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Biology 1201 MIDTERM NOTES.pdf

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Western University
Biology 1202B
Richard Gardiner

Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh Lecture 1- A Tour of the Cell early microscope: antony van leeunwenhoek single lens- ca. 1673 before the invention of microscopes, if you couldn't see it, then it isn’t exist, look long time to discover diseases robert hooke: the cell, first person to coin the word “cell” to describe the tiniest components of living systems i.e plants 1635-1703 royal society wanted the microscope trying to make a better one the electron microscope resolution limit o flight microscope is about 0.2 microns the transmission electron microscope: can fire electrons on living cells for a long time need vacuum, because ti cant travel in air the scanning electron microscope surface of the cell, ex counting hair on bacteria, no light/ electrons minimum res o flight microscope = .2 microns ( best X1000) Cell Fractionation • Take cells apart to study their components • Centrifuge is used to fractionate cells and separate their major organelles • Ultracentrifuges are capable of speeds as fast as 150,000 rpm applying forces over 1 million times the force of gravity 1. put cells in buffer solution 2. put it blender( mechanically breaks down cell) 3. big stuff ( DNA/ nucleic) stay at the bottom 4. supernatant ( w/o big cells ) and cell debris ( pellet) 5. spin supernatant ( contain cell organelles) The Cell Cell = simplest collection of matter which has all the properties of life 1. Lowest hierarchical level which is alive 2. Cell is basic unit of life 3. Cell performs all functions necessary to live and reproduce Virus • Occur in virtually every kind of organism • Some wreck havoc others cause no disease or outward sign of their presence • Often highly specific to host Can reproduce only when they enter a cell * smaller than cells, attack every known cells, protein and nucleic acid not “alive” because it cant reproduce in a growth medium must be attached on host Prokaryotic Cells Pro-before and karyote-nucleus Examples – bacteria, cyanobacteria, mycoplasmas, etc.. • no nucleus(genetic information in area called nucleoid.) • Visible components–plasma membrane, ribosomes, nucleoid, cytoplasm, cell wall( give shape), pili(sense/hear), flagella( movement), mesosomes, photosynthetic membranes * plasma membrane Bacteria size from about 0.1 to about 600 μm over a single dimension. Eukaryotic Cells • Eu - true, karyote - nucleus • Found in four (?) Kingdoms – Protista, Fungi, Animalia, Planta Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh Characteristics • True Nucleus (surrounded with nuclear membrane, contains DNA, • Visible Components – Plasma membrane, cytoplasm, nucleus, ribosomes, organelles, endomembrane system, cytoskeleton( holds the shape of the cell), cell wall, cell matrix, some organelles, flagella, centriole ( mitosis) - plant cells: cytoskeleton, cell walls, chloroplasts, central vacuole (storage space) The nucleus (Eukaryotic cell’s genetic library) • Most of the genes in a eukaryotic cell( mitochondria and chloroplasts) • Averages about 5 microns in diameter. • Separated from the cytoplasm by a double membrane. These are separated by 20-40 nm. • • Where the double membranes are fused, a pore allows large macromolecules and particles to pass through. * mitochondria/chloroplasts cannot grow by themselves, do not have enough gene function but they evolved, symbiotic relationship) The nuclear envelope lined by nuclear lamina, a network of intermediate filaments that • maintain the shape of the nucleus. • Within the nucleus ,theDNA and associated proteins are organized into fibrous material, chromatin. • Appear as diffuse mass. cell prepares to divide, the chromatin fibers coil up to be seen as separate structures, • chromosomes. • In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the nucleolus. • In the nucleolus, ribosomal RNA (rRNA) is synthesized and assembled with proteins from the cytoplasm to form ribosomal subunits. • The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA). Cytoplasm • Has a variable viscosity • Main chemical constituents are water (approx. 80%), nucleic acids, proteins, lipids, carbohydrates, pigments, etc..... Ribosomes build Ribosomes contain rRNA and protein. • A ribosome is composed of two subunits that combine to carry out protein synthesis. • Cells that synthesize large # of proteins (e.g., pancreas) have large # of ribosomes & nuclei. • free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol. • bound ribosomes, are attached to the outside of the endoplasmic reticulum. • *Ribosomes can shift between roles Endomembrane System • internal membranes in a eukaryotic cell are part of the endomembrane system. • These membranes in direct contact/connected via transfer of vesicles, sacs of membrane. • The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane. Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh • There are two regions of ER that differ in structure and function. – Smooth ER looks smooth because it lacks ribosomes. – Rough ER looks rough because ribosomes (bound ribosomes) are attached to the outside, including the outside of the nuclear envelope. Smooth ER • Smooth ER rich in enzymes & metabolic processes. • Synthesize lipids, including oils, phospholipids, and steroids. • Smooth ER also catalyzes step in mobilization of glucose from stored glycogen in the liver. • Other These include alcohol and barbiturates. • Frequent exposure leads to proliferation of smooth ER, increasing tolerance to the target and other drugs. • Muscle cells are rich in enzymes that pump calcium ions from the cytosol to the cisternae. Rough ER • Rough ER is especially abundant in those cells that secrete proteins. – polypeptide is formed by ribosome, threaded into cisternal space through a pore due to protein in the ER membrane. • Secretory proteins are packaged in transport vesicles & go to next step. • Rough ER is also a membrane factory. – Membrane bound proteins are synthesized directly into the membrane. – Enzymes in the rough ER form phospholipids from precursors in the cytosol. – As the ER membrane expands, parts transferred as transport vesicles to other components of the endomembrane system. Lecture 2: A Tour of the Cell - Part Two The Golgi Apparatus • transport vesicles from ER travel to Golgi apparatus for modification of their contents. • center of manufacturing, warehousing, sorting, and shipping. • extensive in cells specialized for secretion. • consists of flattened membranous sacs - cisternae - looking like a sac of pita bread. • membrane of each cisterna separates its internal space from the cytosol • cis side, receives material by fusing with vesicles, trans side, buds off vesicles that travel to other sites. • from cis to trans pole, products from the ER are modified to reach their final state. • make own macromolecules, i.e pectin & Golgi tags, sorts, and packages materials into transport vesicles. Lysosomes single membrane, like a vesicle, contains enzymes(in its membrane proton pump) and ingested material • The lysosome is a membrane-bounded sac of hydrolytic enzymes that digests macromolecules. • Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids. • These enzymes work best at pH 5. – Proteins in the lysosomal membrane pump hydrogen ions from cytosol to lumen of the lysosomes. • Rupturing one or a few lysosomes has little impact on a cell – Massive leakage from lysosomes can destroy an cell - autodigestion (ex tadpole tails are digested by the the lysosomes of the cell) Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh • Lysosomes fuse with food vacuoles, formed when a food item is brought into the cell by phagocytosis i.e when white blood cells detect intruders, the plasma membrane estendds and engulfs the organism (food vacuole) and sends lysosomes to break down the bacteria) • Lysosomes can also fuse with another organelle or part of the cytosol. – This recycling, this process of autophagy renews the cell. Vacuoles • Vesicles (Microbodies) & vacuoles (larger versions) are membrane-bound sacs with varied functions. – Food vacuoles: from phagocytosis, fuse with lysosomes. – Contractile vacuoles: in freshwater protists(filled with salt and solutes, osmotic pressure, increase [] in the inside then outside), pump excess water out of the cell. – Central vacuoles:are found in mature plant cells. Plant Central Vacuole •Membrane around central vacuole, the tonoplast, selective in transport of solutes into central vacuole. •Functions - stockpiling proteins or inorganic ions, depositing metabolic byproducts, storing pigments, and storing defensive compounds against herbivores. Endomembrane system plays a role in the synthesis (and hydrolysis) of macromolecules in the cell. • The various components modify macromolecules for their various functions. Mitochondria and Chloroplasts main energy transformers of cells ( power house) Mitochondria: cellular respiration, make ATP from catabolism of macromolecule in presence of oxygen. Chloroplasts, found in plants and eukaryotic algae, are the site of photosynthesis. * chloroplasts & mitochondria double membrane organelle, enzymes in inner area, bacterial ribosomes. Similarities • dynamic structure, shape is plastic, can reproduce themselves by pinching in two. • are mobile and move around the cell along tracks in the cytoskeleton. Peroxisomes •Found in Plant and Animal cells, Single membrane bound compartment •Enzymes that transfer hydrogen from various substrates to oxygen •This produces hydrogen peroxide which is then