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MEIOSIS

MEIOSIS. Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis produces 4 haploid cells. ***Mitosis produces 2 diploid cells.

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MEIOSIS

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  1. MEIOSIS

  2. Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). • Meiosis produces 4 haploid cells. • ***Mitosis produces 2 diploid cells. • Meiosis I reduces 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division). • Most of the differences between the processes occur during Meiosis I.

  3. Meiosis is a series of two nuclear divisions called meiosis I and meiosis II. • These two divisions are each divided into further phases: -Prophase • Metaphase • Anaphase • Telophase

  4. Meiosis I Meiosis I encompasses four stages: • Prophase I • Metaphase I • Anaphase I • Telophase I

  5. The stages are similar to the stages in mitosis but the largest differences occur in prophase I. • In most cases, at the end of meiosis I, two daughter cells are produced.

  6. Meiosis II Meiosis II also encompasses four stages: • Prophase II • Metaphase II • Anaphase II • Telophase II

  7. At the end of meiosis II, four daughter cells are produced. • Each of these resulting daughter cells is haploid. (n)

  8. Interphase • Before a dividing cell enters meiosis, it undergoes a period of growth called Interphase. • G1 (first gap) phase: The cell increases in mass in preparation for cell div. • S phase: during which DNA is synthesized. • G2 (second gap) phase: the cell synthesizes proteins & continues to increase in size.

  9. In the latter part of interphase, the cell still has nucleoli present. • Nucleus is bound by nuclear envelope and the cell’s chromosome have duplicated but are in the form of Chromatin. • 2 pairs of centrioles formed from the replication.

  10. MEIOSIS-1 • the largest differences between mitosis and meiosis occur in prophase I. • Prophase I is usually longer in duration usually much more complex. • It can take days for prophase I to complete. • It is estimated that prophase I accounts for some 85 - 95 percent of the total time for meiosis.

  11. PROPHASE-1 • Chromosomes condense and attach to the nuclear envelope. • Synapsis occurs (a pair of homologous chromosomes lines up closely together) and a tetrad is formed. Each tetrad is composed of four chromatids. • Synapsis- the pairing of homologous chromosomes during prophase I of meiosis. • Crossing over may occur. • Chromosomes thicken and detach from the nuclear envelope.

  12. Synapsis

  13. PROPHASE-1 • Similar to mitosis, the centrioles migrate away from one another and both the nuclear envelope and nucleoli break down. • Likewise, the chromosomes begin their migration to the metaphase plate. • Chiasma-The places where pairs of homologous chromatids remain in contact during late prophase to anaphase of the first meiotic division.

  14. Metaphase I • Metaphase I is of much shorter duration and complexity when compared to prophase I. • The following changes occur: • Tetrads align at the metaphase plate. • Note that the centromeres of homologous chromosomes are oriented toward the opposite cell poles.

  15. Metaphase I Metaphase I

  16. Anaphase I • Anaphase I is very similar to anaphase in mitosis. The following changes occur: Chromosomes move to the opposite cell poles. • Unlike in mitosis, the homologous chromosomes move to opposite poles yet the sister chromatids remain together.

  17. Anaphase I Anaphase I

  18. Telophase I • The following changes occur: • The spindles continue to move the homologous chromosomes to the poles. • Once movement is complete, each pole has a haploid number of chromosomes. • In most cases, cytokinesis occurs at the same time as telophase I. • At the end of telophase I and cytokinesis, two daughter cells are produced, each with one half the number of chromosomes of the original parent cell.

  19. Telophase I Telophase I

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