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Cell Division Sexual Reproduction = OR Asexual Reproduction = Cell Division: reproduction of cells; “cells come from cells” * Basis of all life 2 Main Roles: 1) development of fertilized egg 2) continuation of life (growth, repair) Prokaryotes = binary fission (split in half) OR Eukaryotes = more complex; more genetic material Prokaryotes = binary fission (split in half) OR Eukaryotes = more complex; more genetic material chromosome: structure which contains DNA (deoxyribonucleic acid) chromatin: long, thin fibers of DNA & protein clumping together to form chromosomes gene: somatic cell: all body cells except egg & sperm; contain chromosomes (humans= 46) Human egg & sperm (gametes) have 23 chromosomes Prior to Cell Division… * All chromosomes duplicate…result in 2 identical parts = sister chromatids (X-shaped) * joined at centromere Sister Chromatids A chromosome and its identical replicated copy, joined at the centromere. When Cells Divide * sister chromatids separate..each goes to separate cell (daughter cell) * Overview of Cell Division * eukaryotic cells divide according to cell cycle cell cycle: sequence of events including time a cell divides until its daughter cell divide 6.4 A time for everything: the cell cycle. Phases in the Cell Cycle 1) Interphase: most of cycle here - chromosomes duplicate - cell grows 2) Mitotic Phase: cell division phase Includes Mitosis & Cytokinesis * Mitosis unique to eukaryotes * Mitosis = continuous process but separated into defined stages Stages of Mitosis 1) Prophase - chromatin fibers coil to form discrete chromosomes - sister chromatids - nuclear membrane breaks near end 2) Metaphase - sister chromatids line up along center of cell Stages of Mitosis 3) Anaphase - sister chromatids separate & migrate to opposite ends of cell 4) Telophase - nuclear membrane reforms around chromosomes Cytokinesis: division of cytoplasm - usually occurs along with telophase - daughter cells separate Mitosis has just one purpose: homologous chromosome: matched pair of chromosomes; same length, genes for same traits at same loci locus (loci = plural): e.g., each chromosome has gene for hair color at same loci, but the gene may be for any color of hair … impt pt = gene results in some color of hair • homologous chromosomes have matching loci & • One chromosome of each pair inherited from mother & father Human Example Somatic cells = 46 chromosomes Sex Chromosomes Human females 1 pair (2 XX) Human male 1 pair (1X, 1Y) • Are human male sex chromosomes homologous? diploid cells: cells with 2 homologous sets of chromosomes in nucleus total # chromosomes = diploid # = 2n human diploid # = 46 (2x23=46) • Humans = diploid animals because most of our cells = diploid (e.g., somatic cell) • But, eggs & sperm are not diploid gametes: egg & sperm cells (sexual reproduction only) haploid cells: cells with 1 homologous set of chromosomes haploid # = n human haploid # = 23 • Human gametes are haploid 6.11 Sperm and egg are produced by meiosis: the details, step-by-step. Mitosis occurs almost everywhere in an animal’s body. Meiosis only occurs in one place. Where? Meiosis starts with a diploid cell. one of the specialized diploid cells in the gonads Meiosis starts with a diploid cell. a homologous pair, or homologues • the maternal and paternal copies of a chromosome Chromosomes are duplicated. sister chromatids • Each strand and its identical duplicate, held together at the centromere. Cells undergoing meiosis divide twice. There are two major parts to meiosis: Meiosis Division 1 Separating the homologues 1. Prophase I The most complex of all of the phases of meiosis Crossing over 2. Metaphase I Each pair of homologous chromosomes moves to the equator of the cell. 3. Anaphase I Beginning of the first cell division that occurs during meiosis The homologues are pulled apart toward opposite sides of the cell. The maternal and paternal sister chromatids are pulled to the ends of the cell in a random fashion. 3. Anaphase I 4. Telophase I and Cytokinesis This phase is marked by the chromosomes arriving at the two poles of the cell. The cytoplasm then divides and the cell membrane pinches the cell into two daughter cells. 4. Telophase I and Cytokinesis Meiosis Division 2 Separating the sister chromatids 5. Prophase II The genetic material once again coils tightly making the chromatids visible under the microscope. It is important to note that in the brief interphase prior to prophase II, there is no replication of any of the chromosomes. 6. Metaphase II The sister chromatids (each appearing as an X) move to the center of the cell. 7. Anaphase II The fibers attached to the centromere begin pulling each chromatid in the sister chromatid pair toward opposite ends of each daughter cell. 8. Telophase II The cytoplasm then divides, the cell membrane pinches the cell into two new daughter cells, and the process comes to a close. Outcome of Meiosis the creation of four haploid daughter cells, each with just one set of chromosomes which contains a completely unique combination of traits Why is there so much variety among species? (e.g., diversity in humans) 1) Independent orientation of chromosomes - in Metaphase I --- way that tetrads line up is due to chance (random) - Results in different possible combinations of chromosomes in gametes - For humans = 8 million possible combos.! 