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In animals each centrosome has 2 centrioles. During the transition to prophase, nucleoli disappear and microtubule arrays (asters) form around centrosomes, which move apart. Chromosomes begin to CONDENSE and become visible. Key point: Enzymes can’t copy condensed DNA to make mRNA or new DNA. Condensation stops EVERYTHING except the ongoing process. Interphase and the interphase-prophase transition are very similar in cells about to undergo meiosos and mitosis
Right: Mitosis Nucleoli have disappeared and asters continue to develop and move to opposite poles. Chromosomes further CONDENSE. Spindle begins to develop. In prometaphase kinetechore and aster microtubles interact, moving chromosomes back and forth. Right: Meiosos Prophase I As in mitosis, nucleoli disappear and asters develop and move to opposite poles. Chromosomes further condense as spindle begins to develop. The big difference is that a process called synapsis links the homologous chromosomes at the chiasmata. Mitosis prophase is the preparative step to separate sister chromatids, while homologous chromosomes are not separated. Meiosos prophase I is the preparative step to separate homologous chromosomes, while sister chromatids remain joined at kinetichores.
Meiosis: In Metaphase I the chromosomes line up on the metaphase plate, but they aren’t separate; they line up as tetrads. The sister chromatids remain joined, and instead the homologous chromosomes get separated during Anaphase I. Mitosis: during Metaphase and Anaphase chromosomes line up separately on equatorial plane and one sister chromatid moves toward each pole. Metaphase is the longest stage of mitosis, lasting about 20 minutes. During Anaphase, microtubules pull the pairs apart so that one of each pair moves towards each pole.
Telophase and cytokinesis in Meiosis I are similar to the corresponding processes in Mitosis. HOWEVER, differences in the preceeding stages result in daughter cells which are haploid but which have replicated DNA (both chromatids are present but there are only 1/2 the chromosomes. Mitosis produces diploid cells with unreplicated DNA. With the completion of telophase. Mitosis is complete and is immediately followed by the completion of cytokinesis, which begins in late telophase. In animals, a cleavage furrow pinches the cell in two.
In animals each centrosome has 2 centrioles. During the transition to prophase, nucleoli disappear and microtubule arrays (asters) form around centrosomes, which move apart. Chromosomes begin to CONDENSE and become visible. Key point: Enzymes can’t copy condensed DNA to make mRNA or new DNA. Condensation stops EVERYTHING except the ongoing process. Interphase and the interphase-prophase transition are very similar in cells about to undergo meiosos and mitosis
Right: Mitosis Nucleoli have disappeared and asters continue to develop and move to opposite poles. Chromosomes further CONDENSE. Spindle begins to develop. In prometaphase kinetechore and aster microtubles interact, moving chromosomes back and forth. Left: Meiosos Prophase II This is the preparative step for the separation of sister chomatids in a haploid cell. There are no homologous chomosomes because the cell is already haploid after Meiosis I. In animals, mitosis prophase is the preparative step to separate sister chromatids in a diploid cell, while homologous chromosomes are not separated. Meiosos prophase I is the preparative step to separate homologous chromosomes to produce a haploid cell, while sister chromatids remain joined at kinetichores.
Meiosis: In Metaphase II the chromosomes line up on the metaphase plate, just as in mitosis, but the cell is haploid. The sister chromatids are pulled apart by forces exerted against microtubules anchored to the kinetochore, and get separated during Anaphase I.The product is a haploid gamete with a single copy of each chromosome. Mitosis: during Metaphase and Anaphase chromosomes line up separately on equatorial plane and one sister chromatid moves toward each pole. Metaphase is the longest stage of mitosis, lasting about 20 minutes. During Anaphase, microtubules pull the pairs apart so that one of each pair moves towards each pole.
Telophase and cytokinesis in Meiosis II are similar to the corresponding processes in Mitosis. HOWEVER, differences in the preceeding stages produce haploid daugher cells with one copy per cell. Animal cell mitosis produces diploid cells with one copy per cell. NOTE: HAPLOID CELLS CAN CERTAINLY DO MITOSIS, WHICH IS THEN ALMOST IDENTICAL TO MEIOSIS II. (But daughters in meiosis are not always identical! Why is this true?) With the completion of telophase. Mitosis is complete and is immediately followed by the completion of cytokinesis, which begins in late telophase. In animals, a cleavage furrow pinches the cell in two. Product is a diploid cell with one copy of each chromosome.
The diploid/haploid transition is the heart of sexual reproduction. We think of the diploid form as the ‘normal’ adult form, but many organisms (e.g., fungi such as sordadia) are haploid except for short-lived reproductive stage. Other organsism (e.g., ferns) have both haploid and diploid adult phases which can look completely different. HAPLOID FERTILIZATION FUSION or MEIOSIS DIPLOID
