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Mendelian Genetics and Plant Sexual Reproduction Lecture 2 R. N. Trigiano and Neal Stewart Alternation of Generations Mitosis and Meiosis Mendelian Genetics Sexual Reproduction in Plants DEFINITIONS: Diploid: Having two sets of chromomosomes or 2N – the sporophyte phase of plants. 2N = 2X = 22 (Flowering dogwood) Haploid: Having one set of chromosomes or N – the gametophyte phase of plants. N = X=11 (Flowering dogwood) Polyploid: Having more than two sets of chromosomes – 2N = 6X = 54 sporophyte; gametophyte N = 3X= 27 (Chrysanthemum) How is Alternation of Generations Accomplished? Mitosis (1 cell 2 cells) and Meiosis (1 cell 4 cells) and Sexual Reproduction where 1 +1 = 2 = 1! Gametophyte (N) mitosis Gametes (N) fuse (sexual reproduction) to form zygote (2N) mitosis Mature Sporophyte (2N) meiosis Spores (n) ALTERNATION OF GENERATIONS Lower Plants Higher Plants SHADED AREA REPRESENT RELATIVE TIME IN DIPLOID OR SPOROPHYTIC PHASE; OPEN = RELATIVE TIME IN HAPLOID OR GAMETOPHYTIC PHASE. 1 MOSS FERN A www.calflora.net silviahartmann.com D E A B D C C B Figure 6.1 Structure of DNA Nitrogenous bases Adenine Thymine Guanine Cytosine Phosphate (PO4) Deoxyribose sugar Figure 2.1 Mitosis A. Chromosome Physical States Nucleus Nucleus Relaxed Chromosome Condensed Chromosome B. Chromosome Conformations Chromatid Centromere C. Homologous Chromosomes Sister Chromatids DNA Synthesis Locus 2 Centromere Locus 1 Chromosome 1 Chromosome 1 Chromosome 1 Chromosome 2 http://nobelprize.org/nobel_prizes/medicine/laureates/2001/cellcycle_eng.jpg http://tainano.com/Molecular%20Biology%20Glossary.files/image011.gif STAGES OF MITOSIS PROPHASE: Shortening and thickening of chromosomes (actually sister chromatids: Movement to the metaphase plate; nucleous disappears; nuclear membrane disappears. METAPHASE: Chromosomes lie on the “ cell plate”. ANAPHASE: Sister chromatids separate and are pulled toward the poles. TELOPHASE: Last phase of mitosis; chromosomes become reorganized into nuclei, nucleolus reappears and nuclear membrane is reformed. CYTOKINESIS: Literally division of the cytoplasm (including organelles such mitochondria, chloroplasts, etc. Essentially, mitosis is “asexual division (formation of somatic cells and builds the plants). Theoretically, all cells resulting from mitosis are “clones” – not quite true, but close enough for our purposes. 1 = Interphase, 2-5 = Prophase; 6 = Early Metaphase; 7 = Late Metaphase; 8 = Anaphase; 9 = Telophase; 10 = Completion of Cytokinesis http://www.vcbio.science.ru.nl/images/cellcycle/mmitosis_onion_zoom.jpg Meiosis DEFINITIONS Meiosis: Two successive divisions (reduction followed by an equal) that reduces the ploidy level from 2n to n or diploid to haploid. Homologous Chromosomes: Chromosomes that associate in pairs in the first division of meiosis: each member of the pair was derived from a different parent. Crossing Over: The exchange of corresponding segments of genetic material between chromatids of homologous chromosomes. See also Synapse and Chiasma Recombination: The process by which offspring can gain combination of genes different from the combinations in either of their parents. Leads to genetic diversity. Figure 2.8. Crossing- over B. A. Synapse – pairing of homologous chromosomes C. Chiasma - Exchange between homologous chromosomes--sister chromatids during Prophase I. Results in recombination and increased diversity Stages of Meiosis Prophase: Same as in mitosis except that crossing over occurs here. Metaphase I: Homologous chromosomes (each chromosome consists of 2 sister chromatids) line up “opposite” each other on the plate. Anaphase I: One of each of the homologous pair of chromosomes are “drawn” to the poles. This essentially reduces the number of chromosomes that are in each cell by half. Diploid Haploid. Telophase I: Haploid cells are established; chromosomes “disappear”. Prophase II: Chromosome condense (sister chromatids are not identical to one another because of crossing over). Metaphase II: Chromosomes (actually sister chromatids) align on the plate. Anaphase II: One sister chromatid (now chromosome) are drawn to each pole. Teleophase II and Cytokinesis: Essentially the same as in mitosis. Meiosis is essentially a “reduction” division followed by an “equal” division. “Tetrad” (4) of cells or spores are formed. Figure 2.7. Mitosis Metaphase Meiosis I Metaphase I Anaphase I Metaphase Plate Anaphase II Cytokinesis Cytokinesis N N Metaphase Plate Metaphase Plate Anaphase Metaphase II 2N 2N Meiosis II N Cytokinesis N Figure 2.2. Gregor Mendel, working with peas, made two innovations to the science of genetics: 1) developed pure lines 2) counted his results and kept statistical notes Also known as transmission genetics Traits are inherited across generations by sexual reproduction Mendelian traits are typically simple qualitative traits SOME MORE DEFINITIONS Pure Line - a population that breeds true for a particular trait [this was an important innovation because any non-pure (segregating) generation would and did confuse the results of genetic experiments] Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual Dominant - the allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F1 generation from the cross of two pure lines. Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F1 generation from the cross of two pure lines and reappears in the F2 generation Co-dominance: a relationship among alleles where both alleles contribute to the phenotype of the heterozygote. Allelic pair - the combination of two alleles that comprise the gene pair http://www.ndsu.edu/instruct/mcclean/plsc431/mendel/mendel1.htm • • • • • • • • • Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote Genotype - the specific allelic combination for a certain gene or set of genes Phenotype - literally means "the form that is shown"; it is the outward, physical appearance of a particular trait Mendel's pea plants exhibited the following phenotypes: - round or wrinkled seed phenotype - yellow or green seed phenotype - red or white flower phenotype - tall or dwarf plant phenotype http://www.ndsu.edu/instruct/mcclean/plsc431/mendel/mendel1.htm Mendel’s Laws Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete e.g. Aa yields gametes with A a Mendel’s Second Law -- the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair (assume no close linkage) Figure 2.4. Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete A. Monohybrid Cross B. F1 Self Fertilization Parent 1 Parent 2 X X YY Gametes: Y yy Y Parent 2 Parent 1 y Yy Yy y F1 Fertilization: Gametes: Y F2 Fertilization: y Y Parent 1 Parent 1 Y Y Y y y Yy Yy YY Yy y Yy Y YY & Yy Parent 2 Parent 2 Yy F1 Hybrid Plants: 100% yellow Yy y y Yy yy F2 Plants: 75% yellow 25% green yy ty TY TY TtYy TtYy ty TtYy TtYy F1 Generation of Dihybid cross Figure 2.5 Independent Assortment – Two or more traits (color and height). A. Dihybrid Cross Parent 1 TTYY Parent 2 X F1 Fertilization: ttyy TtYy F1 Plants: 100% tall/yellow B. F1 Self Fertilization TtYy X TtYy Parent 1 F2 Fertilization: Ratios of F2 Plants: TY Ty tY ty TY TTYY TTYy TtYY TtYy T_Y_ = 9/16 tall/yellow Ty TTYy TTyy TtYy Ttyy T_yy = 3/16 tall/green tY TyYY TtYy ttYY ttYy ttY_ = 3/16 short/yellow ty TtYy Ttyy ttYy ttyy ttyy = 1/16 short/green Parent 2 Co-dominance –Variation to Mendel “laws” Another Monohybrid Cross RR = Red Flower Parent r R Rr -- rr = White Flower Parent r Rr F1 R Rr Rr If Dominance (R) then all should be RED r r R Rr Rr R Rr Rr F1 R R But co-dominance of R and r produces all pink flowers RR r Rr F2 r rR rr 1 Red; 2 Pink; 1 White Flowers and Sexual Reproduction Flower Parts – Only Angiosperms! 5 PIST SEP PET ST 4 AN ST FL ST STL PIST OVY PET OVL SEP REC RAF Sexual Reproduction Definitions Self Incompatibility: Plant that cannot fertilize its female gametes with its own male gametes. Basically two types: Gametophytic in which the pollen grain will not grow when it shares the same incompatibility gene (s) and Sporophytic in which the diploid parent hinders germination and growth of the pollen tube. Both types prevent double fertilization. Outcrossing Species: basically self-incompatible; requires a partner of a different genotype. Pollination: The process in which the pollen is carried from the anther to the stigma.Basically two types: Cross (between different genotypes of plants) and Self (same plant). Double Fertilization: one sperm nucleus unites with egg to form the zygote and restore the sporophytic phase (2N) of the plant and the other sperm nucleus uniting with the polar nuclei of the female gametophyte to form the primary endosperm nucleus. Figure 2.11 Self-incompatibility systems in plants may be gametophytic (a) or sporophytic (b). In gametophytic self-incompatibility, the pollen grain will not grow and fertilize ovules if the female plant has the same self-incompatibility (S) alleles. In sporophytic selfincompatibility, the diploid parent prevents germination of pollen grains that share an allele with the parent. (Adapted from Briggs and Walters 1997). Zygotic Embryo Ontogeny -- Dicot Figure 4.4 Figure 2.12 Figure 2.12 Triangle of U (1935) shows the relationships between several diploid and polyploidy crop species within the Brassica genus. Figure 2.13 Figure 2.13 Hybridization and genetic integration between closely related species allows for the incorporation of genetic material from one species to another. Summary Slide 1 • All plants (and animals) exhibit alternation of generations • For higher plants, the sporophytic (2N) generation is the dominant dominant phase • Meiosis is the driving biological process behind alternation of generations. Summary Slide 2 • Mitosis in an equal division in which the ploidy of the mother cell is maintained • The phases of mitosis are Prophase, Metaphase, Anaphase, and Telophase followed by Cytokinesis. • Meiosis is a reduction division followed by an equal division similar to mitosis. Results in four cells of reduced (haploid) ploidy • Crossing over (exchange of sister chromatid segments occurs in Prophase I and results in genetic recombination. Summary Slide 3 • Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete • Mendel’s Second Law -- the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair (assumes no close linkage) • Expression of alleles can be dominant, recessive or codominant.