OBC CYTOLOGY.pdf ( properties , components and their functions in the cell
- 1. CYTOLOGY DR MUNDIH NOELAR DR AKAWAARASMOSE DR EBAH BECKELY DR LYONGA KHARIM
- 2. OUTLINE PART I Overview of the Cellular Basis of Life The Plasma Membrane The Cytoplasm PART II The Nucleus Cell Growth and Reproduction
- 3. OVERVIEW OFTHE CELLULAR BASIS OF LIFE CELLULAR BASIS OF LIFE
- 4. English scientist Robert Hooke first observed plant cells with a crude microscope in the late 1600s Since the late 1800s, cell research has been exceptionally fruitful and provided us with four concepts collectively known as the cell theory: 1. A cell is the basic structural and functional unit of living organisms. The activity of an organism depends on both the individual and the 2. The activity of an organism depends on both the individual and the collective activities of its cells. 3. According to the principle of complementarity, the biochemical activities of cells are dictated by the relative number of their specific subcellular structures
- 5. 4. Continuity of life has a cellular basis The trillions of cells in the human body include over 200 different cell types that vary greatly in shape, size, and function Spherical fat cells, disc-shaped red blood cells, branching nerve cells, and cubelike cells of kidney tubules cells also vary greatly in length—ranging from 2 micrometers (1/12,000 of an inch) in the smallest cells to over a meter in (1/12,000 of an inch) in the smallest cells to over a meter in the nerve cells that cause you to wiggle your toes. A cell’s shape reflects its function. For example, the flat, tilelike epithelial cells that line the inside of your cheek fit closely together, forming a living barrier that protects underlying tissues from bacterial invasion.
- 6. PROKARYOTIC AND EUKARIOTIC CELLS All cellular organisms fall under two natural groups known prokaryotes and eukaryotes These terms refer to the differences in location of the genetic material In prokaryotes (pro= before, karyon= nucleus) the In prokaryotes (pro= before, karyon= nucleus) the DNA is not enclosed by nuclear membranes. e.g bacteria In eukaryotes (eu= true, karyon = nucleus) then DNA is enclosed in a nuclear membrane e.g protoctists, fungi, plants, animals
- 7. FEATURE PROKARYOTES EUKARYOTES Organisms bacteria Protoctists, fungi, plants, animals Cell size 0.5um diameter 10-100um diameter Form unicellular multicellular Cell division Binary fission no spindle Mitosis, meiosis; spindle formed Genetic material Circular DNA free in cytoplasm; no Linear DNA with proteins and RNA to Genetic material Circular DNA free in cytoplasm; no chromosomes Linear DNA with proteins and RNA to form chromosomes Protein synthesis 70s ribosomes (smaller) 80s ribosomes (larger) organelles Few, without membranes Many membrane bound respiration mesosomes mitochondria
- 9. fluid mosaic model The fluid mosaic model of membrane structure depicts the plasma membrane as an exceedingly thin (7–10 nm) structure composed of a double layer, or bilayer, of lipid molecules with protein molecules dispersed in it. dispersed in it. The proteins, many of which float in the fluid lipid bilayer, form a constantly changing mosaic pattern; hence the name of the model.
- 10. fluid mosaic model The lipid bilayer, which forms the basic “fabric” of the membrane, is constructed largely of phospholipids, with smaller amounts of cholesterol and glycolipids. Each lollipop-shaped phospholipid molecule has a polar “head” that is charged and is hydrophilic (hydro = water, philic = loving), and an uncharged, nonpolar “tail” that is made of two fatty acid chains an uncharged, nonpolar “tail” that is made of two fatty acid chains and is hydrophobic (phobia = hating). The majority of membrane phospholipids are unsaturated (like phosphatidylcholine), a condition which kinks their tails (increasing the space between them) and increases membrane fluidity.
