Ch 2-The CELL Introduction • There are trillions of cells in the body • Cells are the structural “building blocks” of all plants and animals • Cells are produced by the division of preexisting cells • Cells form all the structures in the body • Cells perform all vital functions of the body Introduction • There are two types of cells in the body: • Sex cells • Sperm in males and oocytes in females • Somatic cells • All the other cells in the body that are not sex cells Cellular Anatomy • The cell consists of: • Cytoplasm • Cytosol • Organelles • Plasmalemma • Cell membrane The Cell can be divided into Plasmalemma Cytoplasm Divided into Organelles Cytosol subdivided into Nonmembranous Organelles Membranous Organelles • Cytoskeleton • Mitochondria • Microvilli • Nucleus • Centrioles • Endoplasmic reticulum • Cilia • Flagella • Ribosomes • Golgi apparatus • Lysosomes • Peroxisomes Cellular Anatomy • Anatomical Structures of the Cell • Organelles • Nonmembranous organelles • Membranous organelles Cellular Anatomy • Organelles of the Cell • Nonmembranous organelles • • • • • • Cytoskeleton Microvilli Centrioles Cilia Flagella Ribosomes Figure 2.1 Anatomy of a Typical Cell Microvilli Secretory vesicles Cytosol Lysosome Centrosome Centriole Chromatin Nucleoplasm Nucleolus Nuclear envelope surrounding nucleus Cytoskeleton Plasmalemma Golgi apparatus Mitochondrion Peroxisome Nuclear pores Smooth endoplasmic reticulum Rough endoplasmic reticulum Fixed ribosomes Free ribosomes Cellular Anatomy • Organelles of the Cell • Membranous organelles • • • • • • Mitochondria Nucleus Endoplasmic reticulum Golgi apparatus Lysosomes Peroxisomes Cellular Anatomy • Plasmalemma • A cell membrane composed of: • • • • Phospholipids Glycolipids Protein Cholesterol Figure 2.3 The Plasmalemma Hydrophilic heads Hydrophobic tails Cholesterol EXTRACELLULAR FLUID Glycolipids of glycocalyx Phospholipid bilayer Integral protein with channel Integral glycoproteins Hydrophobic tails Cholesterol Gated channel a The plasmalemma CYTOPLASM Peripheral proteins = 2 nm b Hydrophilic heads Cytoskeleton (Microfilaments) The phospholipid bilayer Cellular Anatomy • Functions of the Plasmalemma • Cell membrane (also called phospholipid bilayer) • Major functions: • Physical isolation • Regulation of exchange with the environment (permeability) • Sensitivity • Cell-to-cell communication/Adhesion/Structural support Cellular Anatomy • Structure of the Plasmalemma • Called a phospholipid bilayer • Composed of two layers of phospholipid • Hydrophilic heads are at the surfaces (inside lining and outside lining) • Hydrophobic fatty acids (tails) “face toward each other” • Outer layer consists of glycolipids and glycoproteins • Glycolipids and glycoproteins form a glycocalyx coating • Inner layer does not consist of glycolipids or glycoproteins Cellular Anatomy • Structure of the Plasmalemma • Composed of protein molecules • Peripheral proteins: attached to the glycerol portions of the fatty acids • Integral proteins: embedded within the cell membrane • Form channels such as gated channels • Channels open and close Cellular Anatomy • Structure of the Plasmalemma • Composed of sterol molecules • Function to maintain fluidity of the membrane • An example is cholesterol Cellular Anatomy • Membrane Permeability of the Plasmalemma • Passive processes • Diffusion • Osmosis • Facilitative diffusion • Active processes • Active transport • Endocytosis • Exocytosis Cellular Anatomy • Membrane Permeability of the Plasmalemma • Passive process: diffusion • Movement of molecules from an area of high concentration to an area of low concentration • Permeablity, concentration gradient, molecule size and charge, temperature affect the rate of movement • Small inorganic ions and small molecules are involved Membrane Permeability: Active and Passive Processes (1 of 6) Diffusion Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. The difference between the high and low concentrations is a concentration gradient. In diffusion, molecules move down a concentration gradient until the gradient is eliminated. Factors Affecting Rate: Extracellular Membrane permeability; magnitude of thefluid concentration