converted to water •made from cytosol not endomembrane system(started in endomembrane system, no DNA/ribosomes within) Cytoskeleton • The cytoskeleton is a network of fibers extending throughout the cytoplasm. • organizes structures and activities of the cell. Structural support • mechanical support, maintains shape of cell, Fibers act like a geodesic dome to stabilize a balance between opposing forces, anchorage for many organelles and cytosolic enzymes, Dynamic - dismantling in one part and reassembling in another to change cell shape. Microtubules Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh • Hollow tubes, 25 nm diameter, Composed of protein tubulin, Cell shape, cell mobility, chromosome movement, organelle movement, act as tracks, guide motor proteins carrying organelles to their destination. Microfilaments • Two intertwined actin strands, 7 nm in diameter, Cell shape, muscle contraction, cytoplasmic streaming, cell motility, cell division Intermediate filaments • Thick cables, 8–12nm, Different proteins (keratin family), Anchorage of nucleus and organelles Centrosomes and Centrioles • In many cells microtubules grow out of the centrosome (region locate near nucleus) • centrosome of animal cell are a pair of centrioles (9 sets of triplet microtubules arranged in a ring) Centrosome • animal cells, the centrosome has a pair of centrioles, each w/nine triplets of microtubules in a ring. • During cell division the centrioles replicate. Cilia • short, cover the cell surface ( hair),0.25 microns in diameter, 2-20 microns long, Beat in oar motion Flagella • A flagellum has an undulatory movement, similar width as cilia but 10- 200 microns long • Force is generated parallel to the flagellum’s axis. Dynein • The bending of cilia and flagella is driven by the arms of a motor protein, dynein. • ATP supplies energy, Dynein arms alternately grab, move, and release the outer microtubules. • Protein cross-links limit sliding and the force is expressed as bending. Cilia and flagella have the same ultrastructure. • Core of microtubules sheathed by plasma membrane. • Nine doublets of microtubules arranged around a pair at the center, the “9 + 2” • Flexible “wheels” of proteins connect outer doublets to each other and to the core. • Outer doublets are also connected by motor proteins, Anchored in the cell by a basal body (centriole) Muscle Cells • Thousands of actin filaments, arranged parallel to one another. • Thicker filaments, composed of motor protein, myosin, interdigitate with the thinner actin fibers. • Myosin molecule travel the actin filament, pulls stacks of actin fibers together and shortening the cell. Cytoplasmic Streaming •plant cells (& others), actin- myosin interactions & sol-gel transformations drive cytoplasmic streaming. Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh Plant Cell Wall • thicker than plasma membrane, plasma membrane(PM) 0.008 microns (not visible in LM) • 0.1 to several microns thick (visible in LM), are strong, pm decides what goes in/out not CW • fibres of cellulose (polymer of glucan), embedded in polysaccharides also pectin (glue individual cells) Plasmodesmata • channel in cell wall, cytoplasm bridges b/w plant cells: transfer, communication, every cell is connected cytoplasmically Intercellular Junctions • Desmosomes: Two cells attached by intercellular filaments, rivet cells together •Tight Junctions: Prevent materials from moving between cells, Form a belt around cell, forms a seal • GapJunctions: Intercellular connections in animal cells, like Plasmodesmata, small molecules can pass Lecture 3- Mitosis (Divide and Conquer) What do all cells require to survive? • A complete set of genetic instructions • produce required molecules, direct life processes • Genetic instructions are coded in the DNA of cells Why do cells divide? • Growth, Repair,Development Cell Cycle grows, adding more cytoplasmic constituents, DNA is replicated, divides into 2 identical • daughter cells Essential Features of Cell Division • Transmit a complete copy of genetic information (DNA) • Transmit materials necessary for cell to survive and use genetic information Prokaryotic Cell • no nucleus, DNA in cytoplasm, no membrane-bound organelles, example: bacteria, binary fission • chromosome a circular loop, attaches to one point on plasma membrane, replicated • replicated chromosome attached to pm at a different nearby point, cell elongates new plasma membrane is added between chromosomes, pushing them towards opposite • ends of cell • plasma membrane grows inward @ center of cell, parent cell is divided into 2 identical daughter cells Eukaryotic Cell • membrane-bound organelles, nucleus (DNA) ex: fungi, protists, plants, animals, divide = mitosis • Mitosis(non sexual) = nuclear chromosomes, Mitochondria & Chloroplasts = DNA (individual) • Strands of linear