Unit 1 - BIO 311D Introductory Biology 21 JAN 2011 Chapter...

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BIO 311D Introductory Biology 21 JAN 2011 Chapter 13 Meiosis - Basis of sexual reproduction Outline: - Key concepts - Heredity - Asexual/sexual reproduction - Roles of meiosis - Meiosis - Origins of genetic variation - Key terms - Conclusion Key concepts - Clones - A group of genetically identical individuals produced by asexual reproduction - Fertilization or syngame - The fusion of gametes - Sexual reproduction dominates the life cycle of nearly all animals and plants. - Crossing over, random segregation, and random fertilization contribute to variation in traits among offspring. Heredity - Traits trasmitted from one generation to the next. Asexual/sexual reproduction Asexual reproduction - Offspring with the same trait because of identical genes. - Achieved by “budding”
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- Outcome is a clone. Sexual reproduction - Offspring with different traits. - Achieved by fertilization into a syngame. - Outcome of meiosis is a haploid gamete with various genetic material (non-identical to parent) - Majority of animals use sexual reproduction. Roles of meiosis - Human life cycle:
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Meiosis - The basis of sexual reproduction - The process of meiosis DNA replication occurs in interphase prior to meiosis Meiosis I: - Prophase (90%) - Chromosomes condense, homologous chromosomes pairing (synapsis) to form tetrads. Crossing over (chiasmata) occurs between homologues - Metaphase - Chromosomes line up at the equator of the cell (randomly). - Anaphase - Homologues separate, sister chromatids remain together (Random segregation/independent assortment) - Telophase - Chromosomes remain condensed No DNA replication between Meiosis I and Meiosis II.
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Meiosis II: (similar to mitosis) Gametes are never identical to each other because of random segregation and crossing over. The stages of Meiotic cell division: Meiosis I Interphase: Chromatin condenses into choromosomes. Prophase: Sister chromatid pairs cross over to form tetrads and make synapses. Non-sister chromatids exchange segments. Crossing over can happen more than once in each homologous pair. Metaphase: Homologous chromosomes line up at center and are attached to microtubules at the kinetochore. Anaphase: Homologous chromosomes separate. But sister chromatids remain attached in each daughter cell. Telophase: ---------------------------------------------
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Meiosis II Prophase: Sister chromatids all line up. Metaphase: Chromosomes line up along equator of cell. Anaphase: Sister chromatids separate Telophase: Result: Each daughter nucleus has only half as many chromosomes as the original nucleus and the chromosomes are not identical.
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Origins of genetic variation 23 pairs of chromosomes. More than 2^23 different possible chromosome combinations after meiosis. 1. Crossing over
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This note was uploaded on 07/12/2011 for the course GOV 365L taught by Professor Liu during the Spring '08 term at University of Texas at Austin.

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Unit 1 - BIO 311D Introductory Biology 21 JAN 2011 Chapter...

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