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Presentation on theme: "MEIOSIS."— Presentation transcript:



3 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.

4 These two divisions are each divided into further phases: -Prophase
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

5 Meiosis I Meiosis I encompasses four stages: Prophase I Metaphase I
Anaphase I Telophase I

6 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.

7 Meiosis II Meiosis II also encompasses four stages: Prophase II
Metaphase II Anaphase II Telophase II

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

9 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.

10 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.

11 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 percent of the total time for meiosis.

12 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.

13 Synapsis

14 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.





19 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.

20 Metaphase I Metaphase I                       

21 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.

22 Anaphase I Anaphase I                       

23 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.

24 Telophase I Telophase I                       


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