BIO 201: Cells and Molecular Biology
BIO 201: Cells and Molecular Biology BIO 201
Popular in Human Anatomy/Physiology I
verified elite notetaker
Popular in Biology
This 25 page Class Notes was uploaded by ASUNursing19 on Thursday February 18, 2016. The Class Notes belongs to BIO 201 at Arizona State University taught by Dr. Penkrot in Winter 2016. Since its upload, it has received 166 views. For similar materials see Human Anatomy/Physiology I in Biology at Arizona State University.
Reviews for BIO 201: Cells and Molecular Biology
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 02/18/16
Cellular Form and Function Concepts of cellular structure Cell surface Membrane transport Cytoplasm o Organelles Molecular biology & genetics Histology Study of Tissues Intercellular Junctions, Glands, and Membranes Tissue Growth, Development, Death and Repair Cell Theory In science, a theory is something that we are very confident about because it has been supported by all known evidence. (In contrast, a hypothesis is something we are not confident about, and set out to test!) A theory is just below a law of science **The cell is the basic structural and functional unit of life** 1 Organismal activity depends on individual and collective activity of cells 2 Biochemical activities of cells are dictated by subcellular structure 3 Continuity of life has a cellular basis o Cells can only come from other cells Robert Hooke (1637 1703) Cytology : scientific study of cells o Began when Robert Hooke coined the word "cellulae" to describe empty cell walls of cork Diversity of Cell Form Structure and function are complementary Development of the Cell Theory Theodor Schwann concluded, about two centuries later, that all animal tissues are made of cells Louis Pasteur established beyond any reasonable doubt that "cells arise only from other cells" o Refuting the idea of spontaneous generation Modern Cell Theory emerged o The cell is the basic structural and functional unit of life o Organismal activity depends on individualistic and collective activity of cells o Biochemical activities of cells are dictated by subcellular structure o Continuity of life has a cellular basiscells are all similar Cell Size Human cell size o Most from 1015 micrometers in diameter o Egg cells (very large) 100 micrometers diameter o Nerve cell is at 1 meter long Longest human cell Too slender to be seen with naked eye Limitations on cell size o Cell growth increases more than surface area o Surface area of a cell is proportional to the square of its diameter o Volume of a cell is proportional to the cube of its diameter Cell Surface Area and Volume Surface area is used for transport Surface area ~ amount cell can transport Volume is filled with cytoplasm Doubling the "size" of a cell o Surface area increases 4 fold o Volume increases 8 fold This ratio limits cell size General Cell Structure Light microscope reveals plasma membrane, nucleus and cytoplasm o Cytoplasm: fluid between the nucleus and surface membrane Resolution (ability to reveal detail) of electron microscopes reveals ultrastructure o Organelles, cytoskeleton, and cytosol (ICF) Magnification and Resolution Magnification makes things look larger Resolution allows for more detail Major Constituents of Cells Plasma (cell) membrane o Surrounds cell o Made of proteins and lipids o Composition and function can very form one region to another Cytoplasm o Organelles o Cytoskeleton o Cytosol (intracellular fluid ICF) Extracellular fluid ECF Membrane Protein Functions Receptors, secondmessenger systems, enzymes, ion channels, carriers, cellidentity markers, celladhesion molecules Hydrophobicity / Hydrophilicity of Transmembrane Proteins Extracellular portion tends to be hydrophilic Intramembrane portion tends to be hydrophobic Anchored to the cytoskeleton just inside plasma membrane Membrane Proteins Peripheral Proteins o Loosely attached to integral proteins o Include filaments on intracellular surface used for plasma membrane support o Function as: Enzymes Motor proteins for shape change during cell division and muscle contraction Celltocell connections Roles of Plasma Membrane Receptors Membrane receptor proteins serve as binding sites for several chemical signals o Contract Signaling : cells that touch recognize each other by each cell's unique surface membrane receptors Used in normal development and immunity o Chemical Signaling : interaction between receptors and ligands (chemical messengers) that cause changes in cellular activities In some cells, binding triggers enzyme activation; in other, it opens chemically gated ion channels Examples of ligands: neurotransmitters, hormones, and paracrines Same ligand can cause different responses in different cells depending on chemical pathway that