BIOL 20A Tamkun (Topic 1-5 Midterm Study Guide)
BIOL 20A Tamkun (Topic 1-5 Midterm Study Guide) BIO 20A
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This 15 page Study Guide was uploaded by Holly Chen on Friday October 7, 2016. The Study Guide belongs to BIO 20A at University of California - Santa Cruz taught by John Tamkun in Fall 2016. Since its upload, it has received 23 views.
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Date Created: 10/07/16
Biology 20A - Cell and Molecular Biology (Revision Notes) Midterm One (13th of October, 2016) • Unit 1 - Chemical Bondings & Water • Unit 2 - Macromolecules: Proteins and Carbohydrates • Unit 3 - Macromolecules: Lipids and NucleicAcids • Unit 4 - Enzymes & Metabolism • Unit 5 - Protein Structure & Diseases Unit 1 - Chemical Bondings & Water 3 Classes of Chemical Bonds • Covalent bonds - a pair of electron is shared by two other atoms, which allows the atom to fill it’s outer valance shell (e.g two hydrogen atoms to form a hydrogen molecule). • Polar covalent bonds. • Non polar covalent bonds. • Molecules can have both polar and non polar covalent bonds. Number of covalent bonds in common elements. Hydrogen 1 Oxygen & Sulfur 2 Nitrogen 3 Carbon (important to all organic chemistry) 4 Phosphorus 5 • Ionic bonds - two oppositely charged ions attract each other. Often when an atom of higher electronegativity takes the electron of an atom with lower electronegativity (e.g Cl taking an electron from Na to form table salt). • Weaker than covalent bonds, but still relatively strong. • Hydrogen bonds - the existence of two polar covalent bonds. Hydrogen bonds are the weakest out of the three. An Introduction on Water • Water covers 75% of the earth’s surface and takes up roughly 95% in mass of cells. • Universal solvent for life, most cells thrive and live in aqueous environments. • The five important properties of water, explanations & examples of uses in biology: 1. Water of cohesive • Amolecule liquid water is linked by multiple hydrogen bonds, which makes the structure of water stronger than most other liquids. • Cohesion help the transportation of water such as in plants, when water is transported from roots to leaves. When water evaporates through a leaf, the hydrogen bonds causes the water molecules leaving the plant to ‘pull’on the molecules behind it to follow. 2. Water has a high heat capacity. • Specific heat of a substances is the amount of heat it absorbs or loses for 1 gram of the substance to change 1 degrees Celsius. • Water, relative to other liquids, is pretty resistant to temperature changes. This is good for sustaining life both in and out of the water, for example some organisms can’t tolerate a big or quick change in temperature. 3. Water has high heat of evaporation. • Heat of vaporization is the quantity of heat a liquid must absorb for 1 gram of it to be converted to gas. • This moderates the Earth’s climate. 4. Water is denser in liquid form than it is in solid form. • Ice floats on water because while other materials contract when becoming a solid, water expands, leaving more space between hydrogen bonds. • If ice sinks, all bodies of water will eventually freeze solid, making life beneath impossible. 5. Water is a good solvent for polar molecules. • Universal solvent. • Important in polar compounds such as biological fluids, the sap of plants, liquid within cells. • Hydrophilic - works well with water (e.g mixing and dissolving. • Hydrophilic interactions: ionic compounds and molecules with polar covalent bonds easily dissolve in water. • Hydrophobic - when non polar substances stick together (usually in aqueous solutions) and exclude water molecules. It does not dissolved in water. • Non polar molecules can’t interact with water, therefore they interact with each other (more of this on later topics). • Important in cells (e.g hydrophilic heads and hydrophobic tails of cell membranes). pHAcids & Bases • Acidic solutions is when [H+] is greater than [OH-] while basic solutions is when [H+] is less than [OH-]. • Acid pH <7 • Basic pH >7 • pH - the concentration of protons in a solution, the scale is logarithmic meaning a small change in pH is a big change in hydrogen ions in the solution. • pH change is essentially the change in concentration of hydrogen ions. • Acids increase proton concentrations while bases decrease proton concentration. The Importance of pH • Protons are important to molecules and their reactions. • Many biological molecules, environments, processes, and animals are sensitive to changes in pH, even small changes can be harmful or lethal (e.g ocean acidification, anything with a calcium carbonate shell such as sea turtles are harmed). Unit 2 - Macromolecules: Proteins & Carbohydrates The Importance of Carbon Chemistry • Organic chemistry - the study of carbon based molecules. While water is most abundant in cells, the rest is carbon-based molecules. • Carbon - the element of life because of it’s diversity and ability to form polymers. • Carbon can make long and complicated molecule rings/branches (silicon comes in next best). • Carbon’s valance of 4 means it’s readily available to make covalent bonds. • Hydrocarbons - a molecule with only hydrogen and carbon atoms. • All bonds are non polar covalent bonds. • Usually hydrophobic (e.g fats and oils). • Diverse, vary in lengths, linear or branch, contains more than one double bonds and rings. • Other elements commonly found in organic molecules - oxygen, nitrogen, sulfur, phosphorus. They are often found in functional groups attached to a hydrocarbon skeleton. Functional groups • Functional groups - a group of atoms usually attached to a carbon skeleton of organic molecules, functionally important and hydrophilic to make organic molecules soluble in water. Functional Group What? Commonly found Hydroxyl groups 1 oxygen atom connected Alcohol OH- through covalent bonding Sugars to 1 hydrogen atom Example: Ethanol Carbonyl groups 1 carbon atom double Organic compounds -C=O bonded with 1 oxygen atom (end of a carbon chain = aldehyde, not = ketone) Carboxyl groups Both carbonyl and Organic acid (carboxyl -COOH hydroxyl groups linked toacid) a carbon atom. It exists in two forms, ionized and not, depending on the H+ ion. Amino groups 1 nitrogen atom attached Amines -NH2 by single bonds to hydrogen atoms Sulfhydral groups Sulfur and hydrogen Thiols -SH atoms are bonded to an R Cysteine (amino acid that group can form disulfide bonds) Phosphate groups Comprised of 1 DNA/ RNA -PO4 2- phosphorus attached to 4 Organic phosphates oxygen Macromolecules • Levels of biology (in order of size): • Elements (e.g carbon, nitrogen, oxygen, hydrogen, phosphorus, sulfur) • Molecules (e.g amino acids) • Macromolecules (e.g proteins, lipids) • Macromolecular complexes, membranes, chromosomes • Viruses, bacteria. • Organelles (e.g mitochondria) • Plants and animal cells. • Chemical composition of a typical cell: • 75% water • 5% ions and other smaller molecules • 20% macromolecules (60% protein / 25% nucleic acid / 10% carbohydrate / 5% lipid). • Macromolecules - polymers built from monomers, large molecules essential for life. The abilities of carbon and functional groups allow a huge diversity of organic molecules (10,000+ organic molecules are present in a typical cell). • Polymers of repeating subunits. • These subunits are called monomers. • Examples: amino acids (monomers) form proteins (polymer) or nucleotides (monomer) form nucleic acids (polymer). • Polymer synthesis and breakdown - bonds between monomers are made by a dehydration process (water removed as bonds form), while bonds are broken by a hydrolysis reaction (water added to break bonds). Macromolecules: Proteins • Proteins - large macromolecules, polymers of one or more long chains of amino acids. The chain can range from a few to hundreds of thousands. • Account for more than 50% dry mass of cells. • Almost all living organisms depend on proteins. • Proteins functions: • Structure - provides support (e.g collagen in tendons). • Regulation - controls gene expression and other processes. • Signaling - hormones, growth factors, receptors, coordination of organism’s activities (e.g insulin). • Movement - muscle contraction, chromosome segregation. • Metabolism - enzymes that increases biochemical reaction rates, selective. • Transport - transportation of material (e.g in cells, through membranes, blood stream). • Amino acids - organic molecule containing both amino group and carboxyl group. They are building blocks for proteins. • Classified by chemical properties of their R groups. • 20 occur naturally. • Linked by peptide bonds to form proteins. R- Groups ofAminoAcids (lecture slide) • Non polar R groups - side chains of mostly hydrocarbon, hydrophobic. • Polar R groups - groups that are able to participate in hydrogen bonding. Hydrophilic. • Acicid R group (negatively charged) - contains a carboxyl group. • Basic R group (positively charged) - contains an amino group. Hydrophilic. Polypeptides • Polypeptides - an organic polymer consisting of amino acids bonded together in a chain to form a protein. • This is when two amino acids are positioned so carboxyl group is adjacent to the amino group. • Polypeptides have a C terminus (carbonyl end) and N terminus. • The formation of peptide bonds eliminates the amino and carboxyl groups, except at the N- and C- termini (and on R groups). • Peptide bond - a