Chapter 3 Cell Biology.pptx

Because learning changes everything. ® Chapter 3: Cell Biology Seeley’s ANATOMY & PHYSIOLOGY Thirteenth Edition Cinnamon VanPutte, Jennifer Regan, Andrew Russo Lecturer: Mr. Rogers...

Because learning changes everything. ® Chapter 3: Cell Biology Seeley’s ANATOMY & PHYSIOLOGY Thirteenth Edition Cinnamon VanPutte, Jennifer Regan, Andrew Russo Lecturer: Mr. Rogers © 2023 McGraw Hill, LLC. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw Hill, LLC.  Lecture Outline The cell is the basic unit of life and is composed of a plasma membrane and the cytoplasm, which includes a nucleus and cytoplasmic organelles. Access the text alternative for slide images. © McGraw Hill, LLC 2  3.1 Functions of the Cell General parts of a cell: Plasma (cell) membrane – outer boundary that allows cell interaction with its external environment. Nucleus – directs cell activities. Cytoplasm – located between plasma membrane and nucleus; contains organelles that perform specific functions. © McGraw Hill, LLC 3  Characteristic functions of the cell Cell metabolism and energy use – includes all chemical reactions that occur in the cell and often involves energy transfer and production of heat. Synthesis of molecules, such as proteins, nucleic acids, and lipids, that are specific for the type of cell. Communication - cells produce and receive electrical and chemical signals. Reproduction and inheritance - each cell contains a complete copy of all the genetic information (D NA) for the individual that determines the structural and functional characteristics of the cell; some cells are specialized as gametes for the transmission of genetic information to the next generation. © McGraw Hill, LLC 4  A Human Cell Access the text alternative for slide images. © McGraw Hill, LLC 5  3.2 How We See Cells Cells are too small to be seen with the unaided eye. Must use microscopes. Light microscope – has a resolution of about 0.1µm Examine tissues (For example, biopsies) and cells. Requires stains. Electron microscope – has a resolution of about 0.1nm. Scanning microscope (SEM). Three-dimensional surface features. Transmission microscope (TEM). View internal structures. Atomic force microscope (AFM). Tiny probe scans sample. Very high resolution. Reveals surface topography. © McGraw Hill, LLC 6  Microscopic Images of Nuclear Pores (a) National High Magnetic Field Laboratory, The Florida State University (b) Courtesy of Werner Franke and Ulrich Scheer; (c) Courtesy of Dr. Martin W. Goldberg Light microscope image of a cell TEM of a single nuclear pore (c) Courtesy of Dr. Martin W. Goldberg (d) Shahin, Schillers & Oberleithner, Institute of Physiology II, Medical Faculty, University of Muenster, Germany SEM of a single nuclear pore Color-enhanced AFM of nuclear pores © McGraw Hill, LLC 7  3.3 Plasma Membrane Functions: A boundary separating the cytoplasmic (intracellular) substances from the extracellular environment of the cells. Encloses and supports the cell contents. Attaches to the extracellular environment or to other cells. The ability to recognize and communicate with other cells. Determines what moves into and out of cells. © McGraw Hill, LLC 8  Production of a membrane potential An electrical charge difference across the plasma membrane that results from the cell’s regulation of ion movement into and out of the cell. There are more positively charged ions along the outside of the plasma membrane, giving it a positive charge. There are more negatively charged ions and proteins on the inside of the plasma membrane, giving it a negative charge. © McGraw Hill, LLC 9  Composition of the Plasma Membrane The plasma membrane is primarily made of lipids and proteins with a very small amount of carbohydrates. Glycocalyx: combinations of carbohydrates and lipids (glycolipids) and proteins (glycoproteins) on outer surface. Access the text alternative for slide images. © McGraw Hill, LLC 10  3.4 Membrane Lipids Phospholipids and cholesterol predominate. Phospholipids: bilayer. Polar heads facing water in the interior and exterior of the cell (hydrophilic); nonpolar tails facing each other on the interior of the membrane (hydrophobic). Cholesterol: interspersed among phospholipids. Amount determines fluid nature of the membrane, providing stability to the membrane. Fluid nature (fluid-mosaic model) provides/allows. Distribution of molecules within the membrane. Phospholipids automatically reassembled if membrane is damaged. Membranes can fuse with each other. Access the text alternative