Stages of meoisis i (1)


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Stages of meoisis i (1)

  1. 1. Powerpoint Templates Page 1 Stages of Meoisis I By: The Golden Group (Group 2)
  2. 2. Powerpoint Templates Page 2 Introduction Meiosis - involves two successive divisions of a diploid (2N) eukaryotic cell of a sexually reproducing organism that result in four haploid (N) progeny cells, each with half of the genetic material of the original cell. Through the mechanisms by which paternal and maternal chromosomes segregate, and the process of crossing-over, genetic variation is produced in the haploid cells.
  3. 3. Powerpoint Templates Page 3 Premeiotic Interphase• Outside the nucleus of animal cells are two centrosomes, each containing a pair of centrioles. The two centrosomes are produced by the duplication of a single centrosome during premeiotic interphase. The centrosomes serve as microtubule organizing centers (MTOCs). Microtubules extend radially from centrosomes, forming an aster. • Plant cells do not have centrosomes. Different kinds of microtubule organizing centers serve as sites of spindle formation.
  4. 4. Powerpoint Templates Page 4 Prophase I • At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope. • Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. • The nucleolus disappears during prophase I.
  5. 5. Powerpoint Templates Page 5 Prophase I Prophase I has so many processes happening that it is usually separated into five stages. They are listed, in order, below with their explanations. • Leptonema During this stage, the chromosomes begin to condense and become visible. Researchers also believe that homologous pair searching begins also at this stage. • Zygonema The chromosomes continue to become denser. The homologous pairs have also found each other and begin to initially align with one another, referred to as 'rough pairing'. Lateral elements also form between the two homologous pairs, forming a synaptonemal complex. • Pachynema Coiling and shortening continues as the chromosomes become more condense. A synapsis forms between the pairs, forming a tetrad.
  6. 6. Powerpoint Templates Page 6 • Diplonema The sister chromatids begin to separate slightly, revealing points of the chiasma. This is where genetic exchange occurs between two non-sister chromatids, a process known as crossing over. • Diakinesis The chromosomes continue to pull apart, but non-sister chromatids are still loosely associated via the chiasma. The chiasma begin to move toward the ends of the tetrad as separation continues. This process is known as terminalization. Also during diakinesis, the nuclear envelope breaks down and the spindle fibers begin to interact with the tetrad. Where chromatids overlap is called a CHIASMATA, and it allows for CROSSING OVER of genetic information between chromosomes
  7. 7. Powerpoint Templates Page 7 Metaphase I• The centrioles are at opposite poles of the cell. • The pairs of homologous chromosomes (the bivalents), now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate. • Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids).
  8. 8. Powerpoint Templates Page 8 Anaphase I• Anaphase I begins when the two chromosomes of each bivalent (tetrad) separate and start moving toward opposite poles of the cell as a result of the action of the spindle. • Notice that in anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is that sister chromatids remain joined after metaphase in meiosis I, whereas in mitosis they separate.
  9. 9. Powerpoint Templates Page 9 Telophase I • The homologous chromosome pairs complete their migration to the two poles as a result of the action of the spindle. Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids. • A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows. • Many cells that undergo rapid meiosis do not decondense the chromosomes at the end of telophase I. Other cells do exhibit chromosome decondensation at this time; the chromosomes recondense in prophase II.
  10. 10. Powerpoint Templates Page 10 The homologous chromosome pairs reach the poles of the cell, nuclear envelopes form around them, and cytokinesis follows to produce two cells.