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meiosis - Meiosis Objectives Explain why meiosis is...

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Unformatted text preview: Meiosis Objectives Explain why meiosis is essential to sexual reproduction List and describe, in order, the principal events of meiosis Identify the stages of meiosis from lily anther slides Use a simulation to demonstrate understanding of the key stages of meiosis Compare and contrast meiosis and mitosis Genetics is the study of heredity and variation. The transmission of traits from one generation to the next is called heredity or inheritance. Your genome is derived from the thousands of genes you inherited from you mother and your father. However, offspring differ somewhat from parents and siblings, demonstrating variation. In plants animals, sperm and ova (unfertilized eggs) transmit genes from one generation to the next. After fertilization (fusion) of a sperm cell with an ovum, genes from both parents are present in the nucleus of the fertilized egg. The organism then develops through mitosis A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism. It starts at the conception of an organism and continues until it produces its own offspring. Cells alternate between haploid and diploid states. This allows for a consistent chromosome number between generations The life cycle of humans and other animals is typical of one major type. Gametes, produced by meiosis, are the only haploid cells. Gametes undergo no divisions themselves, but fuse to form a diploid zygote that divides by mitosis to produce a multicellular organism. Plants and some algae have a different type of life cycle, alternation of generations. This life cycle includes both haploid (gametophyte) and diploid (sporophyte) multicellular stages. Meiosis by the sporophyte produces haploid spores that develop by mitosis into the gametophyte. Gametes produced via mitosis by the gametophyte fuse to form the zygote which produces the sporophyte by mitosis. Many steps of meiosis resemble steps in mitosis. Both are preceded by the replication of chromosomes. However, in meiosis, there are two consecutive cell divisions, meiosis I and meiosis II, that result in four daughter cells. Each final daughter cell has only half as many chromosomes as the parent cell. Interphase This is the only time interphase occurs G1 Growth S DNA replication G2 Centrosomes replicate Prophase I chromosomes condense and homologous chromosomes pair up In a process called synapsis, special proteins hold homologous chromosomes tightly together. Known as a tetrad. At certain areas, the chromatids of homologous chromosomes are crossed (chiasmata) and segments of the chromosomes are traded. Crossing over may or may not happen. But only between homologous chromosomes Spindle fibers form attach to kinetochores on the chromosomes Nuclear membrane disappears Centrosomes move to opposite poles Metaphase I Tetrads are all arranged at the metaphase plate Chromosomes are lined up in homologous pairs Anaphase I Homologous chromosomes separate and are pulled toward opposite poles. Each chromosome still has 2 chromatids Telophase I Centriole duplication Nuclear membrane may reappear Cytokinesis Cleavage furrow or cell plate forms Meiosis II No interphase Prophase II Spindle fibers attach to kinetochore Centrosomes migrate to poles Nuclear membrane disappears NO crossing over NO homologous pairs At metaphase II, the sister chromatids are arranged at the metaphase plate. The kinetochores of sister chromatids face opposite poles. At anaphase II, the centomeres of sister chromatids separate and the now separate sisters travel toward opposite poles. Telophase II Separated sister chromatids arrive at opposite poles. Nuclear membrane reformes Spindle fibers disassemble Cytokinesis Cytoplasm separated with cleavage furrow or cell plate End result 4 haploid daughter cells Procedure p 127-130 Work through in pairs Lily Anthers (Lilium) In group of 4 First view 1st division 1 pair set up Second view 2nd division 1 pair set up Observe models and diagrams on side counter http://biog-101-104.bio.cornell.edu/BioG101_1 http://biog-101-104.bio.cornell.edu/BioG101_1 Summary Meiotic Mix movie Answer questions in lab book p.132-133 Homework No goggles needed Turn in photosynthesis homework Quiz 3 Photosynthesis Mitosis Meiosis ...
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