for slide images. © McGraw Hill, LLC 11  3.5 Membrane Proteins Integral membrane proteins. Extend deeply into membrane, often extending from one surface to the other. Can form channels through the membrane. Peripheral membrane proteins. Attached to integral proteins at either the inner or outer surfaces of the lipid bilayer or to polar heads of phospholipids. Functioning depends on 3-D shape and chemical characteristics. Markers, attachment sites, channels, receptors, enzymes, or carriers. © McGraw Hill, LLC 12  Marker Molecules Glycoproteins or glycolipids. Allow cells to identify one another or other molecules. Distinguish between self- cells and foreign cells. Recognition of oocyte by sperm cell. Intercellular communication. © McGraw Hill, LLC 13  Attachment Proteins Integral proteins. Cadherins – attach cells to other cells. Integrins – function in pairs to attach to extracellular molecules. Sometimes allow communication due to contact with intracellular molecules. © McGraw Hill, LLC 14  Transport Proteins: Specificity and Competition Involve carrier proteins or channels in the cell membrane. Characteristics. Specificity for a single type of molecule based on shape. Competition among molecules of similar shape. Saturation: rate of transport limited to number of available carrier proteins. © McGraw Hill, LLC 15  Saturation of a Transport Protein Access the text alternative for slide images. © McGraw Hill, LLC 16  Transport Proteins Includes channel proteins, carrier proteins, and ATP- powered pumps. © McGraw Hill, LLC 17  Channel Proteins Channel proteins – integral membrane proteins that form tiny channels through membrane. The channel determines the size, shape and charge of what can move through. Hydrophobic regions face outward toward the hydrophobic part of the plasma membrane. Hydrophilic regions face inward and line the tunnel. The charge of the hydrophilic tunnel determines the types of ions that can move through. © McGraw Hill, LLC 18  Leak and Gated Ion Channels Leak ion channels (nongated ion channels): always open. Responsible for the permeability of the plasma membrane to ions when the plasma membrane is at rest. Gated ion channels: opened or closed by certain stimuli. Ligand-gated ion channel: open in response to small molecules that bind to proteins or glycoproteins. Voltage-gated ion channel: open when there is a change in charge across the plasma membrane. Cystic Fibrosis is a genetic disorder that affects chloride ion channels and causes cells to produce thick, viscous secretions. © McGraw Hill, LLC 19  Leak and Gated Membrane Channels © McGraw Hill, LLC 20  Carrier Proteins 1 Also called transporters. Integral proteins move ions from one side of membrane to the other. 1. Specific molecule enters the carrier. 2. Molecule attaches to binding site in carrier. 3. Protein changes shape to transport to the other side. Resumes original shape after transport. © McGraw Hill, LLC 21  Carrier Proteins 2 Carrier proteins come in several forms. Uniporters – moves one ion/molecule. Symporters – move two ions/molecules in the same direction at the same time (cotransport). Antiporters – move two ions/molecules in opposite directions at the same time (countertransport). © McGraw Hill, LLC 22  ATP-Powered Pumps Require the breakdown of ATP. 1. ATP-powered pumps have binding sites for specific ions or molecules and ATP. 2. Hydrolysis of ATP to ADP, releases energy to change the shape of the carrier to move the substance across the membrane. 3. The ion and phosphate are released and the pump resumes its original shape. © McGraw Hill, LLC 23  Receptor Proteins Proteins or glycoproteins in membranes with an exposed receptor site on the outer cell surface. Can attach to specific chemical signal molecules and act as an intercellular communication system. Ligand can attach only to cells with that specific receptor. © McGraw Hill, LLC 24  Receptors Linked to Channel Proteins Receptor molecules linked to channel proteins. Attachment of receptor-specific chemical signals (for example, acetylcholine) to receptors causes change in shape of channel protein. Channel opens or closes; changes permeability of cell to some substances. Certain toxins disrupt normal cell activity by blocking acetylcholine bonding sites. © McGraw Hill, LLC 25  Opening and Closing of Ligand-Gated Channels 1. Acetylcholine binds to the receptor sites linked to a Na+ channel. When the receptor sites are not occupied by ACh, the channel remains closed. 