- 11. fluid mosaic model . Glycolipids (gli″ko-lip′idz), phospholipids with attached sugar groups, are found only on the outer plasma membrane surface and account for about 5% of the total membrane lipid. Their sugar groups, like the phosphate-containing groups of phospholipids, make that end of the glycolipid molecule polar, Their sugar groups, like the phosphate-containing groups of phospholipids, make that end of the glycolipid molecule polar, whereas the fatty acid tails are nonpolar. Some 20% of membrane lipid is cholesterol, which wedges its platelike hydrocarbon rings between the phospholipid tails, decreasing their orderliness and increasing the mobility of the phospholipids
- 12. About 20% of the outer membrane surface contains lipid rafts, dynamic assemblies of saturated phospholipids (which pack together tightly) associated with unique lipids called sphingolipids and lots of cholesterol. fluid mosaic model cholesterol. These quiltlike patches are more stable and orderly and less fluid than the rest of the membrane, and can include or exclude specific proteins to various extents. Because of these qualities, lipid rafts are assumed to be concentrating platforms for molecules needed for cell signaling. (Cell signaling is discussed.)
- 14. Two distinct populations of membrane proteins:integral and peripheral. Proteins make up about half of the plasma membrane by mass responsible for most of the specialized membrane responsible for most of the specialized membrane functions Integral proteins: firmly inserted into the lipid bilayer. Some protrude from one membrane face only, but most are transmembrane proteins that span the entire width of the membrane and protrude on both sides
- 15. Transmembrane proteins Mainly involved in transport. Some cluster together to form channels, or pores, through which small, water-soluble molecules or ions can move, thus bypassing the lipid part of the membrane. membrane. Others act as carriers that bind to a substance and then move it through the membrane. Others are receptors for hormones or other chemical messengers and relay messages to the cell interior (a process called signal transduction).
- 16. Peripheral proteins, Not embedded in the lipid. Attach loosely to integral proteins or membrane lipids and are easily removed without disrupting the membrane. Include a network of filaments that helps support the membrane from its cytoplasmic side. membrane from its cytoplasmic side. Some are enzymes. Others are involved in mechanical functions, e.g changing cell shape during cell division and muscle cell contraction, or linking cells together.
- 17. The glycocalyx Used to describe the fuzzy, sticky carbohydrate-rich area at the cell surface.You can think of your cells as sugar-coated. It is enriched both by glycolipids and by glycoproteins secreted by the cell. Provides highly specific biological markers by which approaching cells recognize each other. Provides highly specific biological markers by which approaching cells recognize each other. E.g a sperm recognizes an ovum (egg cell) by the ovum’s unique glycocalyx, Cells of the immune system identify a bacterium by binding to certain membrane glycoproteins in the bacterial glycocalyx.
- 18. Specializations of the Plasma Membrane Microvilli (mi″kro-vil′i;“little shaggy hairs”) Membrane Junctions
- 19. Microvilli (mi″kro-vil′i;“little shaggy hairs”) Minute, fingerlike extensions of the plasma membrane that project from a free, or exposed, cell surface. Increase plasma membrane surface area tremendously found on the surface of absorptive cells such as intestinal and kidney tubule cells. intestinal and kidney tubule cells. Have a core of actin filaments. (Actin is a contractile protein, but in microvilli it appears to function as a mechanical “stiffener.”)
- 20. Cell junctions SEE HISTOLOGY
- 21. MembraneTransport Substances move through the plasma membrane in essentially two ways—passively or actively. In passive processes, substances cross the membrane without any energy input from the In passive processes, substances cross the membrane without any energy input from the cell. In active processes, the cell provides the metabolic energy (ATP) needed to move substances across the membrane.
- 22. Passive transport Two main ways; Diffusion filtration filtration
- 23. Diffusion is the tendency of molecules or ions to scatter evenly throughout the environment. Recall: All molecules possess kinetic energy and are in constant motion.As molecules move about randomly at high speeds, they collide and ricochet off one another, changing direction with each collision.The ricochet off one another, changing direction with each collision.The overall effect of this erratic movement is that molecules move away from areas where they are in higher concentration to areas where their concentration is lower, so we say that molecules diffuse along, or down, their concentration gradient. The greater the difference in concentration between the two areas, the faster the net diffusion of the particles.
- 24. Simple diffusion. Nonpolar and lipid-soluble substances diffuse directly through the lipid bilayer oxygen, carbon dioxide, and fat-soluble vitamins. Because oxygen concentration is always higher in Because oxygen concentration is always higher in the blood than in tissue cells, oxygen continuously diffuses from the blood into the cells, whereas carbon dioxide (in higher concentration within the cells) diffuses from tissue cells into the blood.
- 25. Facilitated diffusion. Certain molecules, notably glucose and other sugars, amino acids, and ions are transported passively even though they are unable to pass through the lipid bilayer. Instead they move through the membrane by a passive transport process the membrane by a passive transport process called facilitated diffusion the transported substance either (1) binds to protein carriers in the membrane and is ferried across or (2)moves through water-filled protein channels.
