Human Anatomy and Physiology: Cellular Level of Organization (University of Northern Philippines)

University of Northern Philippines College of Arts and Sciences Bachelor of Science in Biology major in Medical Biology HUMAN ANATOMY AND PHYSIOLOGY Cellular Level of Organization Ch...

University of Northern Philippines College of Arts and Sciences Bachelor of Science in Biology major in Medical Biology HUMAN ANATOMY AND PHYSIOLOGY Cellular Level of Organization Chris Paul P. Pagaoa, MSBiol, LPT Instructor, Human Anatomy & Physiology Lecture  OUTLINE OF DISCUSSION II. Cellular Level of Organization 1. Introduction to Cells 2. Smallest Living Units of Life 3. Plasma Membrane 4. Structures and Function of the Nucleus 5. How Substances Enter and Leave the Cell 6. Cell Life Cycle  An Introduction to Cells  Typical Cell Smallest living unit in the body ~0.1 mm in diameter Could not be examined until invention of microscope in 17th century  An Introduction to Cells  Cell theory An Introduction to Cells SMALLEST LIVING UNITS OF LIFE Cell components – Plasma membrane (cell membrane) Separates cell contents from extracellular fluid – Cytoplasm Material between cell membrane and nuclear membrane Colloid containing many proteins Two subdivisions 1. Cytosol » Intracellular fluid 2. Organelles (“little organs”) » Intracellular structures with specific functions SMALLEST LIVING UNITS OF LIFE Organelles – Nonmembranous Not completely enclosed by membranes In direct contact with cytosol Examples: – Cytoskeleton – Microvilli – Centrioles – Cilia – Ribosomes  SMALLEST LIVING UNITS OF LIFE Organelles – Membranous Enclosed in a phospholipid membrane Isolated from cytosol Examples: – Mitochondria – Nucleus – Endoplasmic reticulum – Golgi apparatus – Lysosomes – Peroxisomes  SMALLEST LIVING UNITS OF LIFE Organelles – Microvilli STRUCTURE: membrane extensions containing microfilaments FUNCTION: increase surface area for absorption – Cytoskeleton STRUCTURE: fine protein filaments or tubes – Centrosome » Organizing center containing pair of centrioles FUNCTION: – Strength and support – Intracellular movement of structures and materials SMALLEST LIVING UNITS OF LIFE Organelles – Ribosomes STRUCTURE: RNA and proteins – Fixed: attached to endoplasmic reticulum – Free: scattered in cytoplasm SMALLEST LIVING UNITS OF LIFE Organelles – Peroxisome STRUCTURE: vesicles containing degradative enzymes FUNCTION: – Catabolism of fats/other organic compounds – Neutralization of toxic compounds – Lysosome STRUCTURE: vesicles containing digestive enzymes FUNCTION: – Removal of damaged organelles or pathogens SMALLEST LIVING UNITS OF LIFE Organelles – Golgi apparatus STRUCTURE: stacks of flattened membranes (cisternae) containing chambers FUNCTION: storage, alteration, and packaging of synthesized products – Mitochondria STRUCTURE: – Double membrane – Inner membrane contains metabolic enzymes FUNCTION: production of 95% of cellular ATP  SMALLEST LIVING UNITS OF LIFE Organelles – Nucleus STRUCTURE: – Fluid nucleoplasm containing enzymes, proteins, DNA, and nucleotides – Surrounded by double membrane FUNCTION: – Control of metabolism – Storage/processing of genetic information – Control of protein synthesis  SMALLEST LIVING UNITS OF LIFE Organelles – Endoplasmic reticulum (ER) STRUCTURE: membranous sheets and channels FUNCTION: synthesis of secretory products, storage, and transport – Smooth ER » No attached ribosomes » Synthesizes lipids and carbohydrates – Rough ER » Attached ribosomes » Modifies/packages newly synthesized proteins  PLASMA MEMBRANE Plasma membrane – Selectively permeable membrane that controls: Entry of ions and nutrients Elimination of wastes Release of secretions Structure of the plasma membrane EXTRACELLULAR FLUID Glycocalyx (extracellular Integral protein carbohydrates) with channel Glycolipid Integral glycoproteins = 2 nm CYTOPLASM Integral (transmembrane) proteins Peripheral proteins Cytoskeleton (microfilaments)  PLASMA MEMBRANE Plasma membrane