the receptor is part of When ligand binds, receptor protein changes shape and thereby becomes activated Some activated receptors become enzymes; others act to directly open of close ion gates, causing changes in excitability Activated G proteinlinked receptors indirectly cause cellular changes by activating proteins , which in turn can affect ion channels, activate other enzymes, or cause release of internal second messenger chemicals such as cyclic AMP or calcium Glycocalyx Consists of sugars (carbohydrates) sticking out of cell surface o Some sugars are attached to lipids (glycolipids) and some to proteins (glycoproteins) Every cell type has different patterns of this "sugar coating" o Functions as specific biological markers for celltocell recognition o Allows immune system to recognize "self" vs. "nonself" Cell Junctions Some cells are "free" (not bound to any other cells) o Examples: blood cells, sperm cells Most cells are bound together to form tissues and organs Three ways cells can be bound to each other: o Tight junctions o Desmosomes o Gap junctions Tight Junctions o Integral proteins on adjacent cells fuse to form an impermeable junctions that encircles whole cell o Prevent fluids and most molecules from moving inbetween cells Forces fluidthrough cells o Sometimes describes as "zipperlike" Desmosomes o Rivetlike cell junction formed when linker proteins (cadherins) of neighboring cells interlock like the teeth of a zipper o Linker protein is anchored to its cell through thickened "buttonlike" areas on inside of plasma membrane called plaques o Keratin filaments connect plaques intracellularly for added anchoring strength o Desmosomes allow "give" between cells, reducing the possibility of tearing under tension o Does not restrict flow of fluids between cells Gap Junctions o Transmembrane proteins ( connexons ) form tunnels that allow small molecules to pass from cell to cell o Used to spread ions, simple sugars, or other small molecules between cells o Allows electrical signals to be passed quickly from one cell to next cell Used in cardiac and smooth muscle cells Membrane Permeability Diffusion through lipid bilayer o Nonpolar, hydrophilic, lipidsoluble substance diffuse through lipid layer Diffusion through channel proteins (most things do this) o Water and charged, hydrophilic solutes diffuse through channel proteins in membrane Cells control permeability by regulating number of channel proteins or by opening and closing gates Diffusion Facilitated Diffusion o Certain hydrophobic molecules (i.e.: glucose, amino acids, and ions) are transported passively down their concentration gradient by: Carriermediated facilitated diffusion Substances bind to protein carriers Channelmediated facilitated diffusion Substances move through waterfilled channels CarrierMediated Facilitated Diffusion o Carriers are transmembrane integral proteins o Carriers transport polar molecules, such as sugars and amino acids, that specific are too large for membrane channels Example of specificity: glucose carriers will carry only glucose molecules, nothing else o Binding of molecule causes carrier to change shape, moving molecule in process o Binding is limited by number of carriers present Carriers and saturated when all are bound to molecules and are bust transporting ChannelMediated Facilitated Diffusion o Channels with aqueousfilled cores are formed by transmembrane proteins o Channel transport molecules such as ions or water (osmosis) down their concentration gradient Specificity based on pore size and/or change Water channels are calledaquaporins o Two types: Leakage channels Always open Gated channels Controlled by chemical or electrical signals Osmosis o Movement of solvent, such as water, across a selectively permeable membrane o Water diffuses through plasma membranes Through lipid bilayer (even though water is polar, it is so small that some molecules can sneak past nonpolar phospholipid tails) Through specific water channel called quaporins (AQPs) o Flow occurs when water (or other solvent) concentration is different on the two sides of a membrane Osmolarity: measure of total concentration of solute particles Water concentration varies with number of solute particles because solute particles displace water molecules o When solute concentration goes up, water concentration goes down, and vice versa Water moves by osmosis from area of low solute (high water) concentration to high areas of solute (low water) concentration When solutions of different osmolarity are separated by a membrane permeable to all molecules, both solutes and water cross membrane until equilibrium is reached o Equilibrium : same concentration of solutes and water molecules on both sides, with equal volume on both sides When solutions of