mitosis and meiosis Watch

alevels2k17
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why is meiosis made of two stages whereas mitosis is only one stage......the two stages of meiosis is first seperating the two homologous chromosmes and thus seperating the two sister chromatids... i mean mitosis also requires the serperation of the homologous pair why doesnt it happen???
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zombiejon
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(Original post by NoorL)
why is meiosis made of two stages whereas mitosis is only one stage......the two stages of meiosis is first seperating the two homologous chromosmes and thus seperating the two sister chromatids... i mean mitosis also requires the serperation of the homologous pair why doesnt it happen???
Most basic overview - meiosis results in the generation of 4 haploid cells (not genetically identical to the parent cell), whilst mitosis results in 2 diploid cells (genetically identical to the parent cell).

Mitosis - Each chromosome duplicates its DNA, resulting in the characteristic X shape. The chromosomes then line up along the metaphase plate and anaphase I results in the splitting of the daughter chromosomes. At no point do homologous pairs form a tetrad. Thus, there is no separation of the homologous pair - the sister chromatids are being separated.

Meiosis I - DNA duplication to form the characteristic X shape, and the chromosomes then pair up into homologous pairs, and forms the tetrad. The homologues separate in anaphase I. Daughter cells resulting from this aren't identical due to synapsis/crossing over. Metaphase II leads to the daughter chromosomes lining up along the metaphase plate, and separation of the sister chromatids.
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alevels2k17
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(Original post by zombiejon)
Most basic overview - meiosis results in the generation of 4 haploid cells (not genetically identical to the parent cell), whilst mitosis results in 2 diploid cells (genetically identical to the parent cell).

Mitosis - Each chromosome duplicates its DNA, resulting in the characteristic X shape. The chromosomes then line up along the metaphase plate and anaphase I results in the splitting of the daughter chromosomes. At no point do homologous pairs form a tetrad. Thus, there is no separation of the homologous pair - the sister chromatids are being separated.

Meiosis I - DNA duplication to form the characteristic X shape, and the chromosomes then pair up into homologous pairs, and forms the tetrad. The homologues separate in anaphase I. Daughter cells resulting from this aren't identical due to synapsis/crossing over. Metaphase II leads to the daughter chromosomes lining up along the metaphase plate, and separation of the sister chromatids.
ohh thanks a lot...... so when the cell is inactive theres no such thing as a chromosome right??? it is in the form of single chromatids?
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zombiejon
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(Original post by NoorL)
ohh thanks a lot...... so when the cell is inactive theres no such thing as a chromosome right??? it is in the form of single chromatids?
Chromosomes and chromatids only appear during mitosis or meiosis. Otherwise, DNA is kept in an uncoiled form.

Think of it as a giant spring, sitting in a box (1) connected to another box (2) by a hole. The hole is half the height of the spring, and the spring can't pass to the other box any other way because it's diameter is bigger than the hole. Your only option is to squish the spring down before moving through the hole. However, when you aren't moving the spring from one box to the other, there's no point wasting all that effort keeping it compressed. This is similar to DNA because there is no point expending the energy to keep DNA compressed unless absolutely necessary.
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thermometer
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for meiosis (which produces sex cells/gametes) there are two steps because you need to halve the genetic material. all gametes contain only half of your genetic information, since at fertilisation each parent only contributes half.
however mitosis does not involve halving because all cells produced are identical to the original and contain all your genetic information.
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alevels2k17
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(Original post by thermometer)
for meiosis (which produces sex cells/gametes) there are two steps because you need to halve the genetic material. all gametes contain only half of your genetic information, since at fertilisation each parent only contributes half.
however mitosis does not involve halving because all cells produced are identical to the original and contain all your genetic information.
Thanks alot! Xxx
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Jpw1097
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(Original post by NoorL)
ohh thanks a lot...... so when the cell is inactive theres no such thing as a chromosome right??? it is in the form of single chromatids?
When cells are inactive (or quiescent) you are right that DNA is not in the form of chromosomes however it is not found as chromatids either, it is found as chromatin. Chromatin is just a loose, decondensed form of DNA which is held in this way by proteins called histones. During DNA replication in the S phase of interphase, the DNA replicates but it is still found in the loose chromatin form. During prophase, the DNA becomes condensed and is found in a tightly packed arrangement as chromosomes - no longer chromatin. The DNA then decondenses to form chromatin during telophase.
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