components 1. Glycocalyx  Superficial membrane carbohydrates Components of complex molecules – Proteoglycans (carbohydrates with protein attached) – Glycoproteins (protein with carbohydrates attached) – Glycolipids (lipids with carbohydrates attached) Functions – Cell recognition – Binding to extracellular structures – Lubrication of cell surface  PLASMA MEMBRANE Plasma membrane components (continued) 2. Integral proteins Part of cell membrane Cannot be removed without damaging cell Often span entire cell membrane – = Transmembrane proteins Can transport water or solutes 3. Peripheral proteins Attached to cell membrane surface Removable Fewer than integral proteins Regulatory or enzymatic functions  PLASMA MEMBRANE Plasma membrane structure – Thin (6–10 nm) and delicate – Phospholipid bilayer Mostly comprised of phospholipid molecules in two layers – Hydrophilic heads at membrane surface – Hydrophobic tails on the inside Isolates cytoplasm from extracellular fluid The phospholipid bilayer that forms the plasma membrane Hydrophilic heads Hydrophobic tails Cholesterol  PLASMA MEMBRANE Plasma membrane functions – Physical isolation – Regulation of exchange with external environment – Sensitivity to environment – Structural support – Lipid bilayer provides isolation – Proteins perform most other functions  CYTOSKELETON Cytoskeleton provides an internal protein framework that gives the cytoplasm strength and flexibility  Components: 1. Microfilaments 2. Intermediate filaments 3. Microtubules  Structures of the cytoskeleton Microvilli Microfilaments Plasma membrane Terminal web Microvilli SEM X 30,000 Intermediate filaments Microtubule Secretory vesicle Mitochondrion Endoplasmic reticulum  CYTOSKELETON Cytoskeleton (cellular framework) components 1. Microfilaments 100,000 different proteins Coded in sequence of nucleotides Determines cell structure/function – Usually only one per cell Exceptions: – Multiple: skeletal muscle cell – None: mature red blood cells  NUCLEUS The nucleus directs cellular responses to environmental (ECF) changes – Short-term adjustments Enzyme activity changes – Long-term adjustments Changes in enzymes produced Changes in cell structure from changes in structural proteins Often occur as part of growth, development, and aging  EXTRACELLULAR FLUID (ECF) The role of the nucleus in preserving Plasma membrane homeostasis at the cellular level Binding to SHORT-TERM Changes membrane ADJUSTMENTS in the receptors composition Enzyme of the Diffusion activation or ECF through inactivation membrane channels LONG-TERM ADJUSTMENTS Changes in the bio- chemical processes under way in the cell resulting from the Binding to nuclear synthesis of additional receptors that alter enzymes, fewer genetic activity enzymes, or different enzymes Changes in the DNA in physical structure of nucleus the cell due to altera- tions in the rates or types of structural proteins synthesized CYTOPLASM  NUCLEAR STRUCTURE Nuclear structures and functions – Nuclear envelope Separates nucleus from cytoplasm Double membrane – Perinuclear space (peri-, around) » Space between layers Nuclear pores – Allow passage of small molecules and ions  NUCLEAR STRUCTURE Nuclear structures and functions (continued) – Nucleoplasm Fluid contents of nucleus – Fine filaments (Nuclear matrix) – Ions – Enzymes – RNA and DNA nucleotides – Small amounts of RNA – DNA  NUCLEAR STRUCTURE Nuclear structures and functions (continued) – Nucleoli (singular, nucleolus) Transient, clear nuclear organelles Composed of: – rRNA – Enzymes – Proteins (histones) Form around DNA instructions for forming proteins/RNA Assemble RNA subunits Many found in large, protein-producing cells – Liver – Nerve – Muscle  NUCLEAR STRUCTURE DNA – Instructions for protein synthesis – Strands coiled Wrap around histone molecules forming nucleosomes Loosely coiled (chromatin) in nondividing cells Tightly coiled (chromosomes) in dividing cells – To begin, two copies of each chromosome held together at centromere – 23 paired chromosomes in somatic (general body) cells » One each from mother/father – Carry instructions for proteins and RNA – Also some regulatory and unknown functions The coiled structure of DNA in the nucleus of a nondividing cell Chromatin Nucelosome Histones DNA double Nucleus of nondividing cell helix The tighter coiling of DNA to form chromosomes in dividing cells Centromere Supercoiled region Dividing cell Visible chromosome  PROTEIN SYNTHESIS DNA – Long parallel chains of nucleotides – Two adjacent DNA chains held by hydrogen bonds – Four nitrogenous bases 1. Adenine (A) 2. Thymine (T) 3. Cytosine (C) 4. Guanine (G) – Genetic code (sequence of nucleotides) Triplet code (three nucleotides specify single amino acid)  PROTEIN SYNTHESIS DNA (continued) – Gene Functional unit of heredity All the DNA nucleotides needed to produce a specific protein Size varies (~3003000 nucleotides)  PROTEIN SYNTHESIS Gene activation – Removal of histones and DNA uncoiling – Messenger RNA (mRNA) Assembled by enzymes Connecting complementary RNA nucleotides – (A, G, C, U) Contains information in triplets (codons) Leaves nucleus through pores – Transfer RNA (tRNA) – Contains triplets (anticodons) that bind to mRNA codons Each type carries a specific amino acid linked to form a polypeptide The key events of protein synthesis Uncoiling of the portion of DNA molecule containing an activated gene DNA triplets are exposed to the nucleoplasms Paired DNA strands Enzyme Assembly of an mRNA strand by enzymes The mRNA strand containing the complementary codons passes through a nuclear pore and enters the cytoplasm. Codon on mRNA Binding of transfer RNA (tRNA) molecules carrying a specific amino acid Amino acid tRNA attaches to mRNA Anticodon mRNA strand Codon Linking of amino acids to form a polypeptide Polypeptide methionine-proline-serine-leucine A summary of how DNA codes for a protein The DNA The mRNA The sequence of tRNAs triplets codons determines the determine the determine sequence of amino sequence of the sequence acids in the polypeptide mRNA codons. of tRNAs. or protein.  TRANSCRIPTION Transcription (“to copy” or “rewrite”) – Production of RNA from DNA template – All three types of RNA are formed – Example: mRNA (information for synthesizing proteins)  TRANSCRIPTION Steps of transcription 1. Gene activation Occurs at control segment (1st segment of gene) Template strand (One DNA strand used to synthesize RNA) 2. RNA polymerase (enzyme) Binds to promoter Assembles mRNA strand – Complementary to DNA » Example: (DNA triplet TAC = mRNA AUG) – Hydrogen bonds between nucleotides Events in the process of transcription of mRNA The template The enzyme strand is the RNA polymerase DNA strand that binds to the will be used to exposed promoter synthesize RNA. and, using the triplets as a guide, The segment assembles a at the start of the strand of mRNA. gene is known as the control segment. Triplet 1 1 Complementary Gene triplets Triplet 2 2 2 Triplet 3 3 Triplet 4 4 Gene activation, which results in temporary disruption of the hydrogen bonds between the nitrogenous bases of the two DNA strands Adenine DNA Guanine Cytosine Uracil (RNA) Thymine (DNA) Figure 3.9 1  TRANSCRIPTION Steps of transcription (continued) 3. Transcription ends Stop codon reached mRNA detaches Complementary DNA strands reassociate (hydrogen bonding between complementary base pairs)  Immature mRNA Events in the process of transcription of mRNA (continued) RNA polymerase works only Introns on RNA nucleotides—it can removed attach adenine, guanine, cytosine, or uracil, but never thymine. If the DNA triplet is Exons spliced together TAC, the corresponding to from mature mRNA mRNA codon will be AUG. The production of functional mRNA from immature mRNA mRNA strand Codon 1 Codon 2 Codon 1 Codon 3 Codon 4 (stop codon) RNA nucleotide RNA RNA polymerase polymerase Hydrogen bonding between Conclusion of transcription the nitrogenous bases of when stop codon is reached the template strand