2. The binding of ACh molecules to the receptor sites opens the channels in the plasma membrane allowing to move into the cell. © McGraw Hill, LLC 26  Receptors Linked to G Protein Complexes 1 Alter activity on inner surface of plasma membrane. Leads to intracellular chemical signals that affect cell function. Some hormones function in this way. The G protein complex consists of three proteins: alpha (α), beta (β), and gamma (γ). Drugs with structures similar to specific chemical signals may compete for the receptor sites. © McGraw Hill, LLC 27  Receptors Linked to G Protein Complexes 2 1. When the G protein complex is not interacting with a receptor protein, the α subunit of the G protein complex has GDP attached to it. 2. When a chemical signal binds to the receptor, it becomes associated with the G protein complex. The α subunit releases the GDP and attached to GTP. At this point the α subunit is considered activated. 3. The G protein complex separates from the receptor, and the activated α subunit separates from the β and γ subunits. 4. The activated α subunit can stimulate a cell response in at least three ways: (a) intracellular chemical signals, (b) opening ion channels, and (c) activating enzymes associated with the plasma membrane. Access the text alternative for slide images. © McGraw Hill, LLC 28  Enzymes Enzymes: some act to catalyze reactions at outer/inner surface of plasma membrane. Example: Surface cells of small intestine produce enzymes that digest dipeptides. Some membrane-associated enzymes are always active while other are activated by receptors or G protein complexes. © McGraw Hill, LLC 29  3.6 Movement through the Plasma Membrane The plasma membrane is selectively permeable. Only allows certain substances to pass through it. Must maintain homeostasis though the composition inside and outside the cell are different. Enzymes, other proteins, glycogen and are in higher concentration in the cell cytoplasm. are in higher concentration in the extracellular environment. Cell volume must stay the same even with movement of materials into and out of the cell. Lipid soluble molecules such as O2, CO2, and steroids readily dissolve in the lipid bilayer to pass through the membrane. Large, non-lipid soluble molecules and ions need transport proteins or vesicles to pass through the membrane. © McGraw Hill, LLC 30  Membrane Transport Mechanisms Passive membrane transport – the cell does not expend ATP; movement from higher concentration to lower concentration. Diffusion. Osmosis. Facilitated diffusion. Active membrane transport – AT P is used to move from lower concentration to higher concentration. Active transport. Secondary active transport. Vesicular transport – uses a membrane-bound sac. Endocytosis. Exocytosis. © McGraw Hill, LLC 31  Diffusion Net movement of solutes from an area of higher concentration to lower concentration in solution. Results from the constant random motion of all atoms, molecules, or ions in a solution. Particles move from both low to high concentration and high to low concentration with the greater chance of moving from high to low concentration areas. Concentration gradient: concentration difference between two points. Solutes move down their concentration gradient until an equilibrium is established. Solute and solvent particles will continue to move even once an equilibrium has been established. © McGraw Hill, LLC 32  Diffusion of Salt in a Beaker of Water 1. A salt crystal is placed in a beaker of water. A concentration difference exists between the salt crystal and the water surrounding it. 2. Salt ions, moving randomly, spread through the beaker of water. 3. Eventually, the salt ions will become evenly distributed throughout the water. Access the text alternative for slide images. © McGraw Hill, LLC 33  Factors Affecting the Rate of Diffusion The magnitude of the concentration gradient; the steeper the gradient, the faster diffusion occurs. The temperature of the solution; the higher the temperature, the greater the kinetic energy and the faster diffusion occurs. The size of the diffusing molecules; the larger the particles, the slower the rate of diffusion. The viscosity of the solvent; viscosity is the fluid’s resistance to flow; the more viscous the solvent, the more slowly diffusion occurs. © McGraw Hill, LLC 34  Diffusion Through the Plasma Membrane Access the text alternative for slide images. © McGraw Hill, LLC 35  Osmosis Diffusion of water (solvent) across a selectively permeable membrane. Water moves from an area of low concentration of solute (high amount of water) to an area of high concentration of solute (low amount of water). Aquaporins – water channel proteins. Osmotic pressure: force required to prevent water from moving across a membrane by osmosis. © McGraw Hill, LLC 36  Osmotic Pressure 1. The osmotic pressure of a solution can be determined by placing the solution in a tube that is closed at one end by a selectively permeable membrane and immersing it in distilled water. 