covalent bond resulted from dehydration process and the removal of water. Levels of Protein Structure • Afunctional protein is not a single polypeptide, but one or more polypeptides folding and twisting in a precise way to form a molecule of a distinct shape. • Primary structure - the linear sequence of amino acids, partly dictates the secondary structure. • Secondary structure - small regions of folding of polypeptide backbones because of the polar characteristics of the polypeptide backbone. • α helix: coil, held together by hydrogen bonds. • β sheet: two or more segments of polypeptide chain lying side by side. • Tertiary structure - 3D structure of the protein. This is held together by the interaction between R groups and a disulfide bridge. • Disulfide bridge: resulted from the folding of the protein, bringing two sulfhydryl groups close together. • Quaternary structure - 3D structure of protein complexes (more than 1 polypeptide chain interacting). • Example: collagen, fibrous protein, which is 3 identical helical polypeptides twisting into a large triple helix. • The interaction between side chain R groups and the polypeptide backbone must be intact, if disrupted, the protein denatures and is useless. • Factors that lead to protein denature: heat, temperature, ph, salinity or solvent polarity changes. Macromolecules: Carbohydrates • Carbohydrates - sugars and their polymers. • They function as energy sources (e.g glucose, startch), structural roles (e.g cellulose), and as precursor for other molecules. • Monosaccharides - single sugars (general formula C_nH_2nO_n). • 3 carbon sugar = triose • 5 carbon sugar = pentose • 6 carbon sugar = hexose • Usually nutrients for cells. • Carbon skeleton of sugar serve as raw material for other organic molecules synthesis. • Disaccharide - double sugar (two monosaccharides linked together by glycoside linkage). • Glycoside linkage: covalent bond between two monosaccharides through dehydration process. • Hydrolysis would split it back into 2 monosaccharides. • Example: glucose + fructose = sucrose. • Oligosaccharides - 3+ monosaccharides linked together by glycoside linkage. • Polysaccharides - a macromolecule, 100s-1000s of monosaccharides linked together by glycoside linkage. • Functions include storage of chemical energy (starch in plants), structural roles (cellulose in plants). Unit 3 - Macromolecules: Lipids & Nucleic Acids Macromolecules: Lipids • Lipids - large molecules (not truly polymers) that don’t mix well with water. • Most heterogenous class of molecules. • Major component of cell membranes. • 3 major classes of lipids: fats, phospholipids, steroids Lipids: Fats • Fat - made from glycerol and fatty acids. Hydrophobic due to the non polar C and H bonds in the hydrocarbon chain, water molecules don’t bond with fat. • Glycerol: an alcohol with 3 carbons with a hydroxyl group each. • Fatty acids: made of a long carbon skeleton with a carboxyl group at one end with the rest being a hydrocarbon chain. • To make a fat, 3 fatty acid molecules (can be same or different) are joined to glycerol through ester linkage, creating triacylglycerol, which is non polar. • Functions: • Energy storage. • 1 gram of fat is twice as much energy as 1 gram of a polysaccharide like starch. Saturated v.s unsaturated fat • Saturated or unsaturated refers to the fatty acid’s hydrocarbon chain structure. • Saturated fat - saturated with hydrogen, no double bonds between carbon atoms to allow maximum flexibility of the molecule. • Most animal fats are saturated (e.g lard, butter) • The lack of double bonds allow the molecules to pack together. • Usually solid at room temperature. • Unsaturated fat - has double bonds with less hydrogen, often creating a “bend” or kink in the hydrocarbon chain. • Fats of plants, fish. • Referred to as oils (e.g olive oil, fish oil). • Built with more than one type of unsaturated fatty acids. • Usually liquid at room temperature because the bends and the double bonds prevent tight packing, so it doesn’t solidify. Lipids: Phospholipids • Phospholipids - 1 glycerol molecule + 2 fatty acids + a modified phosphate group. • Major component of cell membranes due to the phosphate group, which is a negatively charged polar head (hydrophilic). Fatty acid chain remains the same with non polar tails (hydrophobic). • Phospholipid’s two different ends behave differently in water. • Phosphate group forms and hydrophilic head that interacts with water. • Hydrocarbon are hydrophobic and don’t interact with water. • When phospholipids as a whole is added to water, double layers called bilayers are formed (hydrophilic on the outside, hydrophobic on the inside). • This forms a boundary between the cell and the