2) Random fertilization (1 egg & 1 sperm) What is probability that 1 of 8 million possible sperm fertilizes 1 of 8 million possible eggs???? Humans = (8 M) * (8 M) = 64 trillion possible combinations of chromosomes due to random fertilization! 3) Crossing Over - can result in genetic recombination genetic recombination: producing gene combinations different from those carried by original chromosomes * During synapsis, tetrad formed – crossing over possible 1) homologous chromatids break at similar locations & chromatids join 2) h. chrom. separate at Anaphase I – crossing over 3) Meiosis II, sister chromatids separate Mendelian Genetics genetics = science of heredity gene: specific region of genetic material (DNA) that provides provides the cell with a “map” Goal: determine patterns of inheritance Mendelian Genetics Gregor Mendel – 1860’s monk significant findings = offspring obtain discrete heritable factors (genes) from their parents Mendelian Genetics Gregor Mendel – 1860’s monk -carefully chose organisms to study (garden pea), controlled pollinations, chose traits that were easy to observe, used statistical methods to analyze data -significant findings = offspring obtain discrete heritable factors (genes) from their parents Terms self-fertilization: plant’s egg fertilized by it’s own pollen cross-fertilization: plant’s egg fertilized by another plant’s pollen (hybridization) P generation: parental generation F1 generation: filial generation; hybrid offspring of the P generation F2 generation: offspring produced by F1 generation via self-fertilization Mendel’s Principles 1) Principle of Segregation – pairs of genes segregate during gamete formation; fertilization pairs genes again monohybrid cross: cross of 2 individuals that differ in 1 trait allele: alternate form of a gene found at same loci of homologous chromosomes 1) Principle of Segregation Ex: Flower color (P = purple, p = white) P = 1 Purple (PP) & 1 white (pp) F1 = all Purple (Pp) F2 = ¾ Purple (PP & Pp) ¼ white (pp) homozygous: identical pair of alleles heterozygous: 2 different alleles for a trait phenotype: physical trait; appearance of organism; expressed as phenotypic ratio genotype: genetic makeup of organism; expressed as genotypic ratio • In the flower color example….. What is the phenotypic ratio? What is the genotypic ratio? ** For monohybrid cross… phenotypic ratio is always 3:1 & genotypic ratio is always 1:2:1 2) Principle of Independent Assortment • each pair of alleles segregates independently during gamete formation dihybrid cross: cross of 2 individuals that differ in 2 traits 2) Principle of Independent Assortment Example P generation: Round (RR) & Yellow (YY) seeds = RRYY Wrinkled (rr) & Green (yy) seeds = rryy Gametes = RY and ry F1 gen: All RrYy (Round & Yellow seeds) Gametes = RY, Ry, rY, ry RY ry Male RrYy Female 2) Principle of Independent Assortment Example (continued) F2 gen: (Do Punnett Square RY RY Ry rY ry Male Ry rY ry Female 2) Principle of Independent Assortment Example (continued) F2 gen: (Do Punnett Square RY RY Ry rY ry Male RRYY Ry rY ry Female 2) Principle of Independent Assortment Example (continued) F2 gen: (Do Punnett Square RY RY Ry rY ry Male Ry rY RRYY RRYy RrYY ry RrYy Female 2) Principle of Independent Assortment Example (continued) F2 gen: (Do Punnett Square RY rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy RrYy rrYy rryy ry Male Ry Rryy Female Probabilities • Probability (chance) of an event occurring ranges from 0 to 1 Probability = 0 = event will not occur Probability = 1 = event will occur always Tossing a Coin What is the probability of getting a “tails”? = 0.5 (1/2) What is the probability of getting a “heads”? = 0.5 (1/2) What is the probability of getting a “heads” or a “tails”? = P(heads) + P(tails) = 0.5 + 0.5 = 1.0 Tossing 2 Coins What is the probability of getting a “heads” on both coins? = P(heads) x P (heads) = (0.5)*(0.5) = 0.25 Flower Color Example F1 = Pp = 0.5 P & 0.5 p gametes F2 = Pp x Pp 1 P (female) x 1 P (male) = 0.5 * 0.5 = 0.25 PP 1 P (female) x 1 p (male) = 0.5 * 0.5 = 0.25 Pp 1 p (female x 1 P (male) = 0.5 * 0.5 = 0.25 Pp 1 p (female) x 1 p (male) = 0.5 * 0.5 = 0.25 pp • What is the probability of getting a heterozygote? • What is the probability of getting a homozygote? Why are some flowers pink? • Complete dominance = dominant & recessive alleles • Incomplete dominance = F1 offspring have phenotype somewhere between that of the 2 parents = both alleles expressed Ex: Flower color (R = red, r = white) P = 1 Red (RR) & 1 white (rr) F1 = all Reddish-White = Pink (Rr) F2 = ¼ Red (RR), ¼ white (rr), ½ pink (Rr) Incomplete Dominance Pleiotropy vs. Polygenic Inheritance • pleiotropy = 1 gene influence many traits e.g., sickle-cell anemia = homozygous recessive disease sickle-cell gene influences: - shape of RBC’s - health of heart, brain, spleen, kidneys • polygenic inheritance = many genes influence 1 trait, e.g., skin color - many genes interact to give diverse skin color ranging very dark to very light Chromosomal Basis Study Slide Study Slide Study Slide