Biology 5A Notes Midterm2
Biology 5A Notes Midterm2 Biol 5A
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Date Created: 06/16/16
CELL STRUCTURE: Nucleus ● Contains cell DNA in the form of linear chromosomes (Mitochondria and chloroplasts also have their own DNA) ● Transcription occurs here and messenger RNAs have to be exported through nuclear pores to the cytoplasm ● Synthesized in the cytoplasm: ○ Protein components of ribosomes ○ Proteins that are involved in regulation of gene expression ○ Proteins involved transcription ● In mitosis, nuclear envelope breaks down, reassembled after chromosomes are moved to daughter cells Ribosomes ● Responsible for carrying out the process of translation (mRNA is translated into a polypeptide chain) ● Consists of many different protein and RNA components (rRNA) ● rRNA and tRNA are RNA molecules that are transcribed from DNA but do not undergo translation to make a protein ● Can be found free in the cytosol or bound to the cytosolic face of the ER Endomembrane System ● Includes many of the cell’s internal membranes that form part of a dynamic network where many macromolecules are synthesized, modified, and moved through the cell using transport vesicles ● Includes nuclear membrane, ER, golgi apparatus, and plasma membrane, and vacuoles and lysosomes ● ER: membrane system continuous with the nuclear envelope and found near the nucleus ○ Smooth ER: not bound to ribosomes ■ carries out metabolic functions (synthesis of lipids and metabolism of carbohydrates) ■ processes toxic drugs by modifying them ■ stores ions like Ca++ so they can be released to trigger other events ○ Rough ER: bound to ribosomes ■ where proteins will be secreted are first made ■ ribosomes are targeted to the ER by specific amino acids in the amino part of the polypeptide chain ■ the polypeptide is synthesized so it is delivered into the ER lumen ■ Many of the proteins are modified by adding carbohydrates to make glycoproteins ■ Proteins are then moved to the golgi using transport vesicles ■ Also where membrane proteins and membranes are made ○ Golgi Apparatus: system of membranes that is similar to the ER except that ribosomes don’t bind it and is not continuous with the ER or nuclear membrane ■ Both ER and Golgi have flattened membrane called cisternae ■ carbohydrates on proteins can be modified and some macromolecules are made ■ “Packaging center”: materials in vesicles enter on the cis face towards the nucleus, get processed in the Golgi, and shipped out on the trans face towards the plasma membrane ■ Products and vesicles that hold them can be tagged to bring them to the right destination ■ Proteins that are secreted from a cell, transport vesicles deliver stuff from the Golgi to plasma membrane, where exocytosis occurs ○ Lysosomes: vesicles that have enzymes that can digest macromolecules by hydrolytic enzymes (hydrolases) ■ internal compartment has a low pH; environment favors digestion ■ can fuse with vesicles that have food that entered the cell by phagocytosis ● brings the lysosomal enzymes and food into the same cellular compartment ■ digest and recycle materials in cell (autophagy) ■ TaySachs disease: results from inherited defect in gene encoding hexosaminidase A, enzyme that breaks down a type of lipid in lysosomes ● it builds up in cells (particularly in the brain); death before age 5 ■ Peroxisomes: type of singlemembrane organelle that has enzymes in a “crystalline core” to break down molecules ● transfers hydrogens from molecules to oxygen to make O 2 2 (peroxide) to detoxify alcohols and digest fatty acids ● diseases that affect peroxisomes result in accumulation of lipids in cells and affects the nervous system and other tissures ○ Vacuoles: other membraneenclosed sacs ■ Plants and fungi have digestive vacuoles similar to animal lysosomes ■ Protists have contractile vacuole that stores water and salts (important for osmotic balance) ■ Plants have a large central vacuole that has cell sap that contains potassium and chloride ions ● can absorb water and contribute to regulation of cell size Mitochondria ● Convert energy from one form to another ● Almost all eukaryotic cells have mitochondria ● Has an inner and outer