gradient; size, charge, and lipid solubility of the diffusing molecules; presence of membrane channel proteins; S temperature ubstances Involved (all cells): Gases, small inorganic ions and molecules, lipid-soluble materials Plasmalemma Example: CO2 When the concentration of CO2 inside a cell is greater than outsid the cell, CO2 diffuses out of the ce and into the extracellular fluid. Cellular Anatomy • Membrane Permeability of the Plasmalemma • Passive process: osmosis • Movement of water molecules from an area of high concentration of water to an area of low concentration of water • Permeability, concentration gradient, and opposing pressure affect the rate of movement • Only water molecules are involved Osmosis Osmosis is the diffusion of water molecules (rather than solutes) across a selectively permeable membrane. Note that water molecules diffusing toward an area of lower water concentration are moving toward an area of higher solute concentration. Because solute concentrations can easily be determined, they are used to determine the direction and force of osmotic water movement. Factors Affecting Rate: Size of the solute concentration gradient; opposing pressure Substances Involved: Water only Example: If the solute concentration outside a cell is greater than the inside the cell, water molecules will move across the plasmalemma into the extracellular fluid. Water Solute Cellular Anatomy • Membrane Permeability of the Plasmalemma • Passive process: facilitated diffusion • Solutes are passively transported by a carrier protein • Concentration gradient, size and charge of the solute, temperature, and number of carrier proteins affect the rate of movement • Glucose and amino acids are involved Membrane Permeability: Active and Passive Processes Facilitated diffusion In facilitated diffusion, solutes are passively transported across a plasmalemma by a carrier protein. As in simple diffusion, the direction of movement follows the concentration gradient. Factors Affecting Rate: Magnitude of the concentration gradient; size, charge, and solubility of the solutes; temperature; availability of carrier proteins Substances Involved (all cells): Glucose and amino acids Plasmalemma Glucose Example: Extracellular fluid Cytoplasm Receptor site Carrier protein Carrier protein releases glucose into cytoplasm Nutrients that are insolublei n lipids or too large to fit through membrane channels may be transported across the plasmalemma by carrier proteins. Many carrier proteins move a specific substance in one direction only, either into or out of the cell, after first binding the substance at a specific receptor site. Cellular Anatomy • Membrane Permeability of the Plasmalemma • Active process: active transport • Solutes are actively transported by a carrier protein regardless of the concentration gradient • ATP, number of carrier proteins affect the rate of movement • Na, K, Ca, and Mg ions are involved Membrane Permeability: Active and Passive Processes Extracellular fluid Active transport 3 Na+ Using active transport, carrier proteins can move specific substances across the plasmalemma despite an opposing concentration gradient. Carrier proteins that Sodium–potassium exchange pump move one solute in one direction and another solute in the opposite direction are called exchange pumps. Factors Affecting Rate: Availability of carrier proteins, solutes, and ATP Substances Involved: Na+, K+, Ca2+, Mg2+ (all cells); other solutes in special cases Cytoplasm 2 K+ ATP ADP Example: One of the most common examples of active transport is the Na–K exchange pump. For each molecule of ATP consumed, three sodium ions are ejected from the cell and two potassium ions are reclaimed from the extracellular fluid. Cellular Anatomy • Membrane Permeability of the Plasmalemma • Active process: endocytosis • Pinocytosis: vesicles bring small molecules into the cell • A variety of stimuli affect the rate of movement (not fully understood) • Extracellular fluid is involved • Phagocytosis: vesicles bring solid particles into the cell • Presence of extracellular pathogens affects the rate of movement • Bacteria, viruses, foreign matter, and cell debris are involved Figure 2.4 Membrane