DNA, Human cells = 46 strands, coiled up, avg = 4cm • Chromatin: Many proteins are bound to DNA, Protect, Packaging, Duplication, Transcription, Regulation Modification, DNA + bound protein = chromatin (50 % DNA) Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh Chromosomes • Chromosomes =condensed chromatin, long thread like, highly condensed in mitosis, DNA + protein During division phase of cell cycle •each chromosome = molecule of DNA, easier to sort and organize DNA into daughter cells Mitosis • Division of somatic cells in eukaryotic, 1 cell into 2 identical daughter cells (cellular reproduction) Ploidy • Organisms have a specific number of sets in diploid and haploid cells • Mitosis and Meiosis( sexual reproduction) lead to different ploidy outcomes • Ploidy: # of pairs of chromosomes in cells • haploid: one copy of each chromosome (n) • diploid: two copies of each chromosome (2n) • Polyploid = more than two complete sets (in plants, not animals) Eukaryotic Cell Cycle G1: Size Increases, Organelles may replicate Normal Growth and Development S: DNA synthesis DNA is replicated, synthesis of proteins associated with DNA ploidy does not change G2: cell prepares for division, synthesis of proteins associated with Mitosis cell committed to divide * each chromosome is replicated b/c in division the cells are divided @ end the ploidy # doesnt change Stages of Mitosis Interphase • Occurs before mitosis, DNA = Chromatin, replicated, Nucleus well defined, Nucleoli present, Centrosomes replicated (replicated centrioles in animal cells), Microtubules extend from centrosomes (called aster [star]), Chromosomes have duplicated but not condensed Prophase •Chromatin = coiled chromosomes, Nucleoli disappear, Mitotic spindle form, Centrosomes move away. Prometaphase •Nuclear envelope fragments, Microtubules connect to chromosomes, Kinetochores have formed, Some microtubules connect with those from the opp. pole Metaphase •Centrosomes @ opp. pole, chromosomes @ metaphase plate, Centromeres are on the metaphase plate, Kinetochores of each chromatid connected to microtubules from different pole Metaphase Anaphase •paired centromeres separate, chromosomes (0.5 of each sister chromatid pair) move to opp. poles, move centromere first, poles of cell move further apart at same time Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh Telophase •Daughter nuclei forms @ 2 poles ,nuclear envelope forms, chromatin become less condensed, Nucleolus forms, Cytokinesis occurs Mitotic Spindle • Arises from 2 microtubule organizing centres (centrosomes) • Microtubules spontaneously arise from tubulin dimmers • Microtubules arising from M O centres have opp. polarity • meet at equatorial plane • Some microtubules are bound together by proteins • Chromatids attach to microtubules via kinetochores Kinetochores • duplicated chromosomes attach to microtubules of opposite polarity •Kinetochores = protein complexes •walk down microtubules toward microtubule organizing centres •Pull chromatids apart after centromere breaks down (beginning of anaphase) Lecture 4- Mitosis Continued... Cytokinesis • process of splitting daughter cells apart, optional (muscle cells, some protistans, plant cells, fungi) Cytokinesis in Animal Cells - analogy: balloon witha finger in the middle ( squeeze) until you have 2 pieces Cytokinesis in Plant Cells - cell walls are hard so cannot squeeze, wall formation Regulation of the Cell Cycle • Timing & rate of cell division in diff. parts of plant/animal are crucial to normal development Liver cells if needed • • Nerve and muscle do not • These cell to cell differences are the result of regulation of the cell cycle cell cycle control system. - These molecules trigger and coordinate key events in the cell cycle. - control cycle has a built clock, also regulated by external adjustments and internal controls. G1 Checkpoint • G1 restriction point: most important, If cell receives go-ahead it will complete cycle and divide, If not it will exit the cycle and switch into a non dividing state called G0 phase G0(permanent G1 arrest) - Specialized cells stop reproduction, to perform specialized functions ex. nerve cells Lecture 4: Meiosis.... Internal and External Clues Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh • Kinetochores: signal when all connected so anaphase does not begin too early and leave a chromosome • Growth factors in media needed, cells crowded stop dividing, animal cells must be anchored to divide Haploid •Unfertilized egg cells: haploid, Sperm cells: haploid, Male: 23 chromosome & female: 23 chromosome •Haploid sperm fuses with haploid egg to get diploid zygote (fertilized egg) Diploid • Almost all body cells are diploid, 46 chromosomes, DO have pairs of homologous chromosomes Asexual Reproduction –inherit all genes from 1 parent, offspring = clone i.e. 1 celled