2. Water molecules move by osmosis through the membrane into the tube, forcing the solution to move up the tube. This creates a hydrostatic pressure that opposes the osmosis, forcing water back out of the tube. 3. At equilibrium, the level in the tube stops changing because the hydrostatic pressure is equal to the osmotic pressure. Access the text alternative for slide images. © McGraw Hill, LLC 37  Comparative terms used to describe osmotic pressures of solutions. Isosmotic: solutions with the same concentrations of solute particles; equal osmotic pressures Solution with a greater concentration of solute is hyperosmotic; has a greater osmotic pressure Solution with a lesser concentration of solute is hypoosmotic; has a lesser osmotic pressure © McGraw Hill, LLC 38  Osmosis and Cells Important because large volume changes caused by water movement disrupt normal cell function. Isotonic: cell neither shrinks nor swells when placed in a solution. Hypertonic: cell shrinks (crenation) when placed in a solution; water moves out of the cell. Hypotonic: cell swells and may rupture (lysis) when place in a solution; water moves into the cell. © McGraw Hill, LLC 39  Effects of Hypotonic, Isotonic, and Hypertonic Solutions on Red Blood Cells David M. Phillips/Science Source Access the text alternative for slide images. © McGraw Hill, LLC 40  Facilitated Diffusion 1 Facilitated diffusion: mediated transport process carried out by carrier/channel proteins; no A TP required. Move large, water soluble molecules or electrically charged molecules across the plasma membrane. Amino acids and glucose in, manufactured proteins out. © McGraw Hill, LLC 41  Facilitated Diffusion 2 1. A glucose molecule enters a carrier protein from the extracellular fluid. The glucose binds to the carrier protein. 2. The carrier protein changes shape and releases the glucose molecule into the cell. The carrier protein then resumes its original shape to transport additional glucose molecules. Access the text alternative for slide images. © McGraw Hill, LLC 42  Active Membrane Transport: Active Transport Requires ATP. Substances can be moved against their concentration gradients (that is, from low to high), allowing the substance to accumulate on one side of the plasma membrane. Rate of transport depends on concentration of substrate, the number of ATP pumps, and amount of ATP. Example: sodium-potassium pump that creates electrical potentials across membranes. © McGraw Hill, LLC 43  Active Transport: Sodium-Potassium Pump 1. Three sodium ions and adenosine triphosphate (ATP) bind to the sodium–potassium pump. 2. Three sodium ions and adenosine triphosphate (ATP) bind to the sodium–potassium pump. 3. The pump changes shape, and the are transported across the membrane and into the extra cellular fluid. 4. Two potassium ions (K+) bind to the pump. 5. Two potassium ions (K+) bind to the pump. 6. The pump changes shape, transporting across the membrane and into the cytoplasm. The pump can again bind to and ATP. Access the text alternative for slide images. © McGraw Hill, LLC 44  Secondary Active Transport Use of potential energy in concentration gradient of one substance (established by primary active transport) to help move another substance. 1. A Na+−K+ pump maintains a concentration of Na+ that is higher outside the cell than inside. 2. Na+ move back into the cell through a transport protein that also moves glucose. The concentration gradient for Na+ provides the energy required to move glucose against its concentration gradient. © McGraw Hill, LLC 45  Vesicular Transport Movement of larger substances by Phagocytosis formation or release of a vesicle. Requires ATP. Types: Endocytosis: movement into cell. Phagocytosis: solid particle is ingested and large vesicle is formed. Pinocytosis: dissolved molecules Transcytosis: movement ingested and small vesicles are through a cell by a formed. combination of endocytosis Exocytosis: movement out of on one surface and exocytosis on the opposite cell. surface. © McGraw Hill, LLC 46  Pinocytosis and Transcytosis Access the text alternative for slide images. © McGraw Hill, LLC 47  Receptor-Mediated Endocytosis 1. Specific molecules in the extracellular fluid bind to receptors in the plasma membrane. 