- 26. Carriers. A carrier is a transmembrane integral protein that shows specificity for molecules of a certain polar substance or class of substances that are too large to pass through membrane channels, such as sugars and amino acids. Although it was initially believed that the integral proteins that act as carriers either flip-flopped or physically crossed the membrane like ferryboats, the either flip-flopped or physically crossed the membrane like ferryboats, the most popular model for this process indicates that changes in the shape of the carrier allow it to first envelop and then release the transported substance, shielding it en route from the nonpolar regions of the membrane (Figure 3.7b). Essentially, the binding site is moved from one face of the membrane to the other by changes in the conformation of the carrier protein.
- 27. Channels. Channels are transmembrane proteins that serve to transport substances, usually ions or water, through aqueous channels from one side of the membrane to the other. Binding or association sites exist within the channels, and the channels are selective due to pore size and the charges of the amino acids lining the channel. amino acids lining the channel. Some channels, the so-called leakage channels, are always open and simply allow ion or water fluxes according to concentration gradients. Others are gated and controlled (opened or closed) by various chemical or electrical signals.
- 28. Osmosis. (oz-mo′sis; osmos = pushing). The diffusion of a solvent, such as water, through a selectively permeable membrane Even though water is highly polar, it passes via osmosis through the lipid bilayer . Water also moves freely and reversibly through water- specific channels constructed by transmembrane Water also moves freely and reversibly through water- specific channels constructed by transmembrane proteins called aquaporins (AQP). Aquaporins are particularly abundant in red blood cells and in cells involved in water balance such as kidney tubule cells
- 29. The extent to which water’s concentration is decreased by solutes depends on the number, not the type, of solute particles, because one molecule or one ion of solute (theoretically) molecule or one ion of solute (theoretically) displaces one water molecule. The total concentration of all solute particles in a solution is referred to as the solution’s osmolarity (oz″mo-lar′ĭ-te).
- 30. The ability of a solution to change the shape or tone of cells by altering their internal water volume is called tonicity (tono = tension). Solutions with the same concentrations of nonpenetrating solutes as those found in cells nonpenetrating solutes as those found in cells (0.9% saline or 5% glucose) are isotonic (“the same tonicity”). Solutions with a higher concentration of nonpenetrating solutes than seen in the cell (for example, a strong saline solution) are hypertonic.
- 31. Solutions that are more dilute (contain a lower concentration of nonpenetrating solutes) than cells are called hypotonic.
- 32. Active Processes Active transport, VesicularTransport (exocytosis and endocytosis) endocytosis)
- 35. Active transport, like carrier-mediated facilitated diffusion, requires carrier proteins that combine specifically and reversibly with the transported substances. Facilitated diffusion always honors concentration gradients because its driving force is kinetic energy. gradients because its driving force is kinetic energy. In contrast, the active transporters or solute pumps move solutes, most importantly ions (such as Na+, K+, and Ca2+),“uphill” against a concentration gradient. To do this work, cells must expend the energy ofATP.
- 36. Primary active transport. the energy to do work comes directly from hydrolysis of ATP The most investigated example of a primary The most investigated example of a primary active transport system is the operation of the sodium-potassium pump (Figure 3.10), for which the carrier is an enzyme called Na+-K+ ATPase.
- 37. Secondary active transport. transport is driven indirectly by energy stored in ionic gradients created by operation of primary active transport pumps. Secondary active transport systems are all coupled systems; they move more than one substance at a time systems; they move more than one substance at a time symport system (sym = same):If the two transported substances are moved in the same direction, an antiport system (anti = opposite, against):transported substances cross the membrane in opposite directions.
- 39. Cytoplasm (“cell-forming material”) is the cellular material between the plasma membrane and the nucleus. It is the site where most cellular activities are accomplished. accomplished. The electron microscope has revealed that it consists of three major elements: the cytosol, organelles, and inclusions.
