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| Meiosis |
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| Meiosis is a cell division occurs in reproductive cells (gametes). The male gamete is known as the sperm in animals and the female gamete is known as the ovum (egg). These gametes, after they are formed, have no future unless they unite to form a zygote, which then grows into an adult organism. The formation of the zygote is termed as fertilization. During fertilization, the nuclei of the two gametes fuse to form the nucleus of the zygote. Now, if the gametes were to have two of each type of chromosome, the zygote would then have four of each type i.e. twice the normal number. Suppose the sperm and the egg cell had diploid number of chromosomes (2n), after fertilization the zygote would have twice the number of chromosomes, deriving 2n from each parent cell (2n+2n=4n). This would lead to the chromosome number doubling with each succeeding generation. By halving the chromosome number meiosis ensures that this does not happen. When gametes with the haploid number of chromosomes unite, the normal diploid condition is restored. |
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 Meiosis-I (Prophase I, Metaphase I, Anaphase I and Telophase I) Chromosome number is halved (n). Two daughter cells are formed. |
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 Meiosis-II (Basically a mitotic division resulting in the formation of four daughter cells with (n) chromosomes |
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| Prophase - I |
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 The chromatin network condenses into chromosomes, which are long and slender threads. Each thread is a single chromatid, and not double as in Mitosis |
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 Two homologous chromosomes come together and lie side by side. Pairing takes place along their entire length |
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 The two homologous chromosomes split longitudinally, and four chromatids are formed |
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 The homologous pairs are connected at some points called chiasmata. At each chiasmata the chromatids exchange chromatin material. Thus, new gene combinations are established. |
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 The nuclear membrane and nucleolus disappear. The nuclear spindle develops and the chromosomes arrange themselves along the equator of the spindle. |
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Metaphase I
| The four chromatids (in two pairs) are attached to spindle fibres by two centromeres and face opposite poles. |
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Anaphase I
| The two pairs separate and move towards the opposite poles of the spindle. |
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| Telophase I - The chromatid pairs (chromosomes) form a compact group at each pole. The two daughter nuclei thus formed contain haploid or (n) chromosomes. |
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| The two daughter cells then undergo meiosis-II, which is actually mitosis. |
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| Meiosis is thus an equational division resulting in four daughter cells with half the number of chromosomes (haploid). |
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