signals for each structure and then practice using the symbols instead of the words.Activity #3: 3D Cell this is a standard project for entry level biology classes, where students use various objects from around the house to design a three dimensional cell. Popular models are made of clay, cardboard, or Styrofoam. An alternative to the game format is to replace the materials found instead of house but rather inside the school premises.Activity #4: Post- IT Cell this model can be build in class using post it notes. The post-its can be drawn on, labeled or cut to particular shapes. This is a good activity for one class period and students can work in groups to discuss their models, cell structures, and functions.Activity 5: What part? This game is connected to the third suggested activity. The end product design of cell will be brought in front of the class. The teacher will ask which part of the cell is he pointing or what is the function of that particular organelles. Students will be compensated once their answer is correct, if wrong, consequences will be given.

IV. KEY CONCEPTSAnaphase: The stage of mitosis or meiosis during which centromeres split and chromatids separate and chromatids move to opposite poles.Bivalent/ Tetrad: A homologous pair of chromosomes in the synapsed, or paired, state during prophase I of the meiotic division.Cell Cycle: The cell cycle is the series of events that take place in a cell leading to its replication. Centromere: It is the primary constriction in chromosome to which the spindle fibres attach during mitotic and meiotic division. It appears as a constriction when chromosomes contract during cell division. Chromatin: Chromatin is the complex of DNA and protein that makes up chromosomes. It is found inside the nuclei of eukaryotic cells, and within the nucleoid in prokaryotes.Chromosomes: Thread like strands of DNA and associated proteins in the nucleus of cells that carry the genes and functions in the transmission of hereditary information.Crossing over: Crossing over is a process in which homologous chromosomes exchange genetic material through the breakage and reunion of two chromatids with the help of enzyme recombinase. Cytokinesis: The division of the cytoplasm of a cell following division of the nucleus that occurs in mitosis and meiosis, when a parent cell divides to produce two daughter cells.Diakinesis: This is the final stage of meiotic prophase I in which the chromatids break at the chiasmata and exchange their parts. Diplotene: This is the stage of the first meiotic prophase, following the pachytene, in which the two chromosomes in each bivalent begin to repel each other and a split occurs between the chromosomes, which are then held together by regions where exchanges have taken place (chiasmata) during crossing over.Karyokinesis: The indirect division of cells in which, prior to division of the cell protoplasm, complicated changes take place in the nucleus, attended with movement of the nuclear fibrils. The nucleus becomes enlarged and convoluted, and finally the threads are separated into two groups, which ultimately become disconnected and constitute the daughter nuclei.Kinetochore: These are disc shaped structures present on the sides of centromere.Leptotene: This is the stage of meiosis in which the chromosomes are slender, like threads.Metaphase: A stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plane of the cell. Metaphase chromosomes are highly condensed, scientists use these chromosomes for gene mapping and identifying chromosomal aberrations.Metaphase plate: The plane of the equator (a plane that is equally distant from the two spindle poles) of the spindle into which chromosomes are positioned during metaphase.Meiosis: This is a special method of cell division, occurring in maturation of the sex cells, by means of which each daughter nucleus receives half the number of chromosomes characteristic of the somatic cells of the species.M Phase: The M Phase represents the phase when the actual cell division or mitosis occurs i.e., during which the chromosomes are condensed and the nucleus and cytoplasm divide.Nonsister chromatids: Nonsister chromatids are not identical to each other as they represent different but homologous chromosomes and they will carry the same type of genetic information, but not exactly the same information.Pachytene: In meiosis, the stage following synapsis (zygotene) in which the homologous chromosome threads (synaptonemal complex) shorten, thicken, and continue to intertwine, and each of the conjoined (bivalent) chromosomes separate into two sister chromatids, which are held together by a centromere, to form a tetrad.Prophase: Prophase is the first stage of mitosis in which chromosomal material becomes untangled during the process of chromatin condensation and the centriole, begins to move towards opposite poles of cell.Sister chromatids: During S phase of the cell cycle the DNA is replicated and an identical copy of the chromatid is made. These identical copy of chromatids are called sister chromatids.S-Phase or Synthesis Phase: The S phase, short for synthesis phase, is a period in the cell cycle during interphase, between G1 phase and the G2 phase. In this phase DNA synthesis or replication occurs.Spindle fibres: It is a group of microtubules that extend from the centromere of chromosomes to the poles of the spindle or from pole to pole in a dividing cell.Synapsis: The pairing of homologous chromosomes along their length; synapsis usually occurs during prophase I of meiosis, but it can also occur in somatic cells of some organisms.Synaptonemal complex: A ribbon like protein structure formed between synapsed homologues at the end of the first meiotic prophase, binding the chromatids along their length and facilitating chromatid exchange.Telophase: The last stage in each mitotic or meiotic division, in which the chromosomes are assembled at the opposite spindle poles, nuclear envelope assembles around the chromosomes and nucleolus golgi complex and endoplasmic reticulum reform.Zygotene: This is the synaptic stage of the first meiotic prophase in which the two leptotene chromosomes undergo pairing by the formation of synaptonemal complexes to form a bivalent structure.