outside environment. Lipids: Steroids • Steroids - a carbon skeleton with four rings, different steroids will have specific functional groups attached to the ring. • Example: cholesterol, common component of animal cell membranes. • Precursor to synthesize other steroids. Macromolecules: NucleicAcids (DNA/RNA) • Nucleic acids - macromolecules of polymers of polynucleotides made from monomers of nucleotides. • Nucleoside - nitrogenous base, a five carbon sugar (pentose) without a phosphate group. • Nucleotides - nitrogenous base, a five carbon sugar (pentose) and one or more phosphate group. Provide energy that drives many chemical reaction in cells. • Purines (adenine and guanine): larger, double ring structure. • Pyrimidines (thymine, uracil and cytosine): single ring structure. T&C for DNA, U&C for RNA. • Polynucleotides - nucleotides are joined together by phosphodiester linkage, making a backbone of repeating patterns of sugar phosphate units. • Central component of molecular biology and life in general (no shit). • DNA- deoxyribonucleic acid. • Blueprint of a cell. • The genetic material inherited from parents. • During cell replication, DNAmolecules are copied and passed along. • DNAis not directly responsible for the day to day operations of the cell. • RNA: ribonucleic acid. • Genes along a DNAmolecule directs the synthesis of RNAor messenger RNA (mRNA), which control a cell’s protein synthesizing mechanisms. • Site of protein synthesis: ribosomes. • General formula: transcription of DNAmakes RNA, translation of mRNAyields protein. Transfer and ribosomal RNAs (mRNAand tRNA) are involved in the process of translation. 5 Carbon Sugar = DNA/RNA • Carbon is numbered 1’through 5’ • Nucleic acids have 3’ends and 5’ends. • 5’end has a phosphate group. • 3’end has a hydroxyl group. • Anucleotide monomer would be added to the 3’end of the polymer. Unit 4 - Enzymes & Cellular Metabolism Metabolism Basics • Metabolism - general term used for all chemical reactions involved in keeping a cell or organism alive. • Metabolic pathway: a molecule being altered in a series of steps with enzymes, resulting in a product. There are two general types: • Catabolic pathway: breaking down molecules to release the energy stored in chemical bonds. The energy will then be free to do work. • Anabolic pathways: synthesis of molecules, consumes energy. • Think of catabolic pathways as downhill and anabolic pathways as uphill. Energy & Thermodynamics • Energy - many definitions, but in this case, it’s the ability to cause change. • Kinetic energy - energy of motion. • Thermal energy - energy that comes from heat, which is the movement of particles. Faster movements generate more heat. • Potential energy - energy the matter has due to location or structure, includes the energy stored in chemical bonds. • Thermodynamics - the study of energy transformation. • 1st Law of Thermodynamics: • Energy can be transferred and transformed, but never created or destroyed. • Example: sunlight is converted into chemical energy by plants. That makes the plant an energy transformer, not energy producer. • 2nd Law of Thermodynamics: • Every energy transfer or transformation increases the entropy of the universe. • Entropy: a measure of disorder. • Example: a bear hunts for food is in motion, therefore the chemical energy from his last meal is being transferred into kinetic energy, producing heat and CO2 by movement and exhaling. This increases the disorder of the universe (not apparent to us). • Laws of thermodynamics allow the prediction of whether or not a reaction will occur spontaneously based on the change in free energy. • ΔG = G (products) - G(reactants) • ΔG =ΔH - TΔS • ΔH is the change in total energy whileΔS is entropy. • ΔG is based on the change in total energy and entropy decrease as the degree of randomness increases. • ΔG < 0 (negative) = energy released, exergonic reaction, spontaneous. • Energy released from exergonic reactions can be converted into other forms of energy. • Example: cellular respiration,ΔG = -686 kcal/mole, exergonic, spontaneous. • ΔG > 0 (positive) = energy consumed, endergonic reaction, not spontaneous. • Example: photosynthesis,ΔG = 686 kcal/mole therefore endergonic and not spontaneous. ATP(adenosine triphosphate) • ATP - contains sugar ribose, nitrogenous base adenine, and a chain of 3 phosphate groups bonded to it. • Cell’s 3 main work process: chemical work, transport work, mechanical work. • Universal energy currency of cells. • Stores chemical energy to do work. • Living organisms use it continuously, it’s a regenerating resource. • Chemical / energy coupling - using an exergonic process to drive an endergonic process. • Example: oxidation of