different osmolarity are separated by a membrane that is permeable only to water, not solutes, osmosis will occur until equilibrium is reached o Same concentration of solutes and water molecule on both sides, with unequal volumes on both sides Movement of water causes pressure: o Hydrostatic Pressure : pressure of water inside cell pushing on membrane o Osmotic Pressure: tendency of water to move into cell by osmosis The more solutes inside a cell, the higher the osmotic pressure A living cell has limits to how much water can enter it Water can also leave a cell, causing cell to shrink Change in cell volume can disrupt cell function, especially in neurons Tonicity o Ability of a solution to change the shape or tone of cells by altering the cells' internal water volume Isotonic solution has same osmolarity as inside the cell, so volume remains unchanged Hypertonic solution has higher osmolarity than inside cell, so water flows out of cell, resulting in cell shrinking Shrinking is referred to as crenation Hypotonic solution has lower osmolarity than inside cell, so water flows into cell, resulting in cell swelling Can lead to cell bursting, referred to alysing Diffusion Rates Factors affecting diffusion rate through a membrane: 1 Temperature ↑ temperature, ↑ motion of particles 2 Molecular Weight larger molecules move slower 3 Steepness of Concentrated Gradient ↑ difference, ↑ rate 4 Membrane Surface Area ↑ area, ↑ rate 5 Membrane Permeability ↑ permeability, ↑ rate Reverse Osmosis Pressure of blood on capillary walls (due to the heart) forces water and solutes into the surrounding tissues o Water and solutes can leak out of capillary inbetween cells Motor Molecules Receptor Mediated Endocytosis Pinocytosis: bringing ("drinking") in small, nonspecific particles Phagocytosis ( phago = to eat) Consumption of a large object such as a bacterium by another cell Uses pseudopods to engulf the object Pinocytosis is similar, but brings small particles into the cell **Pinocytosis & phagocytosis = endocytosis Exocytosis Secreting material out of cell Replacement of plasma membrane removed by endocytosis Transcytosis Transport of material across the cell by capturing it on one side and releasing it (more or less unchanged) on the other Receptormediated endocytosis moves it into cell and exocytosis moves it out the other side Example: how insulin is taken into cell Cytoplasm All cellular material that is located between the plasma membrane and the nucleus o Composed of: Cytosol: gellike solution made up of water and soluble molecules such as proteins, salts, sugars, etc. Inclusions: anything that isn't an organelle or fluid; insoluble molecules; vary with cell type (Examples: glycogen granules, pigments, lipid droplets, vacuoles, crystals) Organelles: metabolic machinery structures of cell; each with specialized function; either membranous or nonmembranous 3.7 Cytoplasmic Organelles Membranous o Mitochondria o Endoplasmic reticulum o Golgi apparatus o Peroxisomes o Lysosomes Nonmembranous o Ribosomes o Cytoskeleton o Centrioles Membranes allow compartmentalization, which is crucial to cell functioning Mitochondria Called the "power house" of cells because they produce most of the cell's energy molecules (ATP) via aerobic (oxygenrequiring) cellular respiration Enclosed by double membranes; inner membrane has many folds, called cristae o Cristae are embedded with membrane proteins that play a role in cellular respiration Mitochondria contain their own DNA, RNA, and ribosomes Resemble bacteria; capable of same type of cell division bacteria use, called fission Evolution of Mitochondrion Mitochondrion most likely evolved from bacteria that invaded another primitive cell, survived in the cytoplasm, and became permanent residents o Its two unit membranes suggests that the original bacterium provided the inner membrane, and the host cell's phagosome provided the outer membrane o Mitochondrial ribosomes more like bacterial ribosomes o Has its own mtDNA (mitochondrial DNA) Small circular molecule resembling bacterial DNA Replicated independently of nuclear DNA How are Mitochondria Passed On? When a sperm fertilized the egg, any mitochondria introduced by the sperm are usually destroyed, and only those provided by the egg are passed on to the developing embryo o Mitochondrial DNA is almost exclusively inherited through the mother Mutates more readily than nuclear DNA o No mechanism for DNA repair o Produces rare hereditary diseases o Mitochondrial myopathy, mitochondrial encephalomyopathy, and others Ribosomes Nonmembranous that are site of protein synthesis Made up of protein and ribosomal RNA (rRNA) Two switchable forms found in cell: o Free ribosomes: free floating; site of synthesis of soluble proteins that function in cytosol