2. The receptors and the bound molecules are taken into the cells as a vesicle begins to form. 3. The vesicle fuses and separates from the plasma membrane, © McGraw Hill, LLC 48  Exocytosis 1. Within the cell, secretions accumulate within vesicles that move to the plasma membrane. 2. The vesicle membrane fuses with the plasma membrane. 3. The vesicle contents are expelled from the cell into the extracellular fluid. (b) JOSE CALVO/Science Source © McGraw Hill, LLC 49  3.7 Cytoplasm Cytoplasm: cellular material outside nucleus but inside plasma membrane. Composed of cytosol, cytoskeleton, cytoplasmic inclusions, organelles. Cytosol is the fluid portion. Dissolved molecules, ions, and suspended molecules of proteins, especially enzymes. © McGraw Hill, LLC 50  Cytoskeleton Supports the cell but allows for movements like changes in cell shape and movements of cilia. Microtubules: hollow tubes made of tubulin protein. Internal scaffold, transport, cell division. Components of centrioles, spindle fibers, cilia, and flagella. Actin filaments (microfilaments). Structure, support for microvilli, contractility, movement. Intermediate filaments: mechanical strength of nerve cell extensions.. Cytoplasmic inclusions: aggregates of chemicals such as lipid droplets, melanin, glycogen, and lipochromes. © McGraw Hill, LLC 51  3.8 The Nucleus and Cytoplasmic Organelles Organelles: small specialized structures with particular functions. Most have membranes that separate interior of organelles from cytoplasm. Related to specific structure and function of the cell. © McGraw Hill, LLC 52  The Nucleus Large membrane-bound structure containing DNA. Consists of nucleoplasm surrounded by nuclear envelope which is a double membrane with many fused areas called nuclear pores that regulate movement into/out of nucleus. DNA contained in nucleus specifies the structure of proteins. RNA serves as an intermediate during protein synthesis and consists of three types: mRNA, rRNA, and tRNA. Nucleolus: dense region(s) within the nucleus where ribosomes are manufactured. © McGraw Hill, LLC 53  Nucleus (b) Don W. Fawcett/Science Source; (c) Bernard Gilula/Science Source Access the text alternative for slide images. © McGraw Hill, LLC 54  Chromosome Structure DNA is found in nucleus, associated with proteins called histones to form chromosomes. Nucleosomes are structural units of chromosomes. During much of cell cycle, chromosomes are dispersed as chromatin. During cell division, chromatin condenses into compact chromosomes. © McGraw Hill, LLC 55  Ribosomes Sites of protein synthesis. Composed of a large and a small subunit. Composed of ribosomal RNA (rRNA) + proteins. Types: Free – synthesize proteins used inside the cell. Attached (to endoplasmic reticulum) – produce integral proteins and proteins secreted from the cell. © McGraw Hill, LLC 56  Production of Ribosomes 1. Ribosomal proteins, produced in the cytoplasm, are transported through nuclear pores into the nucleolus. 2. rRNA, most of which is produced in the nucleolus, is assembled with ribosomal proteins to form small and large ribosomal subunits. 3. The small and large ribosomal subunits leave the nucleolus and the nucleus through nuclear pores. 4. The small and large subunits, now in the cytoplasm, combine with each other and with mRNA during protein synthesis. © McGraw Hill, LLC 57  Endoplasmic Reticulum (ER) Organelle consisting of a network of membranes that is continuous with outer membrane of nuclear envelope; internal spaces are cisternae. Rough ER: has attached ribosomes; where proteins are produced and modified. Smooth ER: no attached ribosomes; manufactures lipids, participates in detoxification, and calcium ion storage. (b) J. David Robertson, from Charles Flickinger, Medical Cell Biology, Philadelphia © McGraw Hill, LLC 58  Golgi Apparatus Flattened membrane sacs, containing cisternae, stacked on each other. Modifies, packages, and distributes proteins and lipids for secretion or internal use. Substances packaged into transport vesicles. (b) Biophoto Associates/Science Source © McGraw Hill, LLC 59  Function of the Golgi Apparatus 1. Proteins produced at the ribosomes attached to the rough ER move into the ER. 2. These proteins are packed into transport vesicles that move to the cis face of the Golgi Apparatus and fuse with the Golgi Apparatus membrane and release the proteins into the cisternae. 