Permeability: Active and Passive Processes (5 of 6) Endocytosis Endocytosis is the packaging of extracellular materials into a vesicle (a membrane-bound sac) for importation into the cell. Pinocytosis In pinocytosis, vesicles form at the plasmalemma and bring extracellular fluid and small molecules into the cell. This process is often called “cell drinking.” Pinocytotic vesicle forming Example: Water and small molecules within a vesicle may enter the cytoplasm through carriermediated transport or diffusion. Cell Factors Affecting Rate: Stimulus and mechanism not understood Substances Involved: Extracellular fluid and its associated solutes Phagocytosis Receptor-mediated endocytosis In phagocytosis, vesicles form at the plasmalemma to bring solid particles into the cell. This process is often called “cell eating.” Factors Affecting Rate: Presence and abundance of extracellular pathogens or debris Substances Involved: Bacteria, viruses, cell debris, and other foreign material Example: Pseudopodium extends to surround object Cell Phagocytic vesicle Extracellular fluid Receptor proteins Cytoplasm Target molecules Vesicle containing target molecules In receptor-mediated endocytosis, target molecules bind to specific receptor proteins on the membrane surface, triggering vesicle formation. Example: Each cell has specific sensitivities to extracellular materials, depending on the kind of receptor proteins present in the plasmalemma. Large particles are brought into the cell when cytoplasmic extensions (called Factors Affecting Rate: pseudopodia) engulf Number of receptors on the the particle and form plasmalemma and the concentration of a phagocytic vesicle. target molecules (called ligands) Substances Involved (all cells): Many examples, including cholesterol and iron ions Cellular Anatomy • Membrane Permeability of the Plasmalemma • Active process: exocytosis • The release of intracellular material to the extracellular area • Requires ATP and calcium ions for movement • Fluid and cellular waste and secretory products are involved Material ejected from cell Exocytosis Exocytosis is the release of fluids and/or solids from cells when intracellular vesicles fuse with the plasmalemma. . Factors Affecting Rate: Stimulus and mechanism incompletely understood; requires ATP and calcium ions Substances Involved (all cells): Fluid and cellular wastes; secretory products are released by some cells Example: Cell Cellular wastes that accumulate in vesicles are ejected from the cell. Cellular Anatomy • Extensions of the Plasmalemma: Microvilli • • • • Fingerlike projections of the plasmalemma Absorb material from the ECF Increase the surface area of the plasmalemma Microvilli can bend back and forth in a waving manner • This movement helps to circulate extracellular fluid • This movement helps absorb nutrients Microvilli Secretory vesicles Cytosol Lysosome Centrosome Centriole Chromatin Nucleoplasm Nucleolus Nuclear envelope surrounding nucleus Cytoskeleton Plasmalemma Golgi apparatus Mitochondrion Peroxisome Nuclear pores Smooth endoplasmic reticulum Rough endoplasmic reticulum Fixed ribosomes Free ribosomes Cellular Anatomy • The Cytoplasm • Term for all of the intracellular material • Cytosol • Consists of the ICF (intracellular fluid) • Consists of nutrients, protein, and waste products • Organelles • These are intracellular structures that perform specific functions Cellular Anatomy • The Cytoplasm • Cytosol • Contains a higher concentration of potassium ions and a lower concentration of sodium ions as compared to the ECF • Consists of a net negative charge • Contains a high concentration of protein • Contains a small quantity of carbohydrates • Contains a large reserve of amino acids and lipids • Contains large amounts of inclusions Cellular Anatomy • The Cytoplasm • Organelles • Nonmembranous organelles • Cytoskeleton Flagella Centrioles Ribosomes • Membranous organelles • Mitochondria Golgi apparatus Nucleus Lysosomes Cilia Endoplasmic reticulum Peroxisomes Cellular Anatomy • Nonmembranous Organelles (details) • The cytoskeleton consists of: • • • • Microfilaments Intermediate filaments Thick filaments Microtubules Cellular Anatomy • Nonmembranous