glucose drives the synthesis ofATP. ATP Hydrolysis • All life depends on theATP cycle, it allows cells to maintain overall low entropy. • The bonds between the phosphate groups ofATP can is broken down by hydrolysis. • Extremely exergonic process. • ATP hydrolysis drives other endergonic reaction in cells. • Example: when the terminal phosphate bond is broken, triphosphate becomes diphosphate and yieldsADP. This is also an example of chemical coupling. How doesATP perform work? • Example: with help of specific enzymes, cell is able to release energy byATP hydrolysis to drive endergonic reactions. • Glutamic acid’s conversion to glutamine is not spontaneous (ΔG positive). • When coupled withATP hydrolysis occurs:ATP phosphorylates glutamic acid, making it unstable, ammonia displaces phosphate group, forms glutamine. • Glutamic acid to glutamineΔG = 3.4 kcal/mol • ATP hydrolysisΔG = -7.3 kcal/mol • Total reactionΔG = -3.9 kcal/mol • Chemical coupling results in the process being spontaneous. How to cells control spontaneous exergonic reactions? • Spontaneous reactions can occur very slowly. • Reactants must be converted to high energy intermediates before being converted into products. • This creates an energy barrier (activation energy is very large). • This is where enzymes come in. Enzymes • Catalyst - a chemical agent that will speed up reactions without being consumed. • Enzyme - a macromolecule that acts as a catalyst. • Enzymes reduces activation energy. • Does not changeΔG • Enzymes can’t force endergonic reaction, exergonic chemical coupling still required. • Specialized to catalyze specific reactions. • An average cell contains thousands of different enzymes. • Without enzymes and enzyme regulations, chemical reactions would happen all over the place, or take too long, or not happen at all. • Cells regulate metabolic pathway by regulating enzyme activity. Substrate Specificity • Substrate - the reactant in which the enzyme operates on. • Overall process: • Enzyme + substrate —> enzyme substrate complex —> enzyme + products. • Each enzyme is specific and can only recognize its specific substrate. • Example: sucrase will act only on sucrose and will not bind to any other disaccharides. • If there are big temperature or pH changes, the enzyme will denature and lose it’s ability to function. • Each enzyme works best at a specific temperature and pH. Enzyme Structure • Active site - a “pocket” on the surface of the enzyme where the catalysis occurs. • Usually formed by a few amino acids. • Rest of the molecule provides framework that determines size and shape of the active site. • Recent studies found active sites change subtly in equilibrium with small differences in free energy for each shape. This is because the shape that best fit the substrate isn’t always the shape with the lowest energy. • Substrate - the reactant in which enzymes operates on. It’s held to the active site usually by weak hydrogen bonds and ionic bonds. • Substrate and active site interact like lock and key. How to Enzymes work? 1. Substrate enters active site. 2. Enzyme alters its shape so the active site folds around the enzyme (induced fit). 3. Substrates are held in the active site through weak bonds such as hydrogen or ionic bonds. 4. Substrate is converted into products. 5. Products released from active site. 6. Active site is now free for new substrates to enter. Factors that regulate of EnzymeActivity • Amounts of enzymes in the cell • Phosphorylation of S/T/Y residues. • Binding of small molecules: • Competitive inhibitors will bind to the active site and compete with substrate. • Non competitive inhibitors, also known as allosteric effectors, will bind to the enzyme away from the active site, which will alter the structure and activity of the enzyme. Unit 5 - Protein Structure & Disease Review of Protein Structure • Afunctional protein is not a single polypeptide, but one or more polypeptides folding and twisting in a precise way to form a molecule of a distinct shape. • Primary structure - the linear sequence of amino acids, partly dictates the secondary structure. • Secondary structure - small regions of folding of polypeptide backbones because of the polar characteristics of the polypeptide backbone. • α helix: coil, held together by hydrogen bonds. • β sheet: two or more segments of polypeptide chain lying side by side. • Tertiary structure - 3D structure of the protein. This is held together by the interaction between R groups and a disulfide bridge. • Disulfide bridge: resulted from the folding of the protein, bringing two sulfhydryl groups close together. • Quaternary structure - 3D structure of protein complexes (more than 1 polypeptide chain