3. The Golgi Apparatus concentrates and/or chemically modifies the proteins by synthesizing and attaching carbohydrate molecules to the proteins to form glycoproteins or attaching lipids to form glycolipids. 4. The proteins are then packaged into vesicles that pinch off from the margins of the trans face of the Golgi Apparatus and are distributed to various locations. 5. Some vesicles contain enzymes used by the cell (lysosomes, peroxisomes, proteasomes) 6. Some vesicles carry proteins to the plasma membrane, where they are secreted from the cell by exocytosis. 7. Other vesicles contain proteins that become part of the plasma membrane. Access the text alternative for slide images. © McGraw Hill, LLC 60  Lysosomes and Peroxisomes Lysosomes. Form at the Golgi Apparatus. Contain hydrolytic enzymes that function in digesting cellular material. Digest material ingested by cell – nutrients and phagocytized bacteria. Digest organelles no longer functional (autophagy). Tay-Sachs Disease is a rare genetic disorder caused by the inability of lysosomal enzymes to break down gangliosides, specialized membrane lipids of neurons. Peroxisomes. Smaller than lysosomes. Contain enzymes to break down fatty acids and amino acids. Hydrogen peroxide (toxic) is a by-product of breakdown. Also contain the enzyme catalase which breaks down hydrogen peroxide into water and oxygen © McGraw Hill, LLC 61  Action of Lysosomes 1. A vesicles forms and enters the cytoplasm as the cell phagocytizes extracellular material. 2. A lysosome, formed at the Golgi Apparatus, migrates to and fuses with the vesicle. 3. The enzymes from the lysosome mix with the material in the vesicle, and digest the material. © McGraw Hill, LLC 62  Proteasomes Proteasomes. Consist of large protein complexes that are not surrounded by membranes. Proteins fold to form a barrel-like structure with enzymatic regions on the inside surface that break down and recycle proteins in cell. Proteins at the ends of the barrel regulate which proteins are taken in for breakdown and recycling. © McGraw Hill, LLC 63  Mitochondria Major site of ATP synthesis. Membranes: Cristae: Infoldings of inner membrane; contain enzymes of the electron transport chain. Matrix: Substance located in space formed by inner membrane; contains the enzymes for the citric acid or Krebs cycle. Mitochondria increase in number when cell energy requirements increase. Mitochondria contain D N A that codes for some of the proteins needed for mitochondria production. (b) EM Research Services/ Newcastle University © McGraw Hill, LLC 64  Centrioles and Spindle Fibers Centrioles located in centrosome, a specialized zone near nucleus. Center of microtubule formation that are involved in cell shape and movement. There are 2 centrioles that are perpendicular to each other made of nine microtubule triplets. Before cell division, centrioles divide, move to ends of cell and organize microtubules called spindle fibers. (b) Biology Media/Science Source © McGraw Hill, LLC 65  Cilia and Flagella Appendages projecting from cell surfaces, capable of movement. Cilia move materials over the surface of cells. For example, mucus in respiratory tract. Made of microtubules in a 9+2 arrangement Movement occurs when microtubules move past each other using ATP; dynein arms that connect adjacent pairs of microtubules, push them past each other. Flagella used for movement by sperm cells. © McGraw Hill, LLC 66  Structure of Cilia and Flagella (b) Biophoto Associates/Science Source; (c) Don W. Fawcett/Science Source © McGraw Hill, LLC 67  Microvilli Extension of plasma membrane that increases surface area; some modified as sensory receptors. Normally many on each cell. One-tenth to one-twentieth the size of cilia. Do not move; supported with actin filaments. (b) Don W. Fawcett/Science Source © McGraw Hill, LLC 68  3.9 Genes and Gene Expression Genes are the functional units of heredity. Heredity is the transmission of genetic traits from parent to offspring. Each gene is a segment of a DMA molecule that specifies the structure of an R NA molecule that can function on its own or produce a protein. The production of R NA and/or proteins from the information stored in DN A is called gene expression. © McGraw Hill, LLC 69  Gene Expression Gene expression involves two steps: 1. Transcription occurs in the nucleus. During this process, information stored in a segment of DNA is used to produce a complementary R NA molecule called mRNA. 2. The mR NA moves to the ribosomes in the cytoplasm where translation occurs. During this process, the nucleotide sequence of the m RNA is used to determine the composition of a polypeptide chain, a precursor to a protein. tRNA is used to carry specific amino acids to the ribosomes. © McGraw Hill, LLC 70  Transcription The strands of DNA are separated. 