1/19/16 Updated Notes
1/19/16 Updated Notes BIOL 11100 - Fundamentals of Biology II
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BIOL 11100 - Fundamentals of Biology II
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This 4 page Class Notes was uploaded by Phoebe Notetaker on Thursday January 21, 2016. The Class Notes belongs to BIOL 11100 - Fundamentals of Biology II at Purdue University taught by Dr. Athena Anderson in Winter 2016. Since its upload, it has received 24 views. For similar materials see Biology in Science at Purdue University.
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Date Created: 01/21/16
1/19/16 Meiosis What’s the difference b/w a centriole, centrosome and centromere? Centrosomes are composed of centrioles which are composed of microtubules. Reproduction Two types: 1. Asexual: Offspring are clones of one parent. 2. Sexual: Offspring are unique, different from both parents. Asexual Reproduction more common in plants and protists than animals 1. Binary fission 2. Budding 3. Runners 4. Rhizomes Sexual Reproduction ● Gametes (sperm, egg): Sex cells produced in gonads of parents, designed to carry half of parents’ genes. ● Somatic cells: Not involved in producing gametes (body cells: liver, heart, skin, etc.). Genes ● Gene: Segment of DNA that codes for a trait. ○ Ex: Eye color ● Allele: Version of a gene. ○ Ex: Brown eyes ● In humans, most genes have two alleles; one inherited from father, the other from mother. Homologous Chromosomes ● Homologous chromosomes have genes coding for the same characteristics, in the same locations (loci). Karyotype ● Karyotype: Picture of chromosomes ● Slide 8 ● For humans, 2n = 46 ● Sex chromosomes are no. 23. ● Other chromosomes are called autosomes. Ploidy ● Denotes the number of copies of genes/chromosomes in organism, abbreviated as a number and the letter “n.” ○ Haploid: 1n, having one copy of genes ○ Diploid: 2n, having two copies of genes ● There are organisms that are triploid, tetraploid, etc. ● From the moment of fertilization, humans are 2n. ● The dilemma: How to sexually produce offspring that are also 2n, with the right number of chromosomes, when all your own cells are 2n? Meiosis ● Meiosis solves the problem by “reductive division,” halving the number of chromosomes in sex cells. ● Cells produced are not identical to parent cell (unlike in m. osis) ● These unique cells add genetic variation to population. ● This is the variation that allows natural selection and evolution to happen. ● Gametogenesis: Gamete formation. ○ Two successive cell divisions ○ No DNA replication in between ○ Produces haploid cells ○ Produces 4 cells instead of 2 (in most cases) Chromosomes ● You have two homologs for each chromosome (diploid = 2n). ● After replication, each has two sister chromatids ○ Slide 14 diagram ● Homologous chromosomes come together during meiosis to form a tetrad; process called synapsis. ● Tetrad has 4 chromatids total ● Blues identical to each other, but might be different from reds. ○ Slide 15 diagram ● Centromere holds chromatids together ● Kinetochore connects centromere w/ microtubule spindle fibers ○ Think about an oreo: The centromere is a black piece, the kinetochore is white stuffing, and the microtubule spindle fibers are a black piece ● Corona aids attachment of kinetochore to microtubules Meiosis I ● Meiosis I is significantly different from mitosis ● Overall purpose is to have increased genetic variation ● Phase names similar to mitosis: ○ Interphase 1, prophase 1, metaphase 1, anaphase 1, telophase 1, cytokinesis. Interphase before Meiosis 1 ● All chromosomes (uncoiled as chromatin) copied twice (bivalent) so tetrads can form later. ● Still considered 2n, not 4n, b/c two chromatids together can be called one chromosome. Meiosis 1, Prophase 1 ● Chromosomes visible ○ Chromatin is wound into chromosomes ● Nuclear membrane disintegrating ● Tetrads form ● Synaptonemal complex [proteins that hold homologs together (synapsis)] ● Crossing over occurs at chiasmata ● Crossing over: Homologs exchange sections b/w nonsister chromatids and form new combinations of genes ○ The two chromosomes get mixed up with each other and then make unique new gene combinations ● Chiasmata [the sections that are seen as crossed over regions] Other forms of adding genetic diversity ● Besides crossing over there is independent assortment of chromosomes and random fertilization. ● Independent Assortment of Chromosomes: At metaphase I, the homologous pairs of chromosomes (consisting of one from the dad and one from the mom) are situated at the metaphase plate. Each pair may orient itself with with its maternal or paternal homolog closer to a given pole (its orientation is 50% likely to the maternal chromosome and 50% likely to get the paternal chromosome). B/c each pair of homologous chromosomes is positioned independently of the other pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair. Meiosis 1, Metaphase 1 ● Microtubules move tetrads to the metaphase plate ● Each tetrad is lined up in the middle of the cell [powered by motor proteins] ● One chromosome in each tetrad faces each pole Meiosis 1, Anaphase 1 ● Microtubules pull tetrads apart ● One chromosome with its two chromatids goes to each pole ● Sister chromatids still attached at centromere ● There is evidence that crossing over has occurred (a distinguishment from mitosis to meiosis) Meiosis 1, Telophase 1 & Cytokinesis ● Chromosomes moved to opposite poles ● All chromosomes have two chromatids ● Nuclear envelope reforms ● Two haploid cells result after cytokinesis ○ By the time cytokinesis occurs, the genetic material/chromatin is in an unwound state ● Each chromosome has two chromatids Meiosis 1 Review, slide no. 24 You see a picture of a cell with clearly visible chromosomes each with two chromatids and at least one chromosome showing evidence of cross over. No tetrads are present. The chromosomes have no spindle fibers attached are not aligned at the equator and are not being pulled apart. There is no cleavage furrow. In what stage of meiosis is this cell? Prophase II Anaphase Metaphase II Telophase I
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