organisms divide, Hydra budding, - variation comes from mutations, reduced ability to evolve SexualReproduction - individual gets 1⁄2 genetic info from one parent & 1⁄2 from other parent, Offspring = different - Sexual reproduction = greater variation, Evolution requires pre-existing genetic variation Life Cycles • events in the organism development and reproduction, animal life cycle = dominant diploid phase • germ line diploid cells will undergo meiosis to produce gametes Sexual Cycle (humans) - Homologous chromosomes = 2 of given matched pair, gene @ specific locus on a specific chromosome - chromosomes = long linear strands of DNA, homologous chromosomes = homologous genes @ homologous loci (but the two genes at the homologous loci need not be identical) Homologous Chromosomes •same size, same gene loci, has diff. alleles, diploid nucleus: 2 sets of homologs (2n), 1 each parent Sex Chromosomes •1 pair of homologous chromosomes is diff, females: homologous pair XX, males XY, Y:13 known genes, X has 179 known genes, X and Y: sex chromosomes, Autosomes: non-sex chromosomes Phases of Meiosis - Meiosis 1: 2n to n (reduction) and Meiosis 2: like mitosis (division), produces 4 haploid cells Brief summary of the Process: homologous chromosome , replication occurs during premiotic interphase, each chromosome consists of 2 sister chromatids, chromosomes pair during prophase I for meiosis, may undergo recombination , first meiotic division separates the holmologs, placing one in each of the two cells resulting form the division. the products have haploid number of chromosome, each chromosomes still contain 2 chromatids.the second meiotic division occurs, separates the sister chromatids, one in each cell ( 4 cells are produced) as a final product of meiosis. Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh synapsis: process where special proteins attach homologous chromosomes tightly together. chiasmata: crossing over points, multiple in human per tetrad, a form of genetic arrangement meiosis 1: Interphase :chromosomes copied = sister chromatids (genetically identical) joined @ centromere, single centrosome is also replicated Prophase1: chromosomes condense, homologous chromosomes pair up = tetrads, synapsis occurs and then recombinations at several sites of the chromatid crossed (chiasmata), segments of the chromosomes are traded. spindle forms from, attaches to kinetochores on the chromosomes, begin to move tetrads around. Metaphase1: the tetrads are all arranged at the metaphase plate, microtubules attached to the kinetochore Anaphase1: the homologous chromosomes separate and are pulled to opposite poles Telophase1: movement of homo. chromosome till haploid set @ each pole (consists linked sister chromatid) cytokenesis occurs Meiosis 2: prophase2: a spindle apparatus forms, attaches to kinetochores of each sister chromatids, moves them spindle from one pole attach to kinetochore of one sister chromatid. metaphase2: sister chromatids arranged at metaphase plate, kinetochores of sis. chromatids face opp. pole. anaphase2: the centromeres of sister chromatids separate,the homologous chromosomes separate and are pulled toward opposite poles, separate sister travel to opposite poles telophase2: separated sister chromatids arrive at opposite poles, nuclei form around the chromatids. Cytokinesis separates the cytoplasm. Mitosis and meiosis have several key differences. - chromosome number reduced by half in meiosis, mitosis: genetically identical to parent and each other. - Meiosis produces cells that differ from the parent and each other. - 3 events unique to meiosis, occur during the first division cycle: - prophase1, homo. chromosomes pair up (synapsis). Protein zipper, the synaptonemal complex, holds homo. chromosomes together tightly. Later in prophase 1, joined homo. chromosomes are visible as tetrad, At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged. - metaphase1 homo. pairs of chromosome, not individual chromosome are aligned on metaphase plate. i.e humans = 23 tetrads - Anaphase1 its homo. chromosome, not sis. chromatid, separate & carried to opp. pole of cell. *Sister chromatids remain attached at the centromere until anaphase 2. Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh *Second meiotic division identical to mitosis. Sexual life cycles produce genetic variation among offspring 1. Independent assortment of homo. chromosome in meiosis 1 & nonidentical sister chromatid in meiosis 2 - Genetic variability due to random orientation of tetrads @ metaphase plate. - 50/50 chance particular daughter cell of meiosis 1 get maternal chromosome of a homo. pair & a 50/50 chance it will get paternal chromosome. - Each homo. pair of chromosomes, positioned independently of other pair at metaphase 1. first meiotic division results in independent assortment of maternal and paternal chromosomes into daughter cells. - combinations possible when chromosomes assort independently into gametes is 2^n, where n is the haploid number of the organism, humans n = 23 2. Crossing over between homologous chromosomes during prophase - Crossing over produces recombinant chromosomes (combine genes inherited from each parent) - early in prophase1 homo. chromosome pair up gene by gene, homo. ports of 2 non sister chromatids trade places. - humans: this occurs 2 or 3 times per chromosome pair, one sister chromatid may undergo diff. patterns of crossing over than its match. - Independent assortment of nonidentical sister chromatids during meiosis 2 increases 3.Random fertilization of an ovum by a sperm. - A sperm can fuse w/any egg, zygote produced by mating of woman and man has unique genetic identity. - An ovum is one of approximately 8 million possible chromosome combinations, sperm=one of 8 million different possibilities - Zygote is composed of 1 in 70 trillion possible combinations of chromosomes Lecture 5- Mendel Part 1 Heredity Historical Perspective • Greek philosopher: male flower --> female open,seed made by diff body part,given to baby time of con. • 1700s Anton van Leeuwenhoek( Father of microscopy), saw“animalcules”in sperm of human + animal • Pangenesis: both sex made“pangenes”every organ moved blood via genitals and then to children • began w/Greeks, influenced biology, till Francis Galton (Darwin’s cousin) disproved theory in 1870’ Gregor Mendel Grew up on a farm in Czech Republic (Austria),1843 entered monastery, University of Vienna, became teacher interest in breeding plants at monastery Why peas ? Peas worked better, basic, 7 traits that gave results Mendel could understand Mendel’s 7 Pairs of Contrasting Pea Traits round/wrinkled ripe seeds, yellow/green (interior, unripe pods), inflated/pinched pods, axial/terminal flowers, long/short stem, purple/white petals Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh Mendel’s Methods • Emasculation(cut of anthers), Cross-pollination, obs. of progeny, Statistical interpretation of the results • cross-pollinate(hybridize)2 true-breeding pea variety, parent=P, hyb. offspring=F1 -> self- pollinate=F2 • quantitative analysis of F2 --> 2 principle of heredity: law of segregation & independent assortment. Terms • Character: a heritable feature Trait: a variant of a character • • True-breeding: organisms which when they breed produce the same characters • Hybridization: mating or crossing of two varieties • Monohybrid cross: cross that tracks the inheritance of a single character F1 Generation: first filial, generation following the parental • • F Generation: second filial, F x F • Alleles: alternate versions of a gene approach 1. Start with true-breeding plants approach 2. Start by studying a single trait approach 3. Create a model approach 4. Compare the model with observation Test Cross * Probabilities are accurate only over infinitely large # of time Summary of Mendel’s Model Adult ( 2N ) traits involve 2 alleles • Alleles segregate in gametes (1N) {no blending of traits} • • LAW OF SEGREGATION Biology Midterm Notes - Test 1 Prof: Gardiner+Waugh • Dominant alleles mask the expression of recessive alleles in heterozygotes • True-breeding adults have identical alleles...they are homozygous • Each gamete contains only one allele • Test cross: breed homo. rec. w/dom. phenotype + unknown genotype, determine identity of unknown allele. How does this relate to meiosis? • Homologous chromosomes may have different alleles for seed colour. law of independent assortment: each pair of alleles segregates into gametes independently • • exp. followed inheritance of flower color/other characters focused on a single character via monohybrid crosses, conduced other exp. in which he followed the inheritance of 2 diff. characters, a dihybrid cross. • one dihybrid cross experiment, Mendel studied the inheritance of seed color and seed shape (analysis 1) • hypothesis: 2 pairs of alleles segregate independently, pres. of 1 spec allele for 1 trait, no effect on pres. of spec. allele for 2nd trait. • In our example, the F1 offspring would still produce yellow, round seeds However, when the F1’s produced gametes, genes would be packaged into gametes with all • possible allelic combinations, 4 classes of gametes (YR, Yr, yR, and yr) produced in = amounts. Analysis 1 Analysis 2 Analysis 3 So Y and R are independent? Do the data fit the independence model? •possibility: 2 characters, passed from parent to baby as package. • Y and R alleles and y and r alleles stay together. •If tit was true, F1 offspring would make yellow, round seeds. •F2 offspring would make 2 phenotypes in 3:1 ratio, like a monohybrid cross but was inconsistent with Mendel’s results. * Mendel re
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