or other organelles o Membranebound ribosomes: attached to membrane of endoplasmic reticulum (ER); site of synthesis of proteins to be incorporated into membranes or lysosomes Endoplasmic Reticulum (ER) Consists of series of parallel, interconnected cisterns flattened membranous tubes that enclose fluidfilled interiors ER is continuous with outer nuclear membrane Two varieties: o Rough ER o Smooth ER Rough ER (protein synthesis!) o External surface appears rough because it is studded with attached ribosomes Site of synthesis of proteins that will be secreted from cell Site of synthesis of many plasma membrane proteins and phospholipids o Proteins enter cisterns as they are synthesized and are modified as they wind through fluidfilled tubes o Final protein is enclosed in vesicle and sent to Golgi apparatus for further processing Smooth ER o Network of looped tubules continuous with rough ER o Enzymes found in its plasma membrane (integral proteins) function in: Lipid metabolism; cholesterol and steroidbased hormone synthesis; making lipids for lipoproteins Absorption, synthesis, and transport of fats Detoxification of certain chemicals (drugs, pesticides, etc.) Converting of glycogen to free glucose Storage and release of calcium Sarcoplasmic reticulum is specialized smooth ER found in skeletal and cardiac muscle cells Peroxisomes Membranous sacs containing powerful detoxifying substances that neutralize toxins o Free radicals: toxic, highly reactive molecules that are natural byproducts of cellular metabolism; can cause havoc to cell if not detoxified o Two main detoxifiers: oxidase uses oxygen to convert toxins to hydrogen peroxide (H 2 2, which is itself toxic; however, peroxisome also contains catalase, which converts H O to harmless water 2 2 Peroxisomes also play a role in breakdown and synthesis of fatty acids Smooth ER and Peroxisomes are involved in detoxification of the body Lysosomes Spherical membranous bags containing digestive enzymes (acid hydrolases) o Considered "safe" sites because they isolate potentially harmful intracellular digestion from rest of cell Digest ingested bacteria, viruses, and toxins Degrade nonfunctional organelles Metabolic functions: break down an release glycogen; break down and release Ca from bone Intracellular release in injured causes cells to digest themselves (olysis) Endomembrane System Consists of membranous organelles discussed so far (ER, Golgi apparatus, secretory vesicles, and lysosomes), as well as the nuclear and plasma membranes These membranes and organelles work together to: 1. Produce, degrade, store, and export biological molecules 2. Degrade potentially harmful substances Inclusions Two kinds of inclusions o Stored cellular products Glycogen granules, pigments and fat droplets o Foreign bodies Viruses, intracellular bacteria, and dust particles and other debris phagocytized by cell Never enclosed in a nit membrane Not essential for cell survival (?) o Brain accumulates them as it ages Cytoskeleton Elaborate network of rods that run throughout cytosol Hundreds of different kinds of proteins link rods to other cell structures Also act as cell's "bones, ligaments, and muscle" by playing a role in movement of cell components Three types (smallest to largest): o Microfilaments o Intermediate filaments o Microtubules Microfilaments o Thinnest of all cytoskeletal elements o Semiflexible strands of proteinactin* o Each cell has a unique arrangement of strands, although share common terminal web Dense, crosslinked network of microfilaments attached to cytoplasmic side of plasma membrane Strengthens cell surface and helps to resist compression o Some are involved in cell motility, changes in cell shape, or endocytosis and exocytosis Intermediate Filaments o Size is inbetween microfilaments and microtubules o Tough, insoluble, ropelike protein fibers o Composed of tetramer (4)fibrilstwisted together, resulting in one strong fiber o Help cell resist pulling forces Filaments attach to desmosome plaques and act as internal guywires o Some have special names Called neurofilaments in nerve cells and keratin filaments in epithelial cells Microtubules o Largest of cytoskeletal elements; consists of hollow tubes composed of protein subunits calledtubulins, which are constantly being assembled and disassembled Most radiate from centrosome area of cell o Determine overall shape of cell and distribution of organelles Many organelles are tethered to microtubules to keep organelles in place Many substances are moved throughout cell by motor proteins , which use microtubules as tracks o Motor proteins : complexes that function in motility Can help in movement of organelles and other substance around cell Use microtubules as tracks to move their cargo on Powered by ATP