Organelles (details) • Microfilaments: consist of actin protein • • • • Anchor cytoskeleton to integral proteins Stabilize the position of membrane proteins Anchor plasmalemma to the cytoplasm Produce movement of the cell or a change in the cell’s shape Cellular Anatomy • Nonmembranous Organelles (details) • Intermediate filaments • Provide strength • Stabilize organelle position • Transport material within the cytosol Cellular Anatomy • Nonmembranous Organelles (details) • Thick filaments: composed of myosin protein • Found in muscle cells: involved in muscle contraction Cellular Anatomy • Nonmembranous Organelles (details) • Microtubules: composed of tubulin protein • Involved in the formation of centrioles • perform a function during cell reproduction • Involved in moving duplicated chromosomes to opposite poles of the cell • perform a function during cell reproduction • • • • Involved in anchoring organelles Involved in moving cell organelles Involved in moving the entire cell Involved in moving material across the surface of the cell The Cytoskeleton Microvilli Microfilaments Anchor plasmalemma to the cytoplasm SEM × 30,000 Plasmalemma b Terminal web A SEM image of the microfilaments and microvil of an intestinal cell. Mitochondrion Intermediate filaments a The cytoskeleton provides strength and structural support for the cell and its organelles. Interactions between cytoskeletal elements are also important in moving organelles and in changing the shape of the cell. Endoplasmic reticulum Microtubule Secretory vesicle LM × 3200 c Microtubules in a living cell, as seen after fluorescent labeling. Cellular Anatomy • Nonmembranous Organelles (details) • Examples of microtubules • Centrioles • Cilia • Flagella Table 2.2 A Comparison of Centrioles, Cilia, and Flagella Centrioles and Cilia Microtubules a A centriole consists of nine microtubule triplets (9 + 0 array). The centrosome contains a pair of centrioles oriented at right angles to one another. Plasmalemma Microtubules Basal body b A cilium contains nine pairs of microtubules surrounding a central pair (9 + 2 array). Power stroke TEM × 240,000 c Return stroke A single cilium swings forward and then returns to its original position. During the power stroke, the cilium is relatively stiff, but during the return stroke, it bends and moves parallel to the cell surface. Cellular Anatomy • Nonmembranous Organelles (details) • Ribosomes • Free ribosomes: float in the cytoplasm • Fixed ribosomes: attached to the endoplasmic reticulum • Both are involved in producing protein Figure 2.1 Anatomy of a Typical Cell Microvilli Secretory vesicles Cytosol Lysosome Centrosome Centriole Chromatin Nucleoplasm Nucleolus Nuclear envelope surrounding nucleus Cytoskeleton Plasmalemma Golgi apparatus Mitochondrion Peroxisome Nuclear pores Smooth endoplasmic reticulum Rough endoplasmic reticulum Fixed ribosomes Free ribosomes Cellular Anatomy • Membranous Organelles (details) • Double-membraned organelles • • • • • • Mitochondria: produce ATP Nucleus: contains chromosomes Endoplasmic reticulum: network of hollow tubes Golgi apparatus: modifies protein Lysosomes: contain cellular digestive enzymes Peroxisomes: contain catalase to break down hydrogen peroxide Cellular Anatomy • Membranous Organelles (details) • Mitochondria • Consist of cristae • Consist of mitochondrial matrix • Produce ATP Figure 2.8 Mitochondria Inner membrane Cytoplasm of cell Cristae Matrix Organic molecules and O2 Outer membrane CO2 ATP Matrix Cristae Enzymes TEM × 61,776 Cellular Anatomy • Membranous Organelles (details) • Nucleus: control center of the cell • • • • • Nucleoplasm Nuclear envelope Perinuclear space Nuclear pores Nuclear matrix Figure 2.9ab The Nucleus Perinuclear space Nucleoplasm Chromatin Nucleolus Nuclear envelope Nuclear pores TEM × 4828 a Nuclear envelope Perinuclear space Nuclear pore b A nuclear pore and the perinuclear space. TEM showing important nuclear structures. Figure 2.9c The Nucleus Inner membrane of nuclear envelope Broken edge of outer membrane Outer membrane of nuclear envelope SEM × 9240 c The cell seen in this SEM was frozen and then broken apart so that internal structures could be seen. This technique, called freeze-fracture, provides a unique perspective on the internal organization of cells. The nuclear envelope and nuclear pores are visible; the fracturing process broke away part of the outer membrane of the nuclear envelope, and the cut edge of the nucleus can be seen. Cellular Anatomy • Membranous Organelles: Nucleus • Chromosomes: • DNA wrapped around proteins called histones • Nucleosomes • Chromatin Chromatin and chromosome structure.