interacting). • Example: collagen, fibrous protein, which is 3 identical helical polypeptides twisting into a large triple helix. • The interaction between side chain R groups and the polypeptide backbone must be intact, if disrupted, the protein denatures and is useless. • Factors that lead to protein denature: heat, temperature, ph, salinity or solvent polarity changes. Sickle Cell Disease • Sickle Cell Disease - an inherited disease where red blood cells contorts into abnormal shapes. • Incurable. • The abnormal cells die early, leaving shortage of healthy red blood cells. • Clogs vessels. • Common inAfricanAmericans (1 in 12 are carriers) • This is due to β globin mutations (glutamic acid replaced by valine) that changes the sequence of β globin. • Valine exposes the hydrophobic region, making molecules crystalize into a fiber and decreases it’s capacity to carry oxygen. Prion Diseases • Prion diseases - transmissible spongiform encephalopathies (TSEs) are rare neurodegenerative disorders, quite rare. • Bizarre link between protein structure and disease. • Affects both human and animal. • Humans: Kuru, CJD • Animals: scrapie, mad cow disease, chronic wasting disease, feline spongiform. • Symptoms: • No early symptoms of infection. • Slow and steady loss of coordination, leading to dementia. • Creates “holes” in brain tissue due to nerve cell death, other cell types try to fill the holes, resulting in more damage. • Always fatal. • There is direct evidence that an infectious agent is the cause of TSE. • Ahealthy animal is injected with brain tissue of sick animal. • Disease has a delay, but will develop, suggesting that a slow virus may be responsible Kuru: a human prion disease • Symptoms of Kuru and TSE are very similar. • Cannibalistic tribes eat their dead relatives’brains as a way of honoring them, cannibalisms stopped in 1957 and the Kuru epidemic ended. Cretzfeldt-Jakob Disease (CJD) • Cretzfeldt-Jakob disease - a condition brought by contact with an infected tissue which causes personality changes, anxiety, depression, and memory loss. Eventually lapse into coma. No cure or treatment. • Sporadic Cretzfeldt-Jakob disease (sCJD): spontaneous, cause unknown, possibly due to inheritance, victims usually 60+ years old. Rare, 1 death per million people per year. • Iatrogenic Cretzfeldt-Jakob disease (iCJD): caused by contact with contaminated surgical instruments, common in corneal transplants and growth hormones from cadavers. Identification of InfectiousAgent - Stanley Prusiner (UCSF) • Prusiner, 1997 Nobel prize winner in Physiology and Medicine, used laboratory assay to identify the infectious agent that causes scrapie. • Crude brain “milkshake” is extracted from infected animal. • Using column filtration, molecules are separated based on physical properties and size. • Fractions are collected and analyzed. • Infectious agent determined. • Results showed it’s a protein, not virus or bacteria, that causes scrapie. • Prusiners work was controversial, others believed the proteins (named proteinaceous infectious particle, or prion) was contaminated with a virus. • Prions (PrP) are present in both healthy and sick animals. • Normal cellular protein = PrPc • Infectious form = PrPsc • The primary sequence of PrPc and PrPsc are identical. • Secondary structure of healthy and infected prion proteins are different, infected proteins lack the α helix found in normal proteins. • PrPsc can change the structure of PrPc in a test tube. • Prion hypothesis: • PrPsc converts other healthy structures of PrPc • Conversion builds up. • Leads to disease. • Prusiner used engineered mice without PrP to test this hypothesis and found that those mice can not be infected. Species Barrier & Mad Cow Disease • Species barrier - prion diseases tend to spread within a species because primary structures of PrPs of different species are related but not identical, PrPsc likes to convert identical PrPc. Mad Cow Disease • Cows are fed meat and bone meal consisting of a mash up of all unused meats and bones from other farmed animals. • This contains animals like sheep with scrapie. • Sheep and cow PrP differs by 7 amino acids. • Scrapie crossed the species barrier from sheep to cow in England in the 1980s, causing millions of people to eat infected beef before the feeding recipe was changed. • Anew form of human CJD, called vCJD, appeared in the 1990s. • Younger victims (average age 27). • Lasts 14 months instead of 4. • Appeared 10 years after first mad cow disease which fits the 10-15 year incubation time. • Now, mad cow disease is rare due to heavy regulations of livestock industry with relatively few cases of vCJD in England.
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