Dynein and kinesin Centrosome and Centrioles Centrosome, which is located near the nucleus, means "cell center" It is a microtubule organizing center, consisting of a granular matrix and rioles a pair of barrelshaped microtubular organelles that lie at right angles to each other o Crucial for cell division Newly assembled microtubules radiate from centrosome to rest of cell o Some microtubules aid in cell division, and some form cytoskeletal track system Centrioles form the basis of cilia and flagella 3.8 Cellular Extensions Certain cell has structures extending from the cell surface: o Cilia and flagella aid in the movement of the cell or of materials across the surface of the cell o Microvilli are fingerlike projections that extend from the surface of the cell to increase surface area Do not move Cilia and Flagella Cilia are whiplike, motile extensions on surfaces of certain cells (such as respiratory cells) o Thousands of cilia work together in sweeping motion to move substances (Example: mucus) across cell surfaces in one direction Flagella are longer extensions that propel the whole cell (Example: tail of sperm) Both structures are made up of microtubules synthesized by centrioles that are called basal bodies because they form the base of each cilium and flagellum Cilia and flagella have "9 + 2" pattern of microtubules (9 sets of double tubes surrounding a central pair of doublets) o Slightly different from 9 +0 pattern of centriole (9 triplets with no tubules in center) Cilia movements alternate between power stroke and recovery stroke; this alteration produces a current at cell surface that moves substances forward Cystic Fibrosis Lungs create thick mucus o Saline layer is missing o Defect of Cl pumps (made but not "installed" in plasma membrane) Pancreas cannot absorb nutrients correctly Microvilli Minute, fingerlike extensions of plasma membrane that project from surface of select cells (Example: intestinal and kidney tubule cells) Used to increase surface area for absorption Have a core of actin microfilaments that is used for stiffening of projections Nucleus Largest organelle; contains the genetic library of blueprints for synthesis of nearly all cellular proteins o Responds to signals that dictate the kinds and amounts of proteins that need to be synthesized Most cells are uninucleate (one nucleus), but skeletal muscle, certain bone cells, and some liver cells are ultinucleate (many nuclei) o Red blood cells are nucleate (no nucleus) 3.9 Structure of the Nucleus The nucleus has three main structures: 1. Nuclear envelope 2. Nucleoli 3. Chromatin Nuclear Envelope o Doublemembrane barrier that encloses the jellylike fluid, the nucleoplasm Outer layer is continuous with rough ER and, like the rough ER, is studded with ribosomes Inner layer, called nuclear lamina, is a network mesh of proteins that maintains nuclear shape and acts as scaffolding for DNA o Nuclear pores allow substances to pass into and out of nucleus; they are guarded by the nuclear pore complex , which regulates transport of specific large molecules Nucleoli o Darkstaining spherical bodies within nucleus that are involved in ribosomal RNA (rRNA) synthesis and ribosome subunit assembly o Associated with nucleolar organizer regions that contain the DNA that codes for rRNA o Usually one or two per cell Chromatin o Consists of 30% threadlike strands of DNA, 60% histone proteins, and 10% RNA o Arranged in fundamental units called ucleosomes , which consist of DNA wrapped around histones Chemical alterations of histones have an effect on DNA and therefore can help regulate gene expression o Chromosomes are condensed chromatin Condensed state helps protect fragile chromatin threads during cell division Interphase G 1(gap 1) : metabolic activity and vigorous growth G 0: cells that permanently cease dividing S (synthesis): DNA replication G 2(gap 2): preparation for division DNA replication (in S phase) o Prior to division, the cell makes a copy of DNA o Doublestranded DNA helices unwind and unzip Replication fork : point where strands separate Replication bubble: active area of replication Each strand acts as a template for a new complementary strand o RNA starts replication by laying down short strand that acts as a primer o DNA polymerase attaches to primer and begins adding nucleotides to form new strand DNA polymerase synthesizes both new strands at one time (one leading and one lagging strand) o DNA polymerase works only in one direction, so leading strand is synthesized continuously; however, because lagging strand is "backwards," it is synthesized discontinuously into segments o Another enzyme, DNA ligase, then splices short segments of discontinuous lagging strand together End result: two identical "daughter" DNA molecules are formed