>>>>>>>>>>>>>>> Threadlike strands of DNA (30%), histone proteins 1 DNA (60%), and RNA (10%) double Arranged in fundamental helix (2-nm diameter) units called nucleosomes Histones pack long DNA Histones molecules; involved in 2 Chromatin (“beads on a string”) gene regulation structure with Condense into barlike nucleosomes bodies called chromosomes when cell Linker DNA starts to divide Nucleosome (10-nm diameter; eight histone proteins wrapped by two winds of the DNA double helix) 3 Tight helical fiber (30-nm diameter) 4 Looped domain structure (300-nm 5 Chromatid diameter) (700-nm diameter) . 6 Metaphase chromosome (at midpoint of cell division) consists of two sister chromatids Chromosome Structure Histones Nucleosome Chromatin in nucleus Nucleus of nondividing cell Loosely coiled nucleosomes, forming chromatin. a In cells that are not dividing, the DNA is loosely coiled, forming a tangled network known as chromatin. DNA double helix Sister chromatids Centromere Kinetochore Supercoiled region Dividing cell Visible chromosome b When the coiling becomes tighter, as it does in preparation for cell division, the DNA becomes visible as distinct structures called chromosomes. Chromosomes are composed of two sister chromatids which attach at a single point, the centromere. Kinetochores are the region of the centromere where spindle fibers attach during mitosis. Cellular Anatomy • Membranous Organelles (details) • Endoplasmic reticulum (ER) • There are two types • Rough endoplasmic reticulum (RER) • Smooth endoplasmic reticulum (SER) Cellular Anatomy • Membranous Organelles (details) • Rough endoplasmic reticulum • Consists of fixed ribosomes • Proteins enter the ER Figure 2.11 The Endoplasmic Reticulum Ribosomes Rough endoplasmic reticulum with fixed (attached) ribosomes Free ribosomes Smooth endoplasmic reticulum Endoplasmic Reticulum Cisternae TEM × 11,000 Ribosomes Nucleus Free ribosomes Small ribosomal subunit Endoplasmic reticulum with attached fixed ribosomes TEM × 73,600 a Both free and fixed ribosomes can be seen in the cytoplasm of this cell. Large ribosomal subunit b An individual ribosome, consisting of small and large subunits. Cellular Anatomy • Membranous Organelles (details) • Smooth endoplasmic reticulum • Synthesizes lipids, steroids, and carbohydrates • Storage of calcium ions • Detoxification of toxins Cellular Anatomy • Membranous Organelles (details) • Golgi apparatus • Synthesis and packaging of secretions • Packaging of enzymes (modifies protein) • Renewal and modification of the plasmalemma Figure 2.12 TEM of the Golgi Apparatus Vesicles Maturing (trans) face Forming (cis) face Golgi apparatus TEM × 83,520 Cellular Anatomy • Membranous Organelles (details) • Lysosomes • Fuse with phagosomes to digest solid materials • Recycle damaged organelles • Sometimes rupture, thus killing the entire cell (called autolysis) Cellular Anatomy • Membranous Organelles (details) • Peroxisomes • Consist of catalases & oxidases • Abundant in liver cells • Catalases convert hydrogen peroxide to water and oxidants(detoxification) Cellular Anatomy • Membrane Flow • This is the continuous movement and recycling of the cell membrane • Transport vesicles connect the endoplasmic reticulum with the Golgi apparatus • Secretory vesicles connect the Golgi apparatus with the plasmalemma • Vesicles remove and recycle segments of the plasmalemma Functions of the Golgi Apparatus (1 of 3) Cisterna Forming (cis) face Synthesis and Packaging of Secretions: Steps Golgi Apparatus Cytoplasm Transport vesicle 2 1 Rough ER Endoplasmic Reticulum mRNA Ribosome Secretory products are packaged into transport vesicles that eventually bud off from the ER. These transport vesicles then fuse to create the forming (cis) face of the Golgi apparatus. Protein and glycoprotein synthesis occurs in the rough endoplasmic reticulum (RER). Some of these proteins and glycoproteins remain within the ER. Functions of the Golgi Apparatus (2 of 3) Plasmalemma Secretory material Packaging of Enzymes for Use in the Cytosol Renewal or Modification of the Plasmalemma Synthesis and Packaging of Secretions Exocytosis at the surface of a cell Plasmalemma Maturing (trans) face Secretory vesicle Synthesis and Packaging of Secretions: Steps 4 3 Cytoplasm TEM × 75,000 Cytoplasm Lysosome Cisterna Secretory vesicle Forming (cis) face Golgi Apparatus The maturing (trans) face generates vesicles that carry materials away from the Golgi apparatus. Each cisterna physically moves from the forming face to the maturing face, carrying with it its associated proteins. This process is called cisternal progression. Intercellular Attachment • Many cells form permanent or temporary attachment to other cells • Attach via cell adhesion molecules (CAMs) • Attach via cellular cement (proteoglycans) • Examples of Intercellular Attachment • Communicating junctions (gap junctions) • Adhering junctions • Tight junctions • Anchoring junctions Intercellular Attachment • Communicating Junctions • Also called gap junctions • Two cells held together via protein called connexon • This protein is a type of channel protein • Attach via cell adhesion molecules (CAMs) • Attach via cellular cement (proteoglycans) Cell Attachments Tight junction Embedded proteins (connexons) Zonula adherens Terminal web Button desmosome Communicating junction b Communicating junctions permit the free diffusion of ions and small molecules between two cells. Hemidesmosome a A diagrammatic view of an epithelial cell showing the major types of intercellular connections. Intercellular Attachment • Adhering Junctions • Tight junctions, also called occluding junctions • Prevent the movement of water and other molecules from passing between the cells Intercellular Attachment • Anchoring Junctions • Zona adherens (adhesion belt) is a sheetlike anchoring material • Provides strong links that cells can shed from the body in sheets (ex. dandruff) • Macula adherens (desmosome) is a small, localized anchoring junction • Most abundant in superficial layers of the skin Cell Attachments Tight junction Interlocking junctional proteins Tight junction Zonula adherens (Anchoring Junction) Terminal web Button desmosome Communicating junction(Gap Junc) Hemidesmosome Zonula adherens c A tight junction is formed by the a A diagrammatic view of an epithelial cell showing the major types of intercellular connections. fusion of the outer layers of two plasmalemmae. Tight junctions prevent the diffusion of fluids and solutes between the cells. Cell Attachments Tight junction Zonula adherens Terminal web Button desmosome Communicating junction Hemidesmosome (Anchoring junc. found in areas with A lot of abrasions(cornea, vagina,esophagus,skin…) a A diagrammatic view of an epithelial cell showing the major types of intercellular connections. d Anchoring junctions attach one cell to another. A macula adherens has a more organized network of intermediate filaments. An adhesion belt is a form of anchoring junction that encircles the cell. This complex is tied to the microfilaments of the terminal web. Involved in wound repair Intermediate filaments (cytokeratin) Cell adhesion molecules (CAMs) Dense area Intercellular cement Intercellular Attachment • Anchoring junctions • Focal adhesions (focal contacts) • Connect intracellular microfilaments to protein fibers • Found in epithelial tissue that migrates during wound repair • Hemidesmosomes • Found in connecting cells that are exposed to a lot of abrasion • Examples are the cornea of the eye, skin, vaginal tissue, oral cavity, and esophagus Cell Attachments Tight junction Zonula adherens Terminal web Button desmosome Communicating junction Hemidesmosome a A diagrammatic view of an epithelial cell showing the major types of intercellular connections. Clear layer Basal Dense lamina layer e Hemidesmosomes attach an epithelial cell to extracellular structures, such as the protein fibers in the basal lamina. Examples