membrane ○ inside: matrix ○ region between membranes: intermembrane space ■ has many folds called cristae, increases surface area Chloroplasts ● Convert energy from one form to another ● found in plants and algae ● Use energy from sunlight to convert C + HO into sugars and organic compounds 2 2 ● Chlorophyll is found in chloroplasts and gives plants the green color ● Within the Chloroplasts is a system of stacked membranous discs (thylakoids; stacks are called grana) ● Inside chloroplasts is the stroma ○ these regions carry out photosynthesis The Endosymbiont Theory ● Chloroplasts and mitochondria are not part of the endomembrane system ● Similar to bacteria: ○ contain their own DNA chromosome ○ have their own transcription and translation, including ribosomes ○ control their own replication and division ● Proposes that an ancestor of eukaryotic cells engulfed a prokaryote cell that was similar to a mitochondrion; the two became dependent on one another. Later, one the the cells engulfed a photosynthetic prokaryote to become a prototypical plantlike cell Cytoskeleton ● Supports the cell and maintains its shape ● Provides a scaffold for motor proteins to move cellular structures ● Helps produce cellular motion ● Made of 3 types of fibers; all function in maintenance of cell shape ○ Microtubules (25 nm diameter): polymers of alpha and betatubulin ■ function in chromosome movements in mitosis and movement of organelles ■ give flagella or cilia their ability to move ○ Microfilaments (7nm): has 2 intertwined strands of actin polymers ■ function in changes in cell shape (formation of pseudopods, cell projection) ■ forms the cleavage furrow during cytokinesis ○ Intermediate filaments (812 nm): made of proteins such as keratin ■ anchor the nucleus and other organelles in place ■ form part of the nuclear lamina METABOLISM: ● Processes can be broken down into pathways that have multiple steps which are catalyzed by enzymes ● In each step, a small number of covalent bonds or functional groups are changed at a time ● Catabolic: pathways that break down larger molecules into smaller ones and release energy ● Anabolic: pathways that build larger molecules and consume energy ○ also called biosynthetic pathways Thermodynamics: ● Kinetic Energy: energy in motion ○ in atoms and molecules, it causes random motion (heat) ● Light energy is used in photosynthesis ● Potential Energy: stored energy ● Chemical Energy: a form of stored energy in life systems (ie. glucose and fats) ● Thermodynamics: the study of transfer of heat ○ First law of thermodynamics: energy can neither be created or destroyed, can only change the form of it ○ Second law of thermodynamics: every transfer of energy increases the entropy (disorder) of the universe ○ Spontaneous processes: processes that occur without an input of energy ■ energetically favorable (objects falling toward earth, molecules diffusing, or larger molecules are broken down into smaller ones) ○ Life systems are ordered but doesn’t violate second law because they increase the overall entropy of the universe ■ organisms take molecules from environment, harness energy, and release smaller molecules and heat ● Gibbs Equation: change in free energy of a system ○ ΔG = ΔH TΔS ■ ΔG: change in free energy, energy that can do work (kcal/mol) ■ ΔH: the change in enthalpy, total energy of the system (kcal/mol) ■ ΔS: change in entropy, energy that cannot do work (kcal/mol K) ■ T: Temperature (K) ■ ΔG = G G productsreactants ■ When ΔG is negative: spontaneous reaction; products will have less free energy than reactants (Exergonic/ exothermic reaction) ■ When ΔG is positive: nonspontaneous reaction; will absorb free energy from surroundings (endergonic/ endothermic reaction) ○ Cellular respiration is spontaneous and releases energy (686 kcal/mol) ■ spontaneous doesn’t mean instantly there is activation energy to overcome, bonds to be broken; when products form, compounds have lower free energy ● Living organisms are “open” systems ○ Isolated (closed) system: system will generate energy to do work only until it reaches equilibrium; no energy is added ○ Open system: constantly obtaining sources of energy from environment and releasing heat