Unit 4, Week 2 of Notes
Unit 4, Week 2 of Notes Bio 190
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This 9 page Class Notes was uploaded by Danielle Francy on Saturday April 23, 2016. The Class Notes belongs to Bio 190 at Towson University taught by Joseph Velenovsky in Fall 2015. Since its upload, it has received 12 views. For similar materials see Intro Biology for Health Professions in Biology at Towson University.
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Date Created: 04/23/16
Unit 4, Week 2 Notes Synapsis: ● Chromatids of homologous chromosomes exchange segments ● Crossing over ● Alleles may differ between homologs ● Rearrangement of genetic information ● Critical ● Key component of genetic variability stemming from sexual reproduction ● Chromosomes further condense ● Centrosomes move away from each other ● Spindle forms between them ● Nuclear envelope breaks into fragments ● Chromosomal tetrads are captured by spindle microtubules ● Moved toward the center of the cell ● Kinetochores Metaphase I: ● Chromosomal tetrads align on metaphase plate ● Midway between the two poles of spindle ● ***Sister chromatids still attached to each other at centromere ● Spindle microtubules attached to kinetochores ● Homologous chromosomes are held together at the sites of crossing over ● Microtubules from one pole vs. other pole ● Opposite movement Anaphase I: ● Chromosomes move toward opposite poles of the cell ● Sister chromatids composing each doubled chromosome remain attached at the centromere ● Only the tetrads split up ● Haploid number of chromosomes move towards each spindle pole ● If this was mitosis, six daughter chromosomes would be moving toward each pole Telophase I: ● Chromosomes arrive at opposite poles ● Each pole has a haploid chromosome set ● Each chromosome is still duplicated ● Each chromosome consists of two sister chromatids ● Cytokinesis usually occurs concurrently with telophase I ● Two haploid daughter cells are formed ● In some species, chromosomes uncoil and the nuclear envelope reforms ● Interphase between meiosis I and meiosis II ● In other species, daughter cells of first meiotic division proceed directly into meiosis II ● Either way, no chromosome duplication occurs between cytokinesis and meiosis II Meiosis II: ● Same as mitosis ● Starts with a haploid cell ● Prophase II ○ Spindle forms ○ Chromosomes are moved towards the middle of the cell ● Metaphase II ○ Aligned on metaphase plate ○ Kinetochores face opposite poles ● Anaphase II ○ Sister chromatids finally separate ○ Individual daughter chromosomes move towards opposite poles ● Telophase II ○ Nuclei envelope reformation ○ Cytokinesis occurs concurrently ○ Four daughter haploid cells Growth, Tissue repair, and Asexual reproduction: ● Genetically identical chromosomes duplicate once during S subphase (interphase) ● One nuclear and cytoplasmic division ● Two identical diploid daughter cells ● 2n=4 ● Mitosis ● No tetrads ● Sister chromatids separate during anaphase Sexual Reproduction: ● Genetically unique haploid daughter cells ● One homologous chromosome of each homologous pair ● Chromosomes duplicate during S subphase (interphase) ● Two nuclear and cytoplasmic divisions ● Four nonidentical haploid cells ***All events unique to meiosis occur during meiosis I. Prophase I: ● Tertrad formation with duplicated homologs ● Four chromatids ● Each pair of sister chromatids joined at their centromeres ● Also linked at sites of crossing over ● Tetrads aligned at metaphase plate ● ***Not individual chromosomes ● Anaphase I tetrads break apart ● Sister chromatids still together ● End of meiosis I ● Two haploid cells but each chromosome still has two sister chromatids ● Meiosis II same as mitosis except for haploid starting ploidy and end products ● Ploidy=haploid or diploid Ways meiosis produces genetic variability: ● 2n=4 ● Red =mother; Blue=father ● Orientation of paired homologous chromosomes is random ● Red may be on top; blue may be on top; red may be on bottom; blue may be on bottom; whether maternal or paternal chromosomes are closer to a given pole is equivalent to flipping a coin ● 50% chance a daughter cell gets a particular paternal chromosome ● 50% chance a daughter cell gets a particular maternal chromosome ● Within a homologous pair ● Possibility A: Both maternal chromosomes on same side of the metaphase plate; both paternal chromosomes on same side of the metaphase plate ● Possibility B: Tetrads oriented differently, maternal and paternal chromosomes on same side of metaphase plate ● Gametes from A can either have two red or two blue chromosomes ● In B, gametes have one red and one blue chromosome ● Half of the gametes have a big blue chromosome and a small red one ● The other half have a big red chromosome and a small blue one ● Four combinations are possible ● All four types produced in equal amounts ● All chromosome pairs orient independently at metaphase I ● X and Y behave as a homologous pair ● Total number of combinations of chromosomes is 2n; where n=the haploid number ● For humans, there’s a total of 8 million combinations ● Every sperm and egg is composed of one of roughly 8 million combinations that were inherited from that organism’s mother and father ● 64 trillion combinations are possible in a zygote ● Random nature of fertilization is important; based on whether maternal or paternal sister chromatids line up on a particular side of the metaphase plate during metaphase I ● Crossing over adds even more variation Metaphase I: ● Independent orientation of metaphase chromosomes ● Level of genes ● This is why it matters if maternal or paternal are on a particular side of the metaphase plate ● Genes at same loci; different allelic versions ● Different alleles on homologous chromosomes is what accounts largely for differences between gametes and therefore offspring ● Part of the equation Crossing Over: ● Exchange of corresponding segments between nonsister chromatids of homologous chromosomes ● Crossing over occurs during prophase I of meiosis ● Tetrads, four chromatids ● Chiasma; sites of crossing over Chiasma: ● Two homologous (nonsister) chromatids are attached to each other ● Crossing over occurs early in prophase I ● Loci are aligned precisely ● Sister chromatids don’t break because they are genetically identical ● Paternal and maternal break at some locus ● Two homologous segments trade places or cross over ● New combinations of maternal and paternal genes (alleles) ● New segments are found ● These came from corresponding homolog ● Crossing over produces recombinant chromosomes Genetic Recombination: ● If no crossing over ever occurred, gametes would consist of two genetic types ● Paternal type ● Maternal/paternal ● In humans, an average of 13 crossover events occurs per chromosome pair ● Our gametes are likely not like our somatic cells in reference to alleles ● Different alleles arise from mutations ● Once they rise, they are shuffled using the three sources ○ How chromosomes are oriented during metaphase I (independent orientation) ○ Random fertilization (8 million and 8 million= 64 trillion) ○ Crossing over ● Abnormal chromosome numbers and major alterations in structure ● Detected using a karyotype; metaphase of mitosis ● Condensed and replicated ● Lymphocytes ● WBC ● Chemical that stimulates mitosis (Mitogen); LPS ● Grown in culture for several days ● Colchicine arrests mitosis by acting as a spindle poison ● Metaphase because it is most highly condensed ● Centrifuged to separate blood cells from fluid ● Fluid is discarded ● Hypotonic solution makes RBCs burst and WBCs swell (chromosomes spread out) ● Centrifugation separates swollen WBCs ● Fluid containing remains of RBCs is poured out ● A preservative is mixed with the WBCs ● Placed on a microscope slide, dried, and stained Heredity: ● Transmission of traits from one generation to the next ● Gregor Mendel (1866) ● Garden peas ● Discrete heritable factors ● Genes may be rearranged but each gene permanently retains its identity ● Garden peas were used because they have short generation times ● Large amount of offspring ● Varieties that are different ● Character ● Heritable feature that varies among individuals flower color ● Purple and white flowers Trait: ● Variant for a character ● Purple and white flowers ● Mendel controlled matings ● Petal encloses reproductive organs well ● Selffertilize ● Pollen grains from stamen carrying sperm land on eggcontaining carpel in the same flower ● Prevents pollen from separate individual ● Mendel sometimes wanted crossfertilization ● Pollen from a different plant fertilizes the carpel of a plant; not own pollen Process: ● Cut immature stamens ● Mendel through this methodology knew parentage of all his plants ● Which characteristic he chose was also very important ● Seven characteristics ● Each of which occurred as two distinct traits ● Ex: flower color, purple and white ● He eventually obtained true breeding varieties ● Selffertilization produces offspring that are identical to the parent ● When he crossed true breeding varieties, he got true breeding purple and true breeding white ● Produces hybrids ● Crossfertilizations called hybridization or a genetic cross ● True breeding P generation ● Hybrid offspring F1 generation ● F1 selffertilize or cross fertilize F2 generation ● Why did Mendel use true breeding varieties? ● Could make clear hypotheses about the results through controlled experiments ● Purple and white flowers ● The results of his experiment led him to prepare several hypotheses related to inheritance ● Monohybrid cross ● True breeding parental plants differ in only one character ● Flower color ● Purple ● White ● Not light purple ● Mendel suggested that heritable factor for white flowers was masked in the first generation by the heritable factor for purple flowers ● Plants in the first generation must have carried two heritable factors ● One for purple; one for white ● Pattern was observed for other six characters ● From his results, four hypotheses were developed ● Use modern term of gene instead of heritable factor ● Alternative versions of genes exist ● These are responsible for variations in inherited characters ● Purple and white called alleles ● Offspring inherit two alleles one from each parent ● Alleles may be different or the same ● Heterozygous, homozygous ● Dominant allele and recessive allele ● Dominant determines trait ● Recessive has no determination in trait ● In heterozygotes only, not homozygotes ● Gametes carry one allele because allele pairs separate during meiosis ● Fertilization restores paired condition of alleles ● P dominant allele ● P recessive allele ● PP, pp ● Allele pairs separate during meiosis is called law of segregation ● F1 hybrids make P and p in equal numbers ● Punnett square shows four possible combinations of alleles that could occur when P and p combine (gametes) ● Phenotype ratio, Genotype ratio ● All seven characteristics exhibited this inheritance pattern ● White flowers disappear in first generation. Reappear in second generation (¼) ● Mendel’s law of segregation ● Pairs of alleles separate during meiosis ● Gametes fuse during fertilization and create allelic pairs ● This law applies to all sexually reproducing organisms ● Separation of homologous chromosomes ● How many genes? ○ 3 ● Genes for the same characters at the same loci ● Same or different alleles ● Alleles of a gene found at same locus on homologous chromosomes Dihybrid Cross: ● Parental varieties that differ in two characters ● Color or shape ● RRYY ● Homozygous plants that had round and yellow seeds ● Rryy ● Homozygous plants that had wrinkled and green seeds ● RRYY produces RY gametes ● rryy produces ry gametes ● First generation must yield hybrid heterozygous RrYy ● Dihybrids ● Phenotype=round and yellow seeds ● Double dominant phenotype ● Crossed first generation plants with each other ● Dependent assortment genes for color and shape inherited together ● Independent assortment ● Genes for color and shape inherited independently from each other Punnett Square for a Dihybrid Cross: ● Four possible gametes ● Each second generation plant inherits one of four possible sperm and one of four possible eggs ● 16 total combinations ● Ratio= 9:3:3:1 ● Alleles segregate independently of each other ● Equivalent to two monohybrid crosses occurring at the same time ● 12 round seeds to 4 wrinkled, 12 yellow to 4 green ● Reduce to 3:1 second generation ration for a monohybrid cross ● Monohybrid cross occurring for each trait ● Mendel repeated dihybrid crosses with different combinations of his seven characters ● Ratio was always about the same 9:3:3:1 ● Each allele pair separates independently during meiosis ● Gamete formation ● Inheritance of one character has no influence on the inheritance of other characters ● Law of independent assortment Testcross: ● Individual of unknown genotype and a homozygous recessive individual for the character of interest ● Phenotype of offspring reveals genotype of parent ● Mendel used testcrosses to verify truebreeding varieties ● Large samples before inheritance patterns could be verified ● Probability that a second generation individual will have the bb genotype ● .5X.5=.25 ● Probability that a second generation individual will be heterozygous for the coatcolor gene ● Two ways this can occur ● B from egg; b from sperm ● B from sperm; b from egg ● .25+.25=.5 ● Trihybrid cross ● Three different characters ● Wildtype traits: ○ Traits that exist often in nature ● Not always dominant allelic traits ● Similarities between how chromosomes behave and how heritable factors behave ● Chromosome theory of inheritance ● Genes occupy specific loci ● Chromosomes undergo segregation and independent assortment ● During meiosis and fertilization ● Simplified diagram ● Chromosomal basis of the law of segregation ● Pairs of alleles separate ● Two alleles separate in anaphase I ● Fertilization then recombines the two alleles at random ● Basis of the law of segregation ● Chromosomal basis of the law of independent assortment ● Alleles do not influence the sorting of other alleles (seed shape and seed color) ● Arrangement of tetrads ● Random chance ● Four gametes