1. RNA polymerase binds at a promoter region utilizing transcription factors. 2. RNA polymerase catalyzes the formation of a mRNA chain using the DNA as a template and following the rules of complimentary base pairing. A with U. C with G. Transcription ends at a terminator sequence and the RNA is released. © McGraw Hill, LLC 71  Gene Complexity The region of DNA between the promoter and the termination of transcription is a gene. Introns are regions of the gene that are not part of the code for a protein. Exons are regions of the gene that are part of the code for a protein. The mRNA with the introns is called pre-mRNA. Introns are removed from the pre-m RNA by spliceosomes that join the remaining pieces of exons to become functional mRNA. In alternative splicing, various combinations of exons are joined into mRNA that allows for different proteins to be made from the same gene. © McGraw Hill, LLC 72  Posttranscriptional Change in mRNA 1. Introns within a pre-mRNA are removed through the activity of spliceosomes. 2. A 7-methylguanosine cap is added to one end of mRNA. 3. A series of adenine nucleotides, called a poly-A tail, is added to the other end. These modifications to the ends of the mRNA ensure that mRNA travels from the nucleus to the cytoplasm and interacts with ribosomes during translation. Access the text alternative for slide images. © McGraw Hill, LLC 73  Genetic Code The genetic code is the information contained in mRNA and relates the nucleotide sequence of mRNA to the amino acid sequence of a protein. The mRNA is arranged into three nucleotide sequences called codons, each of which specifies an amino acid during translation. Although there are only 20 amino acids, 64 possible codons exist; an amino acid can be designated by more than one codon. AUG codes for methionine and is the start codon. UAA, UGA, and UAG do not code for any amino acid and are considered stop codons. © McGraw Hill, LLC 74  Translation Process that occurs on ribosomes. Turns mRNA into a polypeptide. Involves rRNA, tRNA, and mRNA. tRNA anticodons are complements of mRNA codons, and the rRNA catalyzes the formation of a peptide bond between the amino acids at the opposite end of the tRNA. © McGraw Hill, LLC 75  Translation of mRNA to Produce a Protein 1. To start protein synthesis, a ribosome binds to m RNA. The ribosome has two binding sites for t RNA, one of which is occupied by a tRNA with its amino acid. Note that the 1 st codon of m RNA to associate with a t RNA is AUG, the start codon which codes for methionine. The codon of m RNA and the anticodon of t RNA are aligned and joined. The other t RNA binding site is open. 2. By occupying the open t RNA binding site, the next t RNA is properly aligned with m RNA and the other t RNA. 3. An enzyme catalyzes a synthesis reaction to form a peptide bond between 2 amino acids. Note that the amino acids are now associated with only 1 of the t RNAs. 4. The ribosome shifts position by 3 nucleotides along the m RNA. The tRNA without the amino acid is released from the ribosome, and the tRNA with the amino acid takes its position. A t RNA binding site is left open by this shift. Additional amino acids can be added by repeating steps 2 through 4. 5. Eventually, a stop codon in the m RNA, such as U AA, ends the process of translation. At this point, the mRNA and polypeptide chain are released from the ribosome. 6. After a ribosome uses the initial part of m RNA, another ribosome can attach to the m RNA and begin to make another polypeptide. The resulting cluster of ribosomes attached to the same m RNA is called a polyribosome. Each ribosome in a polyribosome produces an identical polypeptide chain, which is an efficient way to make many copies of the same protein using the same m RNA. © McGraw Hill, LLC 76  Posttranslational Protein Processing Many proteins have extra amino acids when first produced and are called proproteins. If the proprotein is going to be an enzyme, it is called a proenzyme. Extra amino acids are enzymatically removed to make a functional protein or enzyme. Many enzymes are formed as proenzymes because the functional enzyme would damage the cell. Other posttranslational processing may include adding side chains, such as polysaccharides or the joining a separate chains