and more simple compounds ■ living systems are never at equilibrium ● With a positive ΔG, cells perform multiple types of work ○ ie) building polymers (chemical work), transporting substrates up a concentration gradient (transport work), and movement (mechanical work) ○ Energy coupling: combine endergonic and exergonic reactions and the overall process is exergonic ■ Most in cells use hydrolysis of ATP ● energy is released when ATP is hydrolyzed to produce ADP + P because of the high amount of negative charge in this region of the molecule ● charges repel and breaking one of the bonds releases energy ● Hydrolysis of ATP is reversible ○ cells harness energy from other sources to create ATP from ADP + P ■ Example of energy coupling: how cells convert glutamic acid to glutamine ● endergonic reaction with ΔG = +3.4 kcal/mol ● couples with ATP hydrolysis so overall ΔG = 7.3 +3.4 = 3.9 ○ exergonic and energetically favorable (spontaneous) ■ In some reactions, the phosphate released from ATP bonds to one of the reactants, forming a phosphorylated intermediate ● intermediate is more unstable than original molecule Enzymes: ● Enzymes: macromolecules that act as catalysts that speed up a reaction without being consumed by it ● Most are proteins and fold into specific shapes that fit the reactants ● Reactants interact with an enzyme’s active site in an “induced fit” mechanism ● When products are formed, products are released ● Chemical reactions involve breaking old bonds and forming new ones ○ Energy of activation A: reactants absorb enough energy to start the reaction ○ Enzymes work by lowering the energy of activation for a reaction (does not change the ΔG) ● Enzymes have optimal conditions in which they will function because they have to fold into a specific shape ○ Temperature, pH, and concentration of other ions affect enzyme function ● Enzyme activity can be regulated ○ cells can decide when to actually make enzymes by determining when the gene for the enzyme is expressed ○ for most enzymes, makes more sense to keep them around in the cell and to regulate their activity so they function only when needed ○ Example: In the synthesis of amino acid isoleucine from threonine, then end product of the pathway, isoleucine, can bind to the first enzyme, inhibiting it. The binding occurs in a different region than the active site which binds to threonine. The enzyme changes shape once binded to isoleucine and cannot bind to threonine. This is an allosteric change. ● Allosteric change: change in an enzyme’s shape that results in a change of function ● Feedback inhibition: regulation of an enzyme by a later product of a pathway in which it participates ● Other ways enzymes can be regulated: ○ Localization of the enzyme to a particular compartment of the cell (enzymes in cellular respiration are in the mitochondria) ○ A requirement of the enzyme to bind a particular activator (not a substrate or product but important for enzyme activity) ○ Competitive inhibition: presence of an inhibitor molecule that competes with the substrate for binding to the active site ○ Noncompetitive inhibition: molecule binds to an enzyme away from the active site CELLULAR RESPIRATION Energy Flow in Ecosystems: ● There is a flow of energy into systems, and a release of waste products and heat ● Entropy of the universe increases even though life systems are highly ordered ● In our ecosystem, sunlight is the ultimate energy source ○ used by plant chloroplasts to fix carbon fro2nto larger organic molecules to generate O 2 ● Cellular respiration in mitochondria harvests the energy in organic molecules, returning the electrons in these molecules to lower energy states and HO, using some of 2 2 the energy to make ATP ○ Plants have to generate ATP for cellular processes from the organic molecules they create themselves Cellular Respiration: ● Redox reactions involve electrons moving from one reactant to another ● In cellular respiration, the electrons in larger organic molecules have higher potential energy than they do in products ● Oxidized: when electrons are removed from something ● Reduced: when electrons are added to something ● Equation of cellular respiration: ○ C6 12 6O2→ 6CO2+6H2 ○ Glucose is oxidized into carbon dioxide ■ lose electrons and carbon skeleton will be broken up and released as carbon dioxide ○ Oxygen is reduced into water ■ accepts electrons from glucose and protons from hydrogen to make water ● Overall G is 686 kcal/mol, so glucose oxidation is spontaneous but it will take some activation energy to get the process started ● Electrons are moved in small steps to release their energy, generating ATP when the electrons reach oxygen in the mitochondria ● Electron carriers: molecules that are temporarily reduced in order to transfer electrons from one process to another ○ NAD+ (nicotinamide adenine dinucleotide) can accept 2 electrons and one proton to become NADH in its reduced form ○ FAD (flavin adenine dinucleotide) can accept 2 electrons and 2 protons to become FADH 2 ■ Note: becoming reduced means accepting electrons and usually also accepting protons; if hydrogens are on a molecule, molecule is in a reduced state ○ NADH and FADH 2represent “reducing power” ■ the ability to donate electrons to other molecules, reducing them, allowing other processes to take advantage of the energy that can be released ● Overview of Cellular Respiration Glycolysis: ● Glycolysis occurs in nearly all living things ● Eukaryotes: occurs in cytosol without oxygen ● Occurs in 2 phases: ○ 2 ATP are consumed and phosphates are directly added to glucose which destabilizes the molecule and gets split into 2 3carbon molecules each with one phosphate ○ the 2 3carbon molecules are both oxidized to make 2 NADH and a second phosphate is added to each. The phosphates become transferred to ADP to create 4 ATP by substratelevel phosphorylation ○ End result: 2 3carbon molecules called pyruvate (pyruvic acid). Produces some energy and reducing power but there is a lot of potential energy stored in pyruvate Pyruvate Oxidation: ● In the presence of oxygen, pyruvate will enter the mitochondria (without oxygen it will undergo fermentation) ● After it is imported by a transport protein (it is highly polar), pyruvate will undergo 3 enzymatic manipulations 1. carboxylic acid group is removed to release a CO 2 2. the 2carbon compound left is then oxidized to form acetate (CH COO), transferring electrons to NAD+ to make 3 NADH 3. Coenzyme A will become covalently bonded to the acetate to make Acetyl CoA. This allows the acetyl group to enter the Citric Acid Cycle for the rest of its oxidation to 2 The Citric Acid Cycle (Krebs Cycle/ Tricarboxylic Acid Cycle) ● Cycle: set of reactions that produces products from reactants and regenerates the starting materials (which act as coenzymes) ● Citric acid cycle removes the rest of the high potential energy electrons from acetyl, making CO 2a small amount of ATP, and a lot of reducing power (NADH and FADH 2 ● Takes place mostly in the mitochondrial matrix ● AcetylCoA enters the pathway, acetyl is added to oxaloacetate, a 4carbon compound, to make citrate (6carbon) ● This is converted to the isomer, isocitrate, and then oxidized to release 1 CO 2nd reduce NAD+ to NADH ● The loss of CO 2results in a 5 carbon compound aketoglutarate. This happens again with CoA, to make 4 carbon compound succinylCoA, and making another CO 2 ● Oxaloacetate is regenerated over the next steps Oxidative Phosphorylation: Electron Transport and Chemiosmosis ● The reducing power from NADH and FADH 2 will be used to generate energy to synthesize ATP (2 parts) 1. electrons will be contributed to complexes of proteins (and other molecules) found in inner mitochondrial membrane. a. Each of the 3 complexes (I, III, and IV) is more electronegative than the previous one and when electrons are transferred to each, energy released is used to pump a proton (H+) from the matrix into the intermembrane space. b. NADH donates electrons to the start of the complex; FADH 2donates electrons farther down. c. Oxidized electron carriers return to the citric acid cycle d. After electrons have almost no energy left, they are finally accepted by oxygen (addition of protons will complete the reduction of oxygen to water) 2. Chemiosmosis generates ATP a. Pumping of protons into intermembrane space creates an electrochemical gradient of protons. Stored charge