and Adenine complementary nucleotides Guanine in the nucleoplasm Cytosine Uracil (RNA) Thymine (DNA) Figure 3.9 2 – 3  TRANSCRIPTION Immature RNA – Contains triplets not needed for protein synthesis – “Edited” before leaving nucleus through pores Introns (removed nonsense regions) Exons (remaining coding segments) Creates shorter, functional mRNA Changing “edits” can produce mRNAs for different proteins  Immature mRNA Introns removed Exons spliced together to form mature mRNA The production of functional mRNA from immature mRNA  TRANSLATION Translation (translate nucleic acids to proteins) – Uses mRNA created in nucleus Leaves via nuclear pores – Occurs in cytoplasm  TRANSLATION Steps of translation 1. mRNA binds to small ribosomal subunit Binding between mRNA and tRNA – mRNA codons with tRNA anticodons 2. Small and large ribosomal subunits assemble around mRNA strand Additional tRNAs arrive – More than 20 kinds » At least one for each amino acid The process of translation NUCLEUS Amino acid tRNA Anticodon tRNA binding sites Entry of mRNA into cytoplasm Start codon mRNA strand Small Large ribosomal ribosomal subunit subunit Binding of mRNA strand to a small Joining of small and large Adenine ribosomal subunit and arrival of the ribosomal subunits around the first tRNA mRNA strand and arrival of Guanine additional tRNAs Cytosine Uracil Figure 3.10 1 – 2  TRANSLATION Steps of translation (continued) 3. Ribosome attaches to next complementary tRNA 4. Ribosome links amino acids forming dipeptide More tRNAs arrive and continue forming polypeptide 5. Stops once stop codon is reached on mRNA Ribosomal subunits detach – Leaves intact mRNA and new polypeptide  The process of translation (continued) Small ribosomal subunit Peptide bond Large ribosomal subunit Completed polypeptide Stop codon Attachment of tRNA with Formation of a depeptide, Completion of polypeptide and anticodon that is complementary release of first tRNA, and detachment of ribosomal to codon on RNA strand arrival of another tRNA subunits Adenine Guanine Cytosine Uracil  TRANSLATION Translation – Produces a typical protein in ~20 seconds – mRNA can interact with other ribosomes and produce more proteins – Multiple ribosomes can attached to a single mRNA strand to quickly produce many proteins  MEMBRANE TRANSPORT Plasma membrane – Acts as a barrier separating cytosol and ECF – Must still coordinate cellular activity with extracellular environment Permeability (determines which substances can cross membrane) – Freely permeable (any substances) – Selectively permeable (some substances cross) – Impermeable (none can pass) » No living cell is impermeable Permeability characteristics of membranes Freely permeable membranes Selectively permeable membranes Impermeable membranes Ions Carbohydrates Ions Carbohydrates Ions Carbohydrates Protein — Protein — Protein — Water Water Water Lipids Lipids Lipids Freely permeable membranes Selectively permeable membranes, Nothing can pass through impermeable allow any substance to pass without such as plasma membranes, permit the membranes. Cells may be impermeable difficulty. passage of some materials and prevent to specific substances, but no living cell the passage of others. has an impermeable membrane.  MEMBRANE TRANSPORT Selectively permeable membranes – Selective based on: 1. Characteristics of material to pass – Size – Electrical charge – Molecular shape – Lipid solubility – Other factors 2. Characteristics of membrane – What lipids and proteins present – How components are arranged  MEMBRANE TRANSPORT Selectively permeable membranes – Types of membrane transport 1. Passive (do not require ATP) – Diffusion – Carrier-mediated transport 2. Active (require ATP) – Vesicular transport – Carrier-mediated transport Characteristics of selectively permeable membranes EXTRACELLULAR Materials may cross FLUID the plasma membrane through active or passive mechanisms. Plasma Passive mechanisms Active mechanisms membrane do not require ATP. require