to make one protein. © McGraw Hill, LLC 77  Regulation of Gene Expression All nucleated cells except germ cells have the full complement of DNA. During development, differentiation occurs and some segments of DNA are turned off in some cells while those segments remain “on” in other cells. During the lifetime of a cell, the rate of protein synthesis varies depending upon chemical signals that reach the cell. Example: thyroxine from the thyroid causes cells to increase their metabolic rate. More thyroxine, higher metabolic rate; less thyroxine, lower metabolic rate. © McGraw Hill, LLC 78  3.10 Cell Cycle Interphase: phase between cell divisions. Replication of DNA. Ongoing normal cell activities. Mitosis: series of events that leads to the production of two cells by division of a mother cell into two daughter cells. Cells are genetically identical. Prophase. Metaphase. Anaphase. Telophase. Cytokinesis: division of cell cytoplasm. Access the text alternative for slide images. © McGraw Hill, LLC 79  Interphase Preparation for cell division Increase in cell size Replication of organelles Replication of DNA Centrioles duplicated Subphases G1 – first gap phase – cells carries out normal activities S – synthesis phase – DNA is replicated G2 – second gap phase – preparation for division Cells in a resting phase that are not actively dividing or that do not divide enter a G0 phase © McGraw Hill, LLC 80  DNA Replication Copy DNA in preparation for mitosis DNA strands separate. The old strands become the templates for the new (complementary) strands to form. Two identical DNA molecules are formed. DNA synthesis catalyzed by DNA polymerase. Leading versus lagging strands. DNA ligase splices Okazaki fragments. All cells (except sex cells) have diploid number of chromosomes. Sex cells have haploid number. © McGraw Hill, LLC 81  Replication of DNA 1. The strands of the DNA molecule separate from each other. 2. Each old strand (dark purple) functions as a template on which a new, complementary strand (light purple) is formed. The base- pairing relationship between nucleotides determines the sequence of nucleotides in the newly formed strands. 3. Two identical DNA molecules are produced. © McGraw Hill, LLC 82  Structure of a Mitotic Chromosome 1. The DNA of a chromosome is dispersed as chromatin in a cell that is not dividing. 2. The DNA molecule unwinds, and each strand of the molecule is replicated. 3. During mitosis, the chromatin from each replicated DNA strand condenses to form a chromatid. The chromatids are joined at the centromere to form a single chromosome. 4. The chromatids separate to form two new, identical chromosomes. The chromosomes will unwind to form chromatin in the nuclei of the two daughter cells. © McGraw Hill, LLC 83  Cell Division Interphase – DNA replication occurs. Phases of mitosis: Prophase – chromatin condenses, centrioles migrate to each pole. Prometaphase – spindle fibers extend from centrioles to centromeres of chromosomes and attach to the kinetochore, nucleolus and nuclear membrane disappear. Metaphase – chromosomes are aligned at the nuclear equator. Anaphase – spindles separate the chromatids that move to opposite poles, cytokinesis begins. Telophase – chromosomes decondense, nuclear envelope reforms, cytokinesis continues. Cytokinesis – cytoplasmic division, separate process from mitosis. © McGraw Hill, LLC 84  Cell Division: Mitosis and Cytokinesis 1 (1,2,4) Ed Reschke/Photolibrary/Getty Images; (3) Photomicrographs by Dr. Conly L. Rieder, Wadsworth Center, Albany, New York 12201-0509 Access the text alternative for slide images. © McGraw Hill, LLC 85  Cell Division: Mitosis and Cytokinesis 2 (5,6,7) Ed Reschke/Photolibrary/Getty Images Access the text alternative for slide images. © McGraw Hill, LLC 86  Apoptosis Programmed cell death to maintain normal cell numbers, removal of excess tissue, removal of damaged or potentially dangerous cells, virus infected cells, potential cancer cells. Regulated by specific genes that produce proteins that initiate cell changes that lead to cell death. Macrophages will ingest the cell fragments. © McGraw Hill, LLC 87  Effects of Aging on Cells Several hypotheses regarding how aging occurs: Cellular clock. Death genes. DNA damage. Free radicals. Mitochondrial damage. © McGraw Hill, LLC 88  End of Main Content Because learning changes everything. ® www.mheducation.com © 2023 McGraw Hill, LLC. All rights reserved. Authorized only for instructor use in the classroom. 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