difference called protonmotive force can be used to do work b. Channel protein, ATP synthase, provides a return path for protons to matrix c. Energy that is released is used to join ADP + P to make ATP ● ATP synthase is a “motor” : had a large rotor that spins around as protons pass through it ○ motion activates catalytic sites that cause generation of ATP from ADP + P i ● Oxidative phosphorylation is where most energy from glucose is used to make ATP ○ Glycolysis makes 2 ATP per glucose, citric acid cycle makes 2 net ATP, 2628 ATP is make by ET and chemiosmosis ○ 1 mole of glucose can make about 3032 moles of ATP Fermentation: ● In order to turn over NAD+ in the absence of oxygen, cells use fermentation ● Alcohol fermentation: carried out by bacteria and yeast ○ pyruvate is converted to a 2carbon compound and CO 2and the 2carbon compound is reduced by NADH to generate ethanol and NAD+ ○ recycles NAD+ so glycolysis can continue but generates ethanol and CO2 ○ used in beer making ● Lactic acid fermentation: used by bacteria, fungi, and animals in the absence of oxygen ○ pyruvate is directly reduced by NADH to generate lactic acid and NAD+ ○ In humans, lactic acid is sent from muscles to the live, where it is converted back into pyruvate Regulation of Cellular Respiration ● Catabolism is regulated at many steps depending on the energy needed by the cell PHOTOSYNTHESIS ● Photosynthesis: process where light energy is converted into chemical energy and stored in the bonds of organic molecules like glucose ● Takes place in cells of the mesophyll layer of leaves ● Equation: 6CO2+ 6H2 + light energy → 6 12 6 6O2 ○ Carbon dioxide becomes reduced to glucose ○ Water becomes oxidized to oxygen ○ Endergonic and has a positive ΔG, energy comes from light ● Takes place in 2 stages: ○ lightdependent reactions (light reactions) ■ takes place in the thylakoid membrane stacks (grana) of chloroplasts ○ lightindependent reactions ■ Calvin cycle: synthesize sugars precursors by adding electrons and protons to CO 2 ● Energy comes from NADPH and ATP that are made from NADP+ and ADP + P by the lightdependent reactions using electrons in water ● When water is oxidized, oxygen is created ■ Takes place in the stroma ● NADPH and NADP+ have the same function except NADPH is found in photosynthesis and has a phosphate group Light Reactions ● Use light energy to help make ATP and NADPH ● Light energy excites electrons to higher energy states which sends them into orbitals have have a higher potential energy (Photoelectric effect) ● Light energy is absorbed by chlorophyll a and b ○ Chlorophyll a and b have different functional groups and have a different wavelength of light they absorb ○ Chlorophyll is a large molecule that had a porphyrin ring with Mg++ in the center and a long hydrocarbon tail ○ Chlorophyll a directly participates in the light reactions ○ Chlorophyll b and other pigments called carotenoids, play an accessory role by protecting leaves from wavelengths of light that are harmful and extending wavelengths that can be used in photosynthesis ○ Chlorophyll does not absorb green which is why they reflect green light giving the plant a green color ○ When chlorophyll is isolated, energy that is absorbed is given off as heat ● Photosystems: a complex of organic molecules and proteins that chlorophyll must be a part of for the excited electrons to be used ○ Two types within the thylakoid membrane ○ In a photosystem, chlorophyll molecules are embedded in a lightharvesting complex ○ Light energy will excite the electrons and travel around the photosystem until they reach the reactioncenter complex and then the primary electron acceptor ● When light hits the lightharvesting complex in photosystem II, electrons are excited to higher energy states ○ the electrons travel around until they reach 2 special chlorophyll molecules in the reactioncenter complex called P680 (680 nm is the optimal wavelength of light that the electrons absorb ○ Electrons travel from the P680 chlorophyll molecules to the primary electron acceptor (just chlorophyll without the Mg++ ion) ■ Low energy electrons from water replace the electrons lost from P680 ■ An enzyme catalyzes the splitting of water that