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Transcript
Mendel and Meiosis Chp 10 Pp. 252-279 Contents • 10-2 Meiosis • 10-1 Mendel Chromosome Rod shaped structures made of DNA and proteins Carrier of genetic material Located in the nucleus Copied and passed from generation to generation Chromosomes Sex chromosomes: chromosomes that determine the sex of an organism Humans X and Y Females: X X Males: X Y Autosomes: all other types of chromosomes Chromosomes Homologous Chromosomes 2 copies of each autosome Same size and shape Carry genes for the same trait Ex. If 1 homologous chromosome contains gene for eye color the other homologous chromosome will too. Homologous Chromosomes Diploid Cell Cell with 2 sets of chromosomes Contains chromosomes for each homologous pair Somatic Cells= Body Cells: Diploid All human cells except sex cells are diploid. One from each parent 2n Humans 2n = 46 Haploid Cell containing one of each kind of chromosome Sperm and egg cells = Gametes 1 set of chromosomes Half the number of chromosomes of diploid n Sperm + Egg = Zygote n + n = 2n Mitosis vs. Meiosis • Mitosis: t = two: Diploid 2n=46 • Meiosis: o = one: Haploid » n = 23 Meiosis Gametes are produced in specialized body cells Produces Sperm and Egg Cells 2 divisions: Meiosis I and Meiosis II Meiosis occurs in sex cells, not body cells Results in 4 daughter cells Each cell has half the chromosomes of the parent Meiosis I Interphase occurs: the cell grows and DNA replicates Meiosis I begins Original cell produces two new cells Prophase I DNA Coils tightly into chromosomes spindle fibers appear Each chromosome lines up next to the homologue Synapis occurs: pairing of homologous chromosomes Tetrad: Each pair of homologous chromosomes Crossing Over •Crossing Over: why we do not look exactly like our parents. Portions of the chromatid breaks off and attaches to adjacent chromatids on the homologous chromosome Permits the exchange of genetic material between maternal and paternal chromosomes •Occurs during Prophase Genetic Recombination Crossing over produces a new mixture of genetic material • Occurs during Prophase Causes of Variation Chromosomes are assorted randomly Crossing over may occur Cells do not have identical genetic info as each other or the parent Good: more chance of survival and evolution Bad: mistakes more likely Metaphase I Tetrads line up randomly along the mid-line Spindle fibers attach to centromeres Anaphase I Homologous chromosomes move to the opposite poles Random separation or Independent Assortment results in separation of maternal and paternal chromosomes. Telophase I Chromosomes reach opposite ends of cell Cytokinesis begins cell is haploid Meiosis II Occurs in each cell formed in Meiosis I Interphase does not occur again Prophase II Spindle fibers form and move the chromosomes to the mid-line of the dividing cell Metaphase II Chromosomes move to the midline of the dividing cell facing opposite poles of the dividing cell Anaphase II Chromatids separate and move to opposite poles of the cell Telophase II Nuclear membrane forms around the chromosomes in each of 4 new cells Cytokinesis II Cytoplasm divides Cell Membrane closes off End Result: Four new cells that contain half of the original cells number of chromosomes 4 sex cells are created Meiosis Animation • Meiosis Animation Haploid = one of each kind Diploid = two of each kind • 2n = diploid n = haploid WHY DO WE NEED HAPLOID? Female gamete 23 Male gamete • EGG 23 SPERM Fertilization restores the diploid number 1n fertilization 1n meiosis 2n 2n Mitosis and cell growth 2n Gametes formed by meiosis haploid reproductive cells humans: meiosis occurs in the testes and ovaries Spermatogenesis: Male Oogenesis: Female One Mature Egg Cell Human Karotype Asexual Reproduction Production of offspring from one parent Does not involve meiosis Offspring are genetically identical to parent Sexual Reproduction Production of offspring through meiosis and the union of sperm and egg Offspring are genetically different form parents Genes are combined in new ways in meiosis Evolutionary advantage is that it enables species to rapidly adapt to new conditions Mitosis vs. Meiosis • Mitosis vs. Meiosis Animation Nondisjunction • Failure of homologous chromosomes to separate properly during meiosis. • Both chromosomes of a homologous pair move to the same pole of the cell. • 1 gamete has an extra chromosome • Or • 1 gamete is missing a chromosome Nondisjunction Animation • Nondisjunction Animation 10-1 Mendel • Gregor Mendel • Austrian monk • Studied how traits are inherited from parents to offspring • Father of heredity • Chose garden peas for his meticulous experiments Garden Peas • Reproduce sexually • W/ male & female gametes (sex cells) • Fertilization results in zygote • Becomes seed • Pollination male pollen transferred to female pistil Monohybrid Cross • Hybrid- offspring of parents with different forms of a trait • Tall or short • crossed true-bred tall plants w/ truebred short plants to get heterozygous offspring which then self-pollinated Some Genes are Dominant • Some Genes Are Dominant click to play Seven Traits of Peas Mendel’s Rules • Alleles- different gene forms • Rule of Dominance – Dominant –observed trait – Recessivedisappearing or hidden trait Mendel’s Rules • Law of Segregation • Every individual has 2 alleles of each gene w/ each gamete receiving 1. • During fertilization, gametes randomly pair to produce four combinations Phenotypes & Genotypes • Appearance= Phenotype • Allele combination= Genotype • Homozygous-Both alleles alike • Heterozygousdifferent alleles, one dominant & one recessive Mendel’s Dihybrid Cross Law of Independent Assortment • Says that genes for different traits are inherited independently from each other. • The alleles can recombine in four different ways. Punnett Squares 1BB: 2Bb:1bb • Shorthand way to find possible genotypes from crossing two known parents. • Two heterozygous parents produce 1 homozygous dominant: 2 heterozygous:1 homozygous recessive