ATP. Diffusion is Carrier-mediated Vesicular transport movement driven transport involves involves the by concentration carrier proteins, and formation of differences. the movement may intracellular be passive or active. vesicles; this is an active process. CYTOPLASM Figure 3 Section 3 2  DIFFUSION Diffusion – Continuous random movement of ions or molecules in a liquid or gas resulting in even distribution Gradient – Concentration difference or when molecules are not evenly distributed – At an even distribution, molecular motion continues but no net movement – Slow in air and water but important over small distances  DIFFUSION In ECF – Water and solutes diffuse freely Across plasma membrane – Selectively restricted diffusion Movement across lipid portion of membrane – Examples: lipids, lipid-soluble molecules, soluble gases Movement through membrane channel – Examples: water, small water-soluble molecules, ions Movement using carrier molecules – Example: large molecules The effects of the plasma membrane, a selectively permeable membrane, on the diffusion of various substances Lipids, lipid-soluble molecules, Water, small water-soluble molecules, EXTRACELLULAR FLUID and soluble gases (O2 and CO2) and ions diffuse through membrane channels. can diffuse across the lipid bilayer of the plasma membrane. Channel Plasma membrane protein Large molecules that cannot fit through the membrane channels and cannot diffuse through the membrane lipids can only cross the plasma membrane when transported by a carrier mechanism. CYTOPLASM  DIFFUSION Factors that influence diffusion rates: – Distance (inversely related) – Molecule size (inversely related) – Temperature (directly related) – Gradient size (directly related) – Electrical forces Attraction of opposite charges (+,–) Repulsion of like charges (+,+ or –,–)  OSMOSIS Osmosis (osmos, a push) – Net diffusion of water across a membrane – Maintains similar overall solute concentrations between the cytosol and extracellular fluid – Osmotic flow Movement of water driven by osmosis – Osmotic pressure Indication of force of pure water moving into a solution with higher solute concentration Hydrostatic pressure – Fluid force – Can be estimate of osmotic pressure when applied to stop osmotic flow Osmotic flow, the movement of water driven by osmosis Volume Applied increased force Volume Original Volumes decreased level equal Water molecules Solute molecules Selectively permeable membrane A selectively permeable membrane At equilibrium, the solute Pushing against a fluid generates separates these two solutions, concentrations on the two hydrostatic pressure. The which have different solute sides of the membrane are osmotic pressure of solution B concentrations. Water molecules equal. Note that the volume is equal to the amount of (small blue dots) begin to cross of solution B has increased hydrostatic pressure, indicated the membrane toward solution B, at the expense of that of by the weight, required to stop the solution with the higher solution A. the osmotic flow. concentration of solutes (larger pink circles). Figure 3.12 1  OSMOSIS Osmolarity (osmotic concentration) – Total solute concentration in an aqueous solution Tonicity – Effect of osmotic solutions on cell volume – Three effects 1. Isotonic (iso-, same + tonos, tension) – Solution that does not cause osmotic flow across membrane  OSMOSIS Tonicity – Three effects (continued) 2. Hypotonic – Causes osmotic flow into cell – Example: hemolysis (hemo-, blood + lysis, loosening) 3. Hypertonic – Causes osmotic flow out of cell – Example: crenation of RBCs  OSMOSIS Importance of tonicity vs. osmolarity: Example – Administering large fluid volumes to patients with blood loss or dehydration Administered solution has same osmolarity as ICF but higher concentrations of individual ions/molecules – Diffusion of solutes may occur across cell membrane – Water will follow through osmosis – Cell volume increases – Normal saline 0.9 percent or 0.9 g/dL of