produces oxygen and protons ○ From the primary electron acceptor, electrons travel down an electron transport chain, making energy that is used to pump protons into the thylakoid space ■ Using the proton motive force, ATP synthase makes ATP by chemiosmosis ○ Light hits photosystem 1 and the electrons go to P700 (chlorophyll at the reaction center complex of photosystem I) ■ Here electrons move to a different primary electron acceptor and down a short electron transport chain where NADP+ reductase reduces NADP+ and adds a proton to make NADPH ● Light reactions use the energy from sunlight to make ATP from ADP + P ro make NADPH from NADP+ and H+, and split water to make O 2 Similarities of ETC in Photosystem II and Cellular Respiration ● In chemiosmosis in mitochondrias and chloroplasts ○ both use energy released in multiple redox reactions (ETCs) to pump protons into a different compartment ○ stored energy is used by ATP synthase that allows the protons to diffuse down the electrochemical gradient and make ATP from ADP +P ○ Difference: cellular respiration gets electrons from glucose; photosynthesis the electrons are from water but are excited to a higher energy by light Calvin Cycle: ● Uses ATP and NADPH from the light reactions to reduce CO2and make sugars ● CO2is brought into the Calvin cycle ● Energy from ATP and NADP will make sugars ○ **End product isn’t glucose: glyceraldehyde3phosphate (G3P), a 3carbon compound used to make sugars (also an intermediate in glycolysis) ● Carbon fixation: reduction of 2o make sugars ● 3 Phases in the Calvin cycle ● Each turn of the cycle brings in a single CO2but 3 turns in the cycle produces a single G3P ○ 3 starting molecules that produce G3P:Ribulose bisphosphate (RuBP) ○ Phase 1: Carbon fixation ■ each of the 5 carbon RuBP becomes covalently joined to a C2 ■ begins the cycle and carbon is joined to a larger skeleton ■ ribulose bisphosphate carboxylase oxygenase (RuBisCo): enzyme that catalyzes this step. (has several polypeptides and one of the most abundant proteins on earth) ○ Phase 2: Reduction ■ the 6 carbon intermediate splits into 2 ■ each of the 3 carbon molecules get a phosphate from 3 ATP and then gets reduced ■ protons and electrons are added from NADPH to make 6 of the 3 carbon compound G3P ■ 3 carbons have gone into the cycle so only one G3P remains and leaves the cycle as a product ○ Phase 3: Regeneration of the C2acceptor (RuBP) ■ the rest of the calvin cycle uses energy from ATP to take the remaining 5 G3P (5 x 3 carbon) and rearranges them into 3 5 carbon RuBP (3 x 5 carbon) at the beginning of the cycle ● Calvin cycle fixes 2into sugar precursors using electrons from NADPH and energy from ATP ● Regenerates NADP+ and ADP + P for use in light reactions ● Does not require light (dark reactions) DNA Nucleic Acids ● First isolated as a material in the nucleus by Meicher in the late 19th century ● Two types of nucleic acids: ○ Ribonucleic acids (RNA) ■ short lived intermediate that is used during gene expression ○ Deoxyribonucleic acid (DNA) ■ carries genetic information that is passed from generation to generation ■ undergoes transcription that make mRNA that undergoes translation to make proteins ● Central Dogma: flow of information from DNA to protein ● Nucleotide: building block of nucleic acid ○ Has 3 parts: ■ Ribose sugar (deoxyribose in DNA) ● Pentose in a ring structure ● Carbons are numbered 1’ (one prime) and so on; 5’ is where the phosphate group is joined ● In RNA, a hydroxyl group is on the 2’ carbon but in DNA it is only a hydrogen ■ Nitrogenous base ● 5 bases: adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U) ● A and G are purines and have 2 rings ● C, T, and U are pyrimidines and have 1 ring ● T is only in DNA, U is only in RNA ■ Phosphate group ■ Sugar + nitrogenous base is a nucleoside ● Deoxynucleoside Triphosphates (dNTPs): building blocks for DNA synthesis inside the cell ○ contains highenergy bonds that can be used to make chemical bonds ● Nucleotides are joined by phosphodiester bonds ○ backbone consists of alternating sugars and phosphates with nitrogenous base attached to the sugar ● Polymerase: enzymes that synthesize DNA or RNA ○ uses nucleoside triphosphates (NTPs or dNTPs) as building blocks and preexisting DNA molecules as a template ● Order of bases along a single DNA chain is the primary structure (or sequence) ○ Secondary structure: the way the single chains interact with each other ● DNA and RNA store genetic information as the order of the bases along the length of the molecules ● Why do cells need 2 systems with different bases? ○ DNA is better for long term information storage than RNA ○ believed that first living organisms used RNA as their genetic material and DNA displaced it because their backbone is more stable and thymine is more resistant to mutation than uracil ● Properties of genetic material: ○ Carries information ○ Capable of being copied ○ Capable of undergoing change over time The Griffith Experiment (1928) ● Federick Griffith studied Streptococcus pneumoniae in mice (bacteria that causes pneumonia) ○ work with 2 strains of the bacteria: smooth and rough ■ Smooth: encased in polysaccharide capsule that allows them to infect and kill ■ Rough: no capsule, mouse can kill off bacteria and survive ■ Heatkilled smooth: heat destroys the ability smooth bacteria has to infect ■ Heatkilled smooth mixed with rough: injection of this combo causes the mice to die of pneumonia ○ Mice were infected with the combination of the 2 and found that they carried living smooth bacteria ■ called this transformation: the dead smooth bacteria transformed the rough, nonlethal bacteria into lethal ones Avery, MacLeod, and McCarty (1944) ● Purified the “active principle” which had all the properties of DNA known at the time ● Added the purified material to different enzymes that destroy RNA, proteins, or DNA (RNAses, proteases, DNAses) to prove it was DNA ○ Exposed the resulting mixtures to rough bacteria ○ Resulted in only DNAses destroyed the ability of the purified material to transform rough cells ■ DNA was the transforming principle The HersheyChase Experiment (1952) ● Bacteriophage T2: virus that infects the bacterium E. coli ○ consists of DNA in a protective coat ○ adheres to the outside of an E. coli bacterium ○ within 30 minutes E. coli cells produce large amounts of virus particles themselves ● Questioned “What part of the bacteriophage is responsible for transferring the information needed to make more T2?” ○ used radioactive atoms to label either proteins or nucleic acids in phage T2 ○ performed infection of E. coli ○ asked what type of molecule (protein or nucleic acid) was getting into the E. coli ● *Recall Isotopes* ○ Some isotopes are unstable and undergo radioactive decay to make energy and/or a subatomic particle in the process ○ Energy from the radioactive decay can be detected ○ Extra neutrons make large macromolecules heavier ● Hershey and Chase grew T2 phage particles on bacteria that had radioactive nutrients with P (to label DNA) orS (to label proteins) ○ Infected E. Coli with phages that had either labeled DNA or labeled proteins ○ Used a blender to separate the phage particles from the cell ○ Cells and phage were put into centrifuge tubes and spun allowing heavier material to stay in the supernatant ○ Looked where the radioactivity went and found that DNA was what was getting into the E. coli allowing them to become “T2 factories” Structure of DNA ● 1953: Francis Crick and James Watson published their model of DNA based on Xray diffraction (from Rosalind and Maurice Wilkins) ○ double stranded, right handed helix ○ 2 pair in antiparallel orientation ○ nitrogenous base A pairs with T and form 2 hydrogen bonds due to functional groups ■ G and C form 3 hydrogen bonds ■ Specific pairing of base pairs explains Chargaff’s rules ■ Chargaff’s rules” the % of A bases is equal to the % or T bases (like %G to %C) ● Physical Properties of DNA: ○ sugar phosphate backbone ○ 10 base pairs per helical turn ○ 3.4 nm per helical turn ○ 0.34 nm per base pair ○ antiparallel pairing of strands ○ A=T, G=C ○ Molecule is aboue 2 nm thinck ● Human genome is 3 billion base pairs long per haploid copy ● Human DNA would be about 1 meter long but cells are microscopic ● DNA is associated with proteins (complex called chromatin) and packed into chromosomes
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