from the original During mitotic cell division, one complete copy will be given to the new cell while one is retained in original cell Process is called emiconservative replication because each new doublestranded DNA is composed of one old strand and one new strand Sister Chromatids After S phase, a single chromosome consists of two sister chromatids o Each chromatid is a complete copy of that chromosome o In anaphase, the sister chromatids split apart pulled to opposite poles by the spindle fibers attached at the kinetochore o Each side is called a chromosome again after split Cell Division Most cells need to replicate continuously for growth and repair purposes o Skeletal, cardiac, and nerve cells do not divide efficiently damaged cells are replaced with scar tissue M (mitotic) phase of cell is phase in which division occurs; consists of 2 distinct events: o Mitosis: division of nongamete cells o Cytokinesis Control of cell division is crucial, so cells divide when necessary, but do not divide unnecessarily M Phase o Mitosis is the division of nucleus, in which the duplicated DNA is distributed to new daughter cells Four stages of mitosis ensure each cell receives a full copy of replicated DNA 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase o Prophase can be broken into two parts: Early prophase 1. Chromatin condenses, forming visible chromosomes 2. Each chromosome and its duplicate (called sister chromatids) are held together by a centromere 3. Centrosome and its duplicate begin synthesizing microtubules that push each centrosome to opposite poles of cell Called the mitotic spindle Other microtubules called asters radiate from centrosome Late Prophase 1. Nuclear envelope breaks up 2. Special microtubules attach to specific area on centromeres called kinetochore and serve to pull chromosomes to center (equator of cell) 3. Remaining nonkinetochore microtubules push against each other, causing poles of cell to move further apart Metaphase Centromeres of chromosomes are precisely aligned at cell's equator The imaginary plane midway between poles is called metaphase plate Anaphase Shortest of all phases Centromeres of chromosomes split simultaneously each sister chromatic now becomes a separate chromosome Chromosomes are pulled toward their respective poles by moto proteins of kinetochores 1. One chromosome of each original pair goes to opposite poles Nonkinetochore microtubules continue forcing poles apart Telophase Begins when chromosome movement stops Each set of chromosomes (at opposite ends of cell) uncoils to form chromatin New nuclear membranes form around each chromatin mass Nucleoli reappear Spindle disappears Nuclear envelope rebuilds Cytokinesis Begins during late anaphase and continues through mitosis Ring of actin microfilaments contracts to form cleavage furrow Two daughter cells are pinched apart o Control of cell division "Go" and "Stop" signals direct when a cell should and should not divide Go signals include: Critical surfacetovolume ratio of cell, when area of membrane becomes inadequate for exchange Chemicals (Example: growth factors, hormones) Stop signals include: Availability of space; normal cells stop dividing when they come into contact with other cells Referred to as contact inhibition o Checkpoints are key events in the cell cycle where cell division processes are checked and, if faulty, stopped until repairs are made G 1checkpoint (restriction point) is the most important of the three major checkpoints If cell does not pass, it enter0G , in which no further division occurs Mitosis Result of mitosis is two identical daughter cells Gametes undergo a similar process called meiosis o Result of meiosis is four daughter cells, each with a different combination of genes 3.11 Protein Synthesis DNA is master blueprint that hold the code for protein synthesis o DNA directs the order of amino acids in a polypeptide A segment of DNA that holds the code for one polypeptide is referred to as a gene o Alternative splicing The code is determined by the specific order of nitrogen bases (Adenine, Guanine, Thymine, Cytosine) in the gene o Code consists of three sequential bases (base triplet code) Example: GGC codes for amino acid proline, whereas GCC codes for arginine o Each triplet specifies the code for a particular amino acid Genes are composed of exons and introns o Exons are part of gene that actually codes for amino acids o Introns are noncoding segments interspersed amongst exons "junk" DNA DNA Nucleotide Structure DNA deoxyribonucleic acid o 46 DNA molecules in the nucleus of most human cells Total length of 2 meters Average DNA molecule ~2 inches long DNA and other nucleic acids are polymers of nucleotides Each nucleotide consists of o One sugar: deoxyribose o One phosphate