- 40. The cytosol (si′to-sol) is the viscous, semitransparent fluid in which the other cytoplasmic elements are suspended. It is a complex mixture with properties of both It is a complex mixture with properties of both a colloid and a true solution. Dissolved in the cytosol, which is largely water, are proteins, salts, sugars, and a variety of other solutes
- 41. the nonmembranous organelles, lack membranes. Examples are the cytoskeleton, centrioles, and ribosomes. However, most organelles are bounded by a membrane similar in composition to the However, most organelles are bounded by a membrane similar in composition to the plasma membrane (minus the glycocalyx), This membrane enables such membranous organelles to maintain an internal environment different from that of the surrounding cytosol
- 42. Mitochondria (mi″to-kon′dre-ah) are threadlike (mitos = thread) or sausage-shaped membranous organelles In living cells they squirm, elongate, and change shape almost continuously. are the power plants of a cell, providing most of its ATP supply. The density of mitochondria in a particular cell reflects that cell’s The density of mitochondria in a particular cell reflects that cell’s energy requirements, and mitochondria are generally clustered where the action is. Busy cells like kidney and liver cells have hundreds of mitochondria, whereas relatively inactive cells (such as unchallenged lymphocytes) have just a few.
- 44. Mitochondria (mi″to-kon′dre-ah) enclosed by two membranes The outer membrane is smooth and featureless, the inner membrane folds inward, forming shelflike cristae (krĭ′ste;“crests”) that protrude into the matrix, the gel-like substance within the matrix, the gel-like substance within the mitochondrion aerobic cellular respiration (a-er-o′bik) They contain their own DNA and RNA and are able to reproduce themselves
- 45. Ribosomes (ri′bo-sōmz) Ribosomes are small, dark-staining granules composed of proteins and a variety of RNA called ribosomal RNA. Each ribosome has two globular subunits that fit together like the body and cap of an acorn together like the body and cap of an acorn Ribosomes are sites of protein synthesis. Some float freely in the cytoplasm; others are attached to membranes, forming a complex called the rough endoplasmic reticulum
- 46. endoplasmic reticulum (ER) (en″do-plaz′mik re-tik′u-lum; “network within the cytoplasm”) Extensive system of interconnected tubes and parallel membranes enclosing fluid-filled cavities, or cisternae (sis-ter′ne), that coils and twists through the cytosol. the cytosol. Continuous with the nuclear membrane and accounts for about half of the cell’s membranes. There are two distinct varieties of ER: rough ER and smooth ER.
- 48. Endoplasmic Reticulum The rough endoplasmic reticulum is a ribosome-studded membrane system. Its cisternae act as sites for protein modification. Its external face acts in modification. Its external face acts in phospholipid synthesis. Vesicles pinched off from the ER transport the proteins to other cell sites.
- 49. Endoplasmic Reticulum Smooth Endoplasmic Reticulum is in communication with the rough ER and consists of tubules arranged in a looping network. Its enzymes (all integral proteins forming part Its enzymes (all integral proteins forming part of its membranes) play no role in protein synthesis. Instead, they catalyze reactions involved with the following processes:
- 50. Lipid metabolism, cholesterol synthesis, and synthesis of the lipid components of lipoproteins (in liver cells) Synthesis of steroid-based hormones such as sex hormones (testosterone-synthesizing cells of the hormones (testosterone-synthesizing cells of the testes are full of smooth ER) Absorption, synthesis, and transport of fats (in intestinal cells) Detoxification of drugs, certain pesticides, and carcinogens (in liver and kidneys)
- 51. Golgi apparatus (gol′je) Consists of stacked and flattened membranous sacs, shaped like hollow dinner plates, associated with swarms of tiny membranous vesicles. Tis the principal “traffic director” for cellular proteins. Its major function is to modify, concentrate, and Its major function is to modify, concentrate, and package the proteins and lipids made at the rough ER. The transport vesicles that bud off from the rough ER move to and fuse with the membranes at its convex cis face, the “receiving” side of the Golgi apparatus.
- 53. Inside the apparatus, the proteins are modified: Some sugar groups are trimmed while others are added, and in some cases, phosphate groups are added. The various proteins are “tagged” for delivery The various proteins are “tagged” for delivery to a specific address, sorted, and packaged in at least three types of vesicles that bud from the concave trans face (the “shipping” side) of the Golgi stack.
- 54. Vesicles containing proteins destined for export pinch off from the trans face as secretory vesicles, or granules, which migrate to the plasma membrane and migrate to the plasma membrane and discharge their contents from the cell by exocytosis
- 55. Lysosomes (“disintegrator bodies”) Lysosomes are spherical membranous organelles containing digestive enzymes Large and abundant in phagocytes, the cells that dispose of invading bacteria and cell debris. dispose of invading bacteria and cell debris. Lysosomal enzymes can digest almost all kinds of biological molecules. They work best in acidic conditions and thus are called acid hydrolases (hi″drah-la′siz).