NaCl Isotonic with blood CARRIER-MEDIATED TRANSPORT Carrier-mediated transport – Hydrophilic or large molecules transported across cell membrane by carrier proteins – Many move specific molecules through the plasma membrane in only one direction Cotransport (>1 substance same direction) Countertransport (2 substances in opposite directions) – Carrier called exchange pump  CARRIER-MEDIATED TRANSPORT Carrier-mediated transport – Three types 1. Facilitated diffusion – Requires no ATP (= passive) – Movement limited by number of available carrier proteins (= can become saturated) 2. Active transport – Requires energy molecule or ATP (= active) – Independent of concentration gradient – Examples: » Ion pumps (Na+, K+, Ca2+, and Mg2+) » Sodium–potassium ATPase Facilitated diffusion EXTRACELLULAR Glucose FLUID molecule Receptor site Glucose released Carrier into cytoplasm protein The shape of the protein then changes, moving the molecule across the plasma membrane. The CYTOPLASM carrier protein then releases the transported molecule into the cytoplasm. Note that this was Facilitated diffusion begins when a specific accomplished without ever creating a continuous molecule, such as glucose, binds to a receptor open channel between the extracellular fluid and site on the integral protein. the cytoplasm. Figure 3.13 1 Active transport Sodium ion concentrations are high in the extracellular fluids, EXTRACELLULAR but low in the cytoplasm. The FLUID distribution of potassium in the body is just the opposite: low in the extracellular fluids and high in the cytoplasm. As a result, Sodium– sodium ions slowly diffuse into potassium the cell, and potassium ions exchange diffuse out through leak pump channels. Homeostasis within the cell depends on the ejection of sodium ions and the recapture of lost potassium ions. The sodium–potassium exchange pump is a carrier protein called sodium–potassium ATPase. It exchanges intracellular sodium CYTOPLASM for extracellular potassium. On average, for each ATP molecule consumed, three sodium ions are ejected and the cell reclaims two potassium ions. The energy demands are impressive: Sodium- potassium ATPase may use up to 40 percent of the ATP produced by a resting cell! Figure 3.13 2 CARRIER-MEDIATED TRANSPORT Carrier-mediated transport (continued) – Three types (continued) 3. Secondary active transport – Transport mechanism does not require ATP – Cell often needs ATP to maintain homeostasis associated with transport Secondary active transport Glucose Sodium + molecule ion Na+–K+ pump To preserve homeostasis, the cell must then expend ATP to pump the arriving sodium ions out of the cell by using the sodium–potassium exchange pump. It thus “costs” the cell one ATP for every three CYTOPLASM glucose molecules it + transports into the cell. The carrier protein then changes shape, opening a path to the cytoplasm and A sodium ion and a glucose releasing the transported materials. It molecule bind to receptor then reassumes its original shape and is sites on the carrier protein. ready to repeat the process. Figure 3.13 3  VESICULAR TRANSPORT Vesicular transport – Materials move across cell membrane in small membranous sacs Sacs form at or fuse with plasma membrane – Two major types (both require ATP) 1. Endocytosis 2. Exocytosis  VESICULAR TRANSPORT Vesicular transport (continued) – Two major types (both require ATP) 1. Endocytosis (into cell using endosomes) a. Receptor-mediated endocytosis 1) Ligand binds to receptor 2) Plasma membrane folds around receptors bound to ligands 3) Coated vesicle forms 4) Vesicle fuses with lysosomes 5) Ligands freed and enter cytosol 6) Lysosome detaches from vesicle 7) Vesicle fuses with plasma membrane again Receptor-mediated endocytosis Receptor-mediated endocytosis begins when materials in the extracellular fluid bind to receptors on the membrane surface. Most receptor molecules are glycoproteins, and each binds to a specific ligand, or target, such as a transport protein or a