V. SUMMARY According to the cell theory, cells arise from preexisting cells. Cells vary in structure in relation to the functions they perform. No matter how different in shape and size cells are, they have the same main part; cell membrane, cytoplasm, and nucleus. The process by which this occurs is called cell division. Any sexually reproducing organism starts its life cycle from a single-celled zygote. Cell division does not stop with the formation of the mature organism but continues throughout its life cycle The stages through which a cell passes from one division to the next is called the cell cycle. Cell cycle is divided into two phases called (i) Interphase a period of preparation for cell division, and (ii) Mitosis (M phase) the actual period of cell division. Interphase is further subdivided into G1, S and G2. G1 phase is the period when the cell grows and carries out normal metabolism. Most of the organelle duplication also occurs during this phase. S phase marks the phase of DNA replication and chromosome duplication. G2 phase is the period of cytoplasmic growth. Mitosis is also divided into four stages namely prophase, metaphase, anaphase and telophase. Chromosome condensation occurs during prophase. Simultaneously, the centrioles move to the opposite poles. The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing. Metaphase is marked by the alignment of chromosomes at the equatorial plate. During anaphase the centromeres divide and the chromatids start moving towards the two opposite poles. Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear membrane reappear. This stage is called the telophase. Nuclear division is then followed by the cytoplasmic division and is called cytokinesis. Mitosis thus, is the equational division in which the chromosome number of the parent is conserved in the daughter cell. In contrast to mitosis, meiosis occurs in the diploid cells, which are destined to form gametes. In sexual reproduction when the two gametes fuse the chromosome number is restored to the value in the parent. Meiosis is divided into two phases meiosis I and meiosis II. In the first meiotic division the homologous chromosomes pair to form bivalents, and undergo crossing over. Meiosis I has a long prophase, which is divided further into five phases. These are leptotene, zygote, pachytene, diplotene and diakinesis. During metaphase I the bivalents arrange on the equatorial plate. This is followed by anaphase I in which homologous chromosomes move to the opposite poles with both their chromatids. Each pole receives half the chromosome number of the parent cell. In telophase I, the nuclear membrane and nucleolus reappear. Meiosis II is similar to mitosis. During anaphase II the sister chromatids separate. Thus at the end of meiosis four haploid cells are formed.

VI. POST-TEST1. Who are the three principal contributors to the cell theory? State their contribution to the development of the theory.2. What enabled cytologists to gain more knowledge and better understanding of the cell theory?3. Describe the events taking place during interphase.4. How does cytokinesis in plant cells differ from that in animal cells?5. What is the significance of meiosis?6. Differentiate plant and animal cells as to their protective covering or envelope?7. Name two organelles which are:a. Present in plant cells but not in animal cellsb. Present in animal cells but not in plant cells8-10 Make an analogy of the functions and structures of the cell and its organelles.

VII. APPENDIXAnswers for Pre-test:1. B2. A3. C4. B5. A6. A7. C8. C9. A10. BAnswers for Post-Test1. Mattias Schleiden (working on plant cells) and Theodore Schwann (working on animal cells) made the generalization that all living things are made up of cells. Meanwhile, Rudolf Virchow came to the conclusion that cells come from pre-existing cells.2. The invention of the microscope, its subsequent improvements and the use of modern techniques in studying cells enabled cytologists to gain more knowledge and better understanding of the cell theory.3. Interphase may be divided into three stages. In Gap 1, all the necessary materials are prepared by the cell. Synthesis phase is where DNA is replicated. In Gap 2, organelles are being replicated. 4. In animal cells, where no rigid wall exists, the cytoplasm becomes shaped like a dumbbell as the result of constriction initiated at the cell's surface, which extends inward. In plants, a new cell wall is built across the middle of the cell and gradually extends outward. 5. It is very essential to understand the significance of this division in the life of an organism. Mitosis results in the production of diploid daughter cells with identical genetic complement usually. The growth of multi-cellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio. A very significant contribution of mitosis is cell repair. 6. In addition to the cell membrane which envelops the cytoplasm of both plant and animal cells, the plant cell has another outer protective covering known as the cell wall.7. Plastids are large vacuoles present in plant cells, whereas lysosomes and centrosomes are found in animal cells.

VIII. BIBLIOGRAPHYRabago Ph.D., Lilia M., Joaquin Ph.D., Crescencia C., Lagunzad Ph.D, Catherine Genevieve B. (2003) Functional Biology: Modular Approach. 68-79.The Grolier Family Encyclopedia. International Edition (2003) p. 211-216.Biologymad. (2012) Retrieved from: http://www.biologymad.com/resources/Ch%201%20-%20Cells.pdfLearning basket Biology. Retrieved from: http://www.ncert.nic.in/html/learning_basket/biology/cc&cd.pdf