group o One nitrogenous base Either pyrimidine (single carbonnitrogen ring) or ine (double ring) Nitrogenous Bases: A, C, T, G, and U Purines: double ring o Adenine (A) o Guanine (G) Pyrimidines: single ring o Cytosine (C) o Thymine (T) DNA bases: ATCG RNA bases: AUGC The Role of RNA RNA is the "gobetween" molecule that links DNA to proteins o RNA copies the DNA code in nucleus, then carries it into cytoplasm to ribosomes All RNA is formed in nucleus RNA differs from DNA o Uracil is substituted for thymine in RNA o RNA has ribose instead of deoxyribose sugar Three types of RNA: 1. Messenger RNA (mRNA) 2. Ribosomal RNA (rRNA) 3. Transfer RNA (tRNA) Messenger RNA o Single stranded o Code from DNA template strand is copied with complementary base pairs, resulting in a strand of mRNA Process is referred to as nscription o mRNA maintains the triplet code (codon) from DNA Ribosomal RNA o Structural component of ribosomes, the organelle where protein synthesis occurs o Along with tRNA, helps to translate message from mRNA into polypeptide Transfer RNA o Carrier of amino acid o Have special areas that contain a specific triplet anticodon ) that allows each tRNA to carry only a specific acid o Anticodon of tRNA will complementary basepair with codon of mRNA at ribosome, adding its specific amino acid to growing polypeptide chain Process is referred to as nslation Protein Synthesis Occurs in two steps: o Transcription DNA information coded in mRNA (happens in nucleus) o Translation mRNA decoded to assemble polypeptides (happens somewhere in the cytoplasm) Transcription Process of transferring code held in DNA gene base sequence to complementary base sequence of mRNA Transcription factors (protein complex) activate transcription by: o Loosening histones from DNA in area to be transcribed so DNA segment can be exposed o Binding to special sequence of gene to be transcribed, called promoter (starting point) Occurs only on DNA template strand o Mediating binding of RNA polymerase , enzyme that synthesizes mRNA, to promoter region Transcription is broken down into three phases: o Initiation RNA polymerase separates DNA strands o Elongation RNA polymerase adds complementary nucleotides to growing mRNA matching sequence of based on DNA template strand Short, 12basepair segment where DNA and mRNA are temporarily bonded is referred to as DNARNA hybrid o Termination Transcription stops when RNA polymerase reaches special termination signal code Processing of mRNA o Newly formed mRNA is then edited and processed before translation can begin Before processing, it is referred to as premRNA o Introns are removed by special proteins called spliceosomes, leaving only exon coding regions Translation Step of protein synthesis where the language of nucleic acids (base sequence) is translated into the language of proteins (amino acid sequence) Process involves: o mRNA o Genetic code o tRNA and ribosomes o Translating events o And sometimes the rough ER Genetic Code Each threebase sequence on DNA (triplet code) is represented by a complementary threebase sequence on mRNA called codon There are 64 possible codons 3 4 bases (A, U, C, G) and three places, so 4 = 64 There are 3 "stop" codons but rest are codons for amino acids There are only 20 possible amino acids, so this means that some amino acids are represented by more than one codon Redundancy helps protect against transcription errors > mutations tRNA tRNA binds a specific amino acid at one end (stem); once amino acid is loaded onto tRNA, molecule is now called aaminoacyltRNA Anticodon at other end (head) is triplet code that determines which amino acid will be bound at stem Example: tRNA with anticodon UAU will only be able to load a methionine amino acid to its stem region Anticodon of tRNa will bind only to codon on mRNA that is complementary Example: if codon is AUA, only a tRNA with anticodon UAU will be able to bond Ribosomes coordinate coupling of mRNA and tRNA Ribosomes contain one binding site for mRNA and three binding sites for tRNA: Aminoacyl site for incoming aminoacyltRNA Peptidyl site for tRNA linked to growing polypeptide chain Exit site for outgoing tRNA Translating Events Translation occurs in three phases that require ATP, protein factors, and enzymes Initiation Elongation Termination Initiation Small ribosomal subunit binds to a special initiator tRNA (methionine) and then to the mRNA to be decoded Ribosome scans mRNA looking for first methionine codon, which is referred to as the start codon When anticodon of initiator tRNA binds to start codon, large ribosomal unit can attach to small ribosomal unit forming a functional ribosome At end of initiation, initiator tRNA is in P site of ribosome, and A and E sites are empty Elongation : involves three steps: 2a. Codon