hormone. Receptors bound to ligands cluster together. After the vesicle Once an area of the EXTRACELLULAR FLUID Ligands membrane plasma membrane has detaches, it become covered with Ligands binding returns to the ligands, it forms to receptors cell surface, grooves or pockets that where its recep- move to one area of the tors become cell and then pinch off Exocytosis Endocytosis available to bind to form an endosome. more ligands. Ligand receptors The endosomes The vesicle produced in this way membrane are called coated detaches from Coated vesicles, because the secondary vesicle they are “coated” by a lysosome. CYTOPLASM protein-fiber network on the inner membrane surface. The lysosomal enzymes then free the ligands The coated vesicles from their fuse with lysosomes receptors, and filled with digestive the ligands enter enzymes. the cytosol by Lysosome diffusion or Ligands active transport. removed  VESICULAR TRANSPORT Vesicular transport (continued) – Two major types (both require ATP) 1. Endocytosis (into cell using endosomes) (continued) b. Pinocytosis (“cell drinking”) » Formation of endosomes with ECF » No receptor proteins involved c. Phagocytosis (“cell eating”) » Produces phagosomes containing solids » Phagocytes or macrophages perform phagocytosis 2. Exocytosis – Vesicle discharges materials into ECF  Pinocytosis begins with the formation of deep grooves or pockets that then pinch off and enter the cytoplasm. The steps are similar to those of receptor-mediated endocytosis, but they occur in the absence of ligand binding. Plasma membrane Completed pinosome Pinocytosis Color enhanced TEM x 20,000  Bacterium Pseudopodium Phagocytosis begins when cytoplas- The vesicular events linking mic extensions called pseudopodia phagocytosis and exocytosis (soo-dō-PŌ-dē-ah; pseduo-, false podon, foot; singular pseudopodium) Phagocytosis surround the object. The pseudopodia then fuse at their Lysosome tips to form a phagosome containing the targeted material. This vesicle then fuses with many lysosomes, whereupon its contents are digested by lysosomal enzymes. Golgi Released nutrients are absorbed. apparatus Exocytosis The residue is then ejected from the cell through exocytosis.  CELL LIFE CYCLE Cell division – Production of daughter cells from single cell – Important in organism development and survival – Cells have varying life spans and abilities to divide Often genetically controlled death occurs (apoptosis) – Two types 1. Mitosis (2 daughter cells, each with 46 chromosomes) 2. Meiosis (sex cells, each with only 23 chromosomes)  CELL LIFE CYCLE Mitosis – Pair of daughter cells half the size of parent cell Grow to size of original cell before dividing – Identical copies of chromosomes in each – Ends at complete cell separation (= cytokinesis) – Followed by nondividing period (= interphase) Cell performs normal activities OR Prepares to divide again – Chromosomes duplicated – Associated proteins synthesized The production of a pair of daughter cells from a single cell division Original cell Cell division Daughter cells  INTERPHASE Phases – G0 (performing normal cell functions) Examples: – Skeletal muscle cells (stay in this phase forever) – Stem cells (never enter G0; divide repeatedly) – G1 (normal cell function plus growth and duplication of organelles) – S (duplication of chromosomes) – G2 (last minute protein synthesis and centriole replication)  INTERPHASE DNA replication – Strands unwind – DNA polymerase binds Assembles new DNA strand covalently linking nucleotides Works only in one direction – One polymerase works continuously along one strand toward “zipper” – One polymerase works away from “zipper” » As “unzipping” occurs, another polymerase binds closer point of unzipping » Two new DNA segments bound with ligases – Two identical DNA strands formed The events in DNA replication, which DNA replication beings when enzymes unwind the strands and disrupt the hydrogen bonds between the occurs during the bases. As the strands unwind, molecules