of hemidesmosomes are the cornea of the eye, skin, vaginal tissue, oral cavity, and esophagus The Cell Life Cycle • Cell reproduction consists of special events • Interphase • Mitosis • • • • Prophase Metaphase Anaphase Telophase • Cytokinesis • Overlaps with anaphase and telophase The Cell Life Cycle • Cell Reproduction (Interphase) • Everything inside the cell is duplicating • Consists of G1, S, and G2 phases • G1: duplication of organelles and protein synthesis • S: Chromosome replication and DNA synthesis and histone synthesis • G2: protein synthesis The Cell Life Cycle INTERPHASE G1 Normal cell functions plus cell growth, duplication of organelles, protein synthesis Indefinite period G0 Specialized cell functions S DNA replication, synthesis of histones G2 Protein synthesis THE CELL CYCLE M MITOSIS AND CYTOKINESIS (See Figure 2.17) DNA Replication DNA polymerase Segment 2 KEY Adenine Guanine Cytosine Thymine DNA nucleotide Segment 1 DNA polymerase Each strand acts as template for complementary Replication of DNA: strand Leading strand synthesized continuously Lagging strand synthesized discontinuously into Free nucleotides DNA polymerase Old (parental) strand acts as a segments template for synthesis of new Chromosome DNA ligase splices short segments of discontinuous strand strand together Leading strand Two new strands (leading and lagging) synthesized in opposite directions Old DNA Enzymes unwind Replication the double helix and fork expose the bases Lagging strand Replication bubble RNA polymerase separates DNA strands Adenine Thymine Cytosine Guanine DNA polymerase Old (template) strand DNA polymerase begins adding nucleotides at RNA primer and it works only in one direction. DNA polymerase continues from primer Synthesizes one leading, one lagging strand The Cell Life Cycle • Cell Reproduction (Mitosis) • Prophase • The first phase of mitosis • Metaphase • Paired chromatids line up in the middle of the nuclear region • Anaphase • Paired chromatids separate to opposite poles of the cell • Telophase • Two new nuclear membranes begin to form Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (2 of 6) Early Prophase Early mitotic spindle Aster Chromosome consisting of two with sister chromatids Chromosomes become visible, each two chromatids joined at centromere Centrosomes separate and migrate toward opposite poles © 2013 Pearson Education, Inc. Mitotic spindles and asters form Centromere Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (3 of 6) Late Prophase . Spindle pole Polar microtubule Fragments of nuclear envelope Kinetochore Kinetochore microtubule Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (4 of 6) Metaphase Spindle Metaphase plate Centromeres of chromosomes aligned at equator Plane midway between poles called metaphase plate Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (5 of 6) Anaphase Daughter chromosomes Shortest phase Centromeres of chromosomes split simultaneously—each chromatid becomes a chromosome Chromosomes (V shaped) pulled toward poles by motor proteins of kinetochores Polar microtubules continue forcing poles apart Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (6 of 6) Telophase Cytokinesis Nuclear envelope forming Begins when chromosome movement stops Two sets of chromosomes uncoil to form chromatin New nuclear membrane forms around each chromatin mass Nucleoli reappear Spindle disappears . Nucleolus forming Contractile ring at cleavage furrow CYTOKINESIS: Begins during late anaphase Ring of actin microfilaments contracts to form cleavage furrow Two daughter cells pinched apart, each containing nucleus identical to original Mitosis Interphase Prophase Early prophase Nuclear membrane Metaphase Anaphase Telophase Cytokinesis Late prophase Chromosomal microtubules Centromere Nucleus Daughter cells Spindle fibers Centrioles (two pairs) Chromosome with two sister chromatids Astral rays Chromosomal microtubules Metaphase plate Daughter chromosomes Cleavage furrow The Cell Life Cycle • Cell Reproduction (Cytokinesis) • Cell membrane begins to invaginate, thus forming two new cells • Many times this phase actually begins during anaphase • This is the conclusion of cell reproduction