- 57. Lysosomes function as a cell’s “demolition crew” by Digesting particles taken in by endocytosis, particularly ingested bacteria, viruses, and toxins Degrading worn-out or nonfunctional organelles Performing metabolic functions, such as glycogen breakdown and release Breaking down nonuseful tissues, such as the webs between the fingers and toes of a developing fetus and the uterine lining during menstruation Breaking down bone to release calcium ions into the blood
- 58. Lysosomal rupture results in self-digestion of the cell, a process called autolysis (aw″tol′ĭ-sis). Autolysis is the basis for desirable destruction of cells.
- 59. Peroxisomes (pĕ-roks′ĭ-sōmz; “peroxide bodies”) Are membranous sacs containing a variety of powerful enzymes, the most important of which are oxidases and catalases. Oxidases use molecular oxygen (O2) to detoxify harmful substances, including alcohol and harmful substances, including alcohol and formaldehyde. However, their most important function is to neutralize dangerous free radicals, highly reactive chemicals with unpaired electrons that can scramble the structure of biological molecules.
- 60. Oxidases convert free radicals to hydrogen peroxide, which is also reactive and dangerous but is quickly converted to water by catalase enzymes peroxisomes look like small lysosomes peroxisomes look like small lysosomes they are self-replicating organelles formed by a simple pinching in half of preexisting peroxisomes. Unlike lysosomes, they do not arise by budding from the Golgi apparatus.
- 61. The cytoskeleton, “cell skeleton,” An elaborate series of rods running through the cytosol. This network acts as a cell’s “bones,” “muscles,” and “ligaments” by supporting cellular structures and providing the machinery to and “ligaments” by supporting cellular structures and providing the machinery to generate various cell movements Three principal types microtubules, microfilaments, and intermediate filaments
- 62. Assignmnent: Describe the different portions of the cytoskeleton Describe the cellular extensions and Describe the cellular extensions and distinguish between them, giving examples of their locations and functions in the human body
- 63. PART II THE NUCLEUS AND CELL THE NUCLEUS AND CELL DIVISION
- 64. THE NUCLEUS
- 65. nucleus (nucle = pit, kernel). The nucleus can be compared to a computer, design department, construction boss, and board of directors—all rolled into one. Contains the instructions needed to build nearly all the body’s proteins. Contains the instructions needed to build nearly all the body’s proteins. Additionally, it dictates the kinds and amounts of proteins to be synthesized at any one time in response to signals acting on the cell.
- 66. Most cells have only one nucleus, but some, including skeletal muscle cells, bone destruction cells, and some liver cells, are multinucleate (mul″tĭ-nu′kle-āt); all of our body cells are nucleated.The exception is mature red blood cells, whose nuclei are ejected before the cells enter the bloodstream. mature red blood cells, whose nuclei are ejected before the cells enter the bloodstream. These anucleate (a-nu′kle-āt; a = without) cells cannot reproduce and therefore live in the bloodstream for only three to four months before they begin to deteriorate.
- 67. three recognizable regions or structures: the nuclear envelope (membrane), nucleoli, nucleoli, and chromatin
- 68. the nuclear envelope A double membrane barrier separated by a fluid-filled space (similar to the mitochondrial membrane). Outer nuclear membrane continuous with the rough ER of the cytoplasm and studded with ribosomes on its external face. external face. Inner nuclear membrane lined by the nuclear lamina, a network of lamins (rod-shaped proteins of the intermediate filament class), that maintains the shape of the nucleus and acts as a scaffold to organize DNA in the nucleus
- 69. At various points, the two layers of the nuclear envelope interconnect to form the edges of nuclear pores. An intricate complex of proteins, called a pore complex, lines each pore forming an aqueous transport channel and regulating the entry and exit of large particles into and out of the nucleus. The nuclear envelope encloses a jellylike fluid called nucleoplasm (nu′kle-o-plazm) in which other nuclear elements The nuclear envelope encloses a jellylike fluid called nucleoplasm (nu′kle-o-plazm) in which other nuclear elements are suspended. Like the cytosol, the nucleoplasm contains dissolved salts, nutrients, and other essential solutes.
