Mitosis & meiosis

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Mitosis & meiosis

  1. 1. BIO 101 – Laboratory August 2009 This only serves as your guide. You still have to read your manuals EXERCISE V Cell Growth and Division2 General Processes of Cell Division:1. Karyokinesis – organization of DNA strands into chromosomes & separation of chromosomes (nuclear division)2. Cytokinesis – division of the cell body (cytoplasm & its organelles)Cell Cycle events that occur from the completion of one division until the beginning of the next division constitute the cell cycle Interphase Stage – interval between divisions devoted to growth and metabolism the time for chromosomal replication 1. G1 (1st Gap) – initiation of new cycle Cells may become non-dividing or continue through G1 2. S Phase – replication of the DNA of each chromosome; DNA Synthesis 3. G2 (2nd Gap) – volume of cell has doubled, DNA has been replicated, mitosis is initiatedMITOSIS a form of cell division resulting in the production of two cells, each with the same chromosome & genetic complement as the parent cell somatic cell division occurs on cells during embryonic development, growth, repair of injured tissues & replacement of worn-out tissues Specimen used: Allium cepa root tips (plant cell) Root tips → region of active cell division (meristematic) Most cells are in Interphase and Prophase stage Whitefish blastula (animal cell) Blastula → embryonic stage in the development of a fertilized eggMEIOSIS the process that produces haploid gametes in diploid organisms the cell undergoes two consecutive divisions – Meiosis I & Meiosis II four daughter cells result from the divisions, in contrast to the two daughter cells produced from mitotic division 2 divisions of meiosis are preceded by only one duplication of chromosomes → each of the 4 daughter cells resulting from meiosis has only half as many chromosomes as the starting cells – a single haploid set of chromosomes Specimen used: Grasshopper testis (animal cell) -1-
  2. 2. BIO 101 – Laboratory August 2009 This only serves as your guide. You still have to read your manuals MITOSIS Allium cepa – root tips Interphase Cell is undergoing respiration and synthesis of DNA, RNA, and protein in preparation for mitosis Interphase Prophase Chromosomes become distinguishable as long, thin tubules in the nucleus Nuclear membrane begins to break down Chromosomes are distributed in the cytoplasm as coiled mass Each chromosome is composed of 2 separate strands – sister chromatids which are identical Sister chromatids are joined together at a region of attachment, centromere Prophase Within this region is the kinetochore from which projects the spindle to the 2 poles of the cell Metaphase Double stranded chromatids are arranged near the center of the cell at right angles to the long axis of the spindle fibers forming the metaphase plate Metaphase Anaphase Chromatids are pulled by the microtubules to opposite poles of the cell This stage is recognized by the V-shaped chromosomes in 2 groups at opposite sides of the cell Anaphase ends when separated chromatids, now called chromosomes reached the opposite poles Anaphase Telophase Cell plate starts as a fine line across the center of a cell When complete, the cell plate divides the original cell into 2 daughter cells Nuclei begins to reorganize; chromosomes uncoil & become longer & thinner, nuclear membrane re-forms, and nucleoli begins to reappear Karyokinesis is completed at the end of telophase and cytokinesis begins Telophase -2-
  3. 3. BIO 101 – Laboratory August 2009 This only serves as your guide. You still have to read your manuals MITOSIS Whitefish blastula Interphase with distinct nucleus bounded by a nuclear membrane with identifiable nucleolus presence of centrosome (or cell center) containing the centrioles Interphase Prophase – stage of preparation 2 pairs of centioles start to move apart to opposite poles Microtubules – radiate from each pair of centrioles like spokes on a wheel forming a configuration called Aster disappearance of the nuclear membrane appearance of spindle fibers (microtubules + spindle) short chromosomes become visibly doubled → each chromosome consists of 2 sister chromatids Prophase Metaphase – stage of separation chromosomes move & align themselves along the equatorial plane forming the metaphase plate Metaphase Anaphase – stage of migration pair of chromatids are pulled apart by microtubules of the spindle and become daughter chromosomes then they are pulled apart toward opposite poles of the cell Anaphase Telophase – stage of nuclear reconstruction & cytosome division spindle disappears, 2 daughter nuclei are organized nucleoli appear, nuclear membranes are formed cleavage furrow; cytokinesis takes place results into 2 daughter cells with the same genetic material as the parent cell. Telophase -3-
  4. 4. BIO 101 – Laboratory August 2009 This only serves as your guide. You still have to read your manuals MEIOSIS I The resulting cell at the end of Meiosis I is reduced from the diploid to the haploid condition1. Prophase I Homologous chromosomes – each are composed of two chromatids Substages: Leptotene – chromosomes are long, thin, optically single threads with little coiling Zygotene – homologous chromosomes begin to pair (synapsis) and lie adjacent to one another to form bivalents Pachytene – exchange of genetic components called crossing over is manifested by “X” arrangements (chiasmata) of portions of homologous chromosomes; portions of each of the paired chromosomes separates into 2 sister chromatids (structure at this stage is then called a tetrad). Diplotene – chromosomes clump on the center of the cell Diakinesis – chromosomes pairs as well separated through the cell. Nucleolus generally disappears and nuclear membrane begins to break down2. Metaphase I Homologous chromosomes (called tetrads) line up side by side at the metaphase plate Spindle fibers join the centromeres to the poles Nuclear membrane is completely gone3. Anaphase I Homologous chromosomes in each pair separate from each other and move to opposite poles of the cell4. Telophase I - Chromosomes are regrouped at the poles as in mitosis. -4-
  5. 5. BIO 101 – Laboratory August 2009 This only serves as your guide. You still have to read your manuals MEIOSIS II Equational division resulting to four haploid cells1. Prophase II Similar to mitosis, it involves the contraction and coiling of chromosomes Synapsis and crossing over does not occur anymore but the remnants of the crossing over from prophase I is carried over. Chromosomes are already reduced from diploid to haploid2. Metaphase II Chromosomes line up at the metaphase plate; spindle fibers join the chromosomes to the opposite poles3. Anaphase II Sister kinetochores move separately to the poles pulling with them the chromatids to which they are attached (sister chromatids move to opposite poles).4. Telophase II Interphase nuclei are reconstituted and cell walls are laid down to give 4 cells known as a tetrad. -5-

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