of DNA S phase of interphase polymerase bind to the exposed nitrogenous bases. This enzyme (1) promotes bonding between the nitrogenous bases of the DNA strand and complementary DNA nucleotides dissolved in the nucleoplasm and (2) links the nucleotides by covalent bonds. As the two original stands gradually separate, DNA polymerase binds to the strands. DNA polymerase can work in only one direction along a strand of DNA, but the two strands in a DNA molecule are oriented in opposite directions. The DNA polymerase bound to the upper strand shown here adds nucleotides to make a single, continuous complementary copy that grows toward the “zipper.” Segment 2 DNA nucleotide Segment 1 DNA polymerase on the lower strand can work only away from the zipper. So the first DNA polymerase to bind to this strand must add nucleotides and build a complementary DNA strand moving from left to right. As the two original strands continue to unzip, additional nucleotides are continuously being exposed to the nucleoplasm. The first DNA polymerase Adenine on this strand cannot go into reverse; it Thus, a second DNA polymerase must bind closer can only continue to elongate the strand Guanine to the point of unzipping and assemble a comple- it already started. Cytosine mentary copy (segment 2) that grows until it “bumps into” segment 1 created by the first DNA Thymine polymerase. The two segments are then spliced together by enzymes called ligases (LĪ-gās-ez; liga, to tie). Duplicated DNA double helices  MITOSIS Mitosis – Division and duplication of cell’s nucleus – Phases 1. Prophase (pro-, before) – Paired chromosomes tightly coiled » Chromatid (each copy) » Connected at centromere with raised area (kinetochore) – Replicated centrioles move to poles » Astral rays (extend from centrioles) » Spindle fibers (interconnect centriole pairs)  The events in mitosis Chromatids The centrioles Microtubules extend outward have replicated, from each pair of centrioles: and the pairs astral rays extend into the now move to cytoplasm, whereas spindle opposite sides fibers interconnect the of the nucleus. centriole pairs. Kinetochore The kinetochore of The nuclear each chromatid membrane becomes attached Centrioles in Nucleus disintegrates to a spindle fiber. centrosome during this period. Interphase, which Prophase, the first phase of mitosis precedes mitosis  MITOSIS Mitosis (continued) – Phases (continued) 2. Metaphase (meta, after) – Chromosomes align at metaphase plate 3. Anaphase (ana-, apart) – Chromatids separate » Drawn along spindle apparatus 4. Telophase (telo-, end) – Cells prepare to enter interphase – Cytoplasm constricts along metaphase plate (= cleavage furrow) – Nuclear membranes re-form – Chromosomes uncoil The events in mitosis (continued) The two chromatids are now pulled As the chromatids approach apart and drawn to opposite ends of the ends of the spindle the cell along the spindle apparatus, the cytoplasm apparatus (the complex of spindle constricts along the plane of fibers). Anaphase ends when the the metaphase plate, forming chromatids arrive near the centrioles a cleavage furrow. at opposite ends of the cell. Daughter CYTOKINESIS cells Metaphase plate Metaphase Anaphase Telophase, the final Cytokinesis phase of mitosis  MITOSIS Cytokinesis (cyto-, cell + kinesis, motion) – Begins with formation of cleavage furrow – Continues through telophase – Completion marks end of cell division  TUMORS AND CANCER Cancer – Illness that disrupts normal rates of cell division – Characterized by permanent DNA sequence changes (= mutations) – Most common in tissues with actively dividing cells Examples: skin, intestinal lining – Compete with normal cells for resources  TUMORS AND CANCER Cancerous tumor (neoplasm; mass of cells) types 1. Benign – Remain in original tissue 2. Malignant – Accelerated growth due to blood vessel growth and supply to the area – Invasion (cells migrating into surrounding tissues) – Metastasis (formation of secondary tumors)