- 70. Nucleoli (nu-kle′o-li;“little nuclei”) dark-staining spherical bodies found within the nucleus.They are not membrane bounded. Typically, there are one or two nucleoli per nucleus, but there may be more. Sites where ribosome subunits are assembled
- 71. chromatin (kro′mah-tin) composed of about 30% DNA, which is traditionally called our genetic material, about 60% globular histone proteins (his′tōn), and about 10% RNA chains, newly formed or forming. When a cell is preparing to divide, the chromatin threads coil When a cell is preparing to divide, the chromatin threads coil and condense enormously to form short, barlike bodies called chromosomes (“colored bodies”). Chromosome compactness avoids entanglement and breakage of the delicate chromatin strands during the movements that occur during cell division.
- 73. THE CELL LIFE CYCLE Cell Growth and Reproduction
- 75. The Cell Life Cycle encompasses two major periods: Interphase, in which the cell grows and carries on its usual activities, carries on its usual activities, and cell division, or the mitotic phase, during which it divides into two cells
- 76. Interphase Period from cell formation to cell division stage between cell divisions Interphase is divided into G1, S, and G2 subphases (the Gs stand for gaps before and after the S (the Gs stand for gaps before and after the S phase; S is for synthetic). In all three subphases, the cell grows by producing proteins and organelles; however, chromatin is reproduced only during the S subphase
- 77. During G1 (gap 1), the cell is metabolically active, synthesizing proteins rapidly and growing vigorously. The most variable phase in terms of length. In cells that divide rapidly, G1 typically lasts several minutes to hours; In those that divide slowly, it may last for days or even years. In those that divide slowly, it may last for days or even years. virtually no activities directly related to cell division occur. However, as G1 ends, the centrioles start to replicate in preparation for cell division.
- 78. During the S phase, DNA is replicated, ensuring that the two future cells being created will receive identical copies of the genetic material. material. New histones are made and assembled into chromatin. One thing is sure: Without a proper S phase, there can be no correct mitotic phase.
- 79. The final phase of interphase, called G2, is brief. Enzymes and other proteins needed for division are synthesized and moved to their proper sites. By the end of G2, centriole replication (begun in G ) is complete. 2 G1) is complete. The cell is now ready to divide. Throughout S and G2, the cell continues to grow and carries on with business as usual.
- 80. Cell Division Mitotic meiotic
- 81. cell division, also called the M (mitotic) phase of the cell life cycle , involves two distinct events: mitosis (mi-to′sis; mit = thread; osis = process), or division of the nucleus, and cytokinesis (si-to-kĭ-ne′sis; kines = and cytokinesis (si-to-kĭ-ne′sis; kines = movement), or division of the cytoplasm. A somewhat different process of nuclear division called meiosis (mi-o′sis) produces sex cells (ova and sperm) with only half the number of genes found in other body cells.
- 82. Mitosis The series of events that parcel out the replicated DNA of the mother cell to two daughter cells four phases: prophase, metaphase, anaphase, and telophase a continuous process, with one phase merging smoothly into the next
- 83. Prophase – the first and longest stage of mitosis Early prophase – chromatin threads condense into chromosomes Chromosomes are made up of two threads Chromosomes are made up of two threads called chromatids Chromatids are held together by the centromere Centriole pairs separate from one another The mitotic spindle forms
- 85. Late prophase – centrioles continue moving away from each other Nuclear membrane fragments Nuclear membrane fragments
- 86. Metaphase – the second stage of mitosis Chromosomes cluster at the middle of the cell Centromeres are aligned along the equator Anaphase – the third and shortest stage of mitosis Centromeres of chromosomes split
- 88. Telophase – begins as chromosomal movement stops Chromosomes at opposite poles of the cell uncoil Resume their thread-like extended-chromatin form A new nuclear membrane forms A new nuclear membrane forms Cytokinesis – completes the division of the cell into two daughter cells
- 90. G0 Cells that permanently cease dividing are said to be in the G0 phase.
- 91. Checkpoints Regulatory proteins act as checkpoints to control the phases of the cell cycle Cancer cells result from cells that lack Cancer cells result from cells that lack these control mechanisms and hence replicate widely and these cells become harmful to the host
- 92. MEIOSIS See
- 93. assignment Distingusih between Mitosis and Meiosis