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biol139-lecture15-2011 - Next topic Next Meiosis Meiosis pp...

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Unformatted text preview: Next topic Next Meiosis Meiosis pp 93 - 111 pp pp 46 - 52 iGenetics Somatic Cells: Somatic • mitotically dividing and G-arrested cells • make up vast majority of individual’s tissues • diploid in nature (2n) NB* n is the number of chromosomes in a gamete Germ Cells: Germ • pockets of specialized cells (2n) • produce gametes (egg/sperm) • incorporated into ovaries and testis • undergo meiosis meiosis • produce haploid gametes (n) (one of each chromosome) haploid Meiosis : An overview Meiosis Diploid germ cell nucleus ONE round of DNA synthesis Replication of DNA (2n) • Homologous chromosomes pair • Exchange of genetic material 1 n Centromere remains INTACT 2 gametes n Germ cells undergo two successive nuclear divisions of a diploid nucleus to produce gametes - meiosis I and meiosis II Separation of duplicated chromosome pairs 2n→n Separation of sister chromatids Meiosis I = Reduction Stage -2n to n = 1 cell to 2 cells • chromosomes have replicated -2 sister chromatids ** Centromere remains INTACT during Meiosis I ** • Homologous chromosomes: • pair • recombine genetic material • segregate into 2 separate nuclei (n) 4 stages: • prophase I, metaphase I, anaphase I + telophase I Longest phase: 5 substages: leptotene, zygotene, pachytene diplotene and diakinesis Prophase I : Leptotene -thin thread Prophase Leptotene Looks like mitosis prophase • Chromosomes have duplicated • centrosomes have duplicated .chromosomes become thickened, threadlike and visible .chromosomes .the nucleolus begin to disappear .centrosomes begin to move to each pole and produce spindle fibers spindle • During leptotene, threadlike chromosomes begin to condense, thicken, and appear long and thin condense, • Become visible as discrete structures • Although chromosomes have duplicated, sister Although chromatids of each chromosome are not yet visible chromatids Prophase I : Zygotene -paired Prophase • condensed chromosomes seek out homologous partner • homologous chromosomes “zipper” together in an intimate association (synapsis) • “zipper” is a protein structure called synaptonemal complex synaptonemal chromosomes are maximally condensed prior to synapsis • centrosomes reached their respective poles, producing spindle fibers Tetrad (bivalent) formation -Zygotene: Homologous chromosomes • homologous (synapsed) chromosomal pair called a bivalent or tetrad Prophase I : Pachytene -thick or fat Prophase Individual chromatids visible Shortened, condensed Crossing-over -Genetic exchange between nonsister Genetic nonsister chromatids of a homologous pair, occurs chromatids of each synapsed chromosome pair called a bivalent or tetrad During pachytene, recombination nodules appear along recombination the synaptonemal complex nodules facilitate the exchange of DNA at various points Crossing-over results in a mixing of genetic material between paternal and maternal homologous chromosomes (non-sister chromatids of homologous chromosomes) • crossing-over occurs during every meiosis crossing-over • sites vary from meiosis to meiosis sites • # of different outcomes is very large of Prophase I : Diplotene Prophase •chromosomes begin to move apart • but held together at cross-over but points (X-like appearance) points .synaptonemal complex dissolves .tetrads of 4 chromatids are visible, as they begin to move apart .crossover points appear as chiasmata (X-like), which hold nonsister chromatids together During diplonema, the tetrad appears to pull apart During slightly, but remains connected at crossover sites, slightly, called chiasmata chiasmata Tetrad Crossing-over maternal and paternal exchange of information can result in new allelic combinations Prophase I : Diakinesis -moving apart Diakinesis Chromatids thicken and shorten the nuclear membrane and the nucleolus break down nucleolus spindles form and spindles attach to kinetochores attach Chiasmata begin to terminalize (move towards the ends) Chiasmata terminalize During diakinesis further condensation of the During chromatids occurs. Non-sister chromatids that have exchanged parts by crossing-over remain have loosely associated at chiasmata End of prophase I End Metaphase I Metaphase • a single functional kinetochore attaches single to sister chromatids to Tetrads line up along the metaphase plate Tetrads metaphase • maternal/paternal face opposite poles (random) Each chromosome of a homologous pair Each homologous attaches to fibers from opposite poles attaches opposite Sister chromatids attach to fibers from the same pole by single kinetochore Sister same • a single functional kinetochore single attaches to sister chromatids sister Each chromosome of a homologous pair attaches to fibers from opposite poles opposite Sister chromatids attach to fibers from the same pole by single kinetochore Sister same Anaphase I Anaphase The centromere does not split between sister chromatids The not chiasmata removed from the pulling apart of the homologs Homologous chromosomes move to the opposite poles. Telophase I Telophase haploid number of chromosomes sister chromatids attached at centomere .nuclear membranes form around the chromosomes that have moved to the poles that .cytokinesis follows .cytokinesis Interkinesis Interkinesis Similar to interphase, but no chromosomal Similar no duplication takes place Meiosis II -separation of sister chromatids -2 cells to 4 cells Prophase II Chromosomes condense; centrioles move to the poles Metaphase II Metaphase Two differences from Mitosis: • # of chromosomes now n • sister chromatids no longer identical sister Chromosomes align at the metaphase plate; sister chromatids attach to spindle fibers from opposite poles Anaphase II Centromeres split and sister chromatids move to opposite poles poles Telophase II Chromosomes begin to uncoil; Chromosomes nuclear envelopes and nucleoli renuclear form Cytokinesis Cytokinesis The cytoplasm divides, forming four new cells (gametes) Mitosis vs. meiosis Mitosis Mitosis vs. meiosis Mitosis Genetic Significance of Meiosis: Genetic • meiosis generates cells with half (n) the number of chromosomes found in diploid cells (2n) and fusion of the haploid nuclei restores chromosome number to 2n Crossing-over between maternal and paternal Crossing-over chromatid pairs during meiosis I generates still more variation in the final combinations In Metaphase I, each maternal or paternal derived chromosome has an equal chance of aligning on one or other side of metaphase plate • each nucleus generated by meiosis will have some each combination of maternal and paternal chromosomes combination Mendel’s Law of Independent Assortment is based on this fact Yellow can combine in a gamete with green just as easily as with orange ...
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