recognition: tRNa binds complementary codon in A site of ribosome 2b. Peptide bond formation: Ribosomal enzymes transfer and attach growing polypeptide chain from tRNA in P site over to amino acid of tRNA in A site 2c. Translocation : ribosome shifts down three bases of mRNa, displacing tRNAs by one position tRNA in A site moves into P site tRNA in P site moves into E site tRNA in E site is ejected from ribosome Once A site is empty, a new tRNA can enter, bringing its amino acid cargo, and while process starts over After a portion of mRNA is "read," additional ribosomes may attach to already read part and start another round of translations of same mRNA Polyribosome is a multiple ribosomemRNA complex that produces multiple copies of same protein Termination When one of three stop codons (UGA, UAA, UAG) on mRNA enters A site, translation ends Proteins release factor binds to stop codon, causing water to be added to chain instead of another tRNA Causes release of polypeptide chain as well as separation of ribosome subunits and degradation of mRNA Final polypeptide product will be further processed by other cell structures into functional 3D protein Rough ER A short amino acid segment, called the ER signal sequence , present on a growing polypeptide chain, signals associated ribosome to dock on rough ER surface Signalrecognition particle (SRP) on ER directs mRNAribosome complex where to dock Once docked, forming polypeptide enters ER Sugar groups may be added to protein, and its shape may be altered Protein is then enclosed in vesicle for transport to Golgi apparatus Summary: From DNA to Proteins Complementary base pairing directs transfer of genetic information in DNA into amino acid sequence of protein o DNA triplets are coded to mRNA codons o mRNA codons are basepaired with tRNA anticodons to ensure correct amino acid sequence Anticodon sequence of tRNA is identical to DNA sequence, except uracil is substituted for thymine 3.12 Apoptosis, Autophagy, and Proteasomes Cells that have become obsolete or damaged need to be taken out of system Autography (selfeating) is the process of disposing of nonfunctional organelles and cytoplasmic bits by forming autophagosomes , which can then be degraded by lysosomes Unneeded proteins can be marked for destruction by ubiquitins o Proteasomes disassemble ubiquitintagged proteins, recycling the amino acids and ubiquitin Apoptosis , also known as programmed cell death causes certain cells (examples: cancer cells, infected cells, old cells) to neatly selfdestruct o Process begins with mitochondrial membranes leaking chemicals that activate enzymes called caspases o Caspases cause degradation of DNA and cytoskeleton, which leads to cell death o Dead cell shrinks and is phagocytized by macrophages Stem Cells Stem cell: unspecialized cell capable of reproducing itself and giving rise to more specialized cells o Totipotent : can give rise to an entire organism or any type of cell in the body (e.g., zygote, clone) o Pluripotent : can produce any type of tissue, but not an entire organism o Multipotent: can produce a limited range of cell types; often divide asymmetrically (e.g., hematopoietic stem cells produce blood and bone marrow cells) Developmental Aspects of Cells All cells of body contain same DNA, but not all cells are identical or carry out same function Chemical signals in embryo channel cells into specific developmental pathways by turning some genes on and others off Developmental of specific and distinctive features in cells is called cell differentiation Cell Destruction and Modified Rates of Cell Division Organs are well formed and functional before birth, but we need cell division for growth Cell division in adults is needed to replace shortlived cells and repair wounds Hyperplasia is accelerated growth that increases cell numbers when needed Atrophy is a decrease in size that results from loss of stimulation or use Cell Aging The mechanism of aging is a mystery, but there are several theories: o Wear and tear theory : a lifetime of chemical insults and free radicals have cumulative effects o Mitochondrial theory of aging : free radicals in mitochondria diminish energy production o Immune system disorders : autoimmune responses, as well as progressive weakening of immune response o Genetic theory : cessation of mitosis and cell aging are programmed into genes Telomeres are strings of nucleotides that protect ends of chromosomes (like caps on shoestrings) Every time a cell divides, the telomere shortens, so telomeres may act like an hourglass on how many times a cell can divide Telomerase is an enzyme that lengthens telomeres Found in germ cells of embryos but absent in adult cells, except for cancer cells
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'