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Meiosis occurs in the production of gametes—the sperm of the males and the eggs of the females. When a sperm fertilizes an egg, a zygote is produced with the appropriate number of chromosomes for the species—in humans (and potatoes) the zygote and the somatic (body) cells produced from it have 46 chromosomes. This is the diploid (2n) number of chromosomes, half of which have come from the sperm nucleus, half from the egg. The sperm and egg are haploid ( n); they carry half the number of chromosomes of the body cells (in humans, 23 in each sperm and egg). The diagram shows the clear structure and process of Meiosis.
MEIOSIS 1:
Meiosis I has two main purposes:
It is the reduction division, meaning it reduces the number of chromosomes in half, making the daughter cells haploid (when the parent cell was diploid).
It is during meiosis I that most of the genetic recombination occurs.
Phases:
Prophase I
Just like in mitosis, during prophase, DNA condensation occurs, the nuclear envelope and nucleoli disappear, and the spindle starts to form. The big difference is what is going on with the chromosomes themselves.
As DNA condensation proceeds and the chromosomes first become visible, they are visible as tetrads. So, tetrads become visible during prophase.
Metaphase I
It is during prophase I and metaphase I that genetic recombination is occurring.
Anaphase I
Tetrads pull apart and chromosomes with two chromatids move toward the poles.
Telophase I
Chromosomes with two chromatids decondense and a nuclear envelope reforms around them. Each nucleus is now haploid.
MEIOSIS 2:
At the end of meiosis I, each chromosome still had two chromatids. That is double the amount of DNA that a cell should have. So, the entire reason to go through meiosis II is to reduce the amount of DNA back to normal, to split the chromosomes so that each daughter cell has only one chromatid per chromosome (the normal genetic content).
Phases:
Prophase II
Chromosomes with two chromatids become visible as they condense (and the nuclear envelope and nucleoli disappear, and the spindle is forming).
Metaphase II
Chromosomes with two chromatids line up at the equator. The spindle is fully formed.
Anaphase II
Chromosomes split, so that a chromosome with only one chromatid heads toward each pole.
Telophase II
Chromosomes with only one chromatid decondense and get surrounded by new nuclear envelopes. The four daughter cells are now all haploid and have the right amount of DNA. They are ready to develop into sperm or eggs now.
The diagram b
MEIOSIS 1:
Meiosis I has two main purposes:
It is the reduction division, meaning it reduces the number of chromosomes in half, making the daughter cells haploid (when the parent cell was diploid).
It is during meiosis I that most of the genetic recombination occurs.
Phases:
Prophase I
Just like in mitosis, during prophase, DNA condensation occurs, the nuclear envelope and nucleoli disappear, and the spindle starts to form. The big difference is what is going on with the chromosomes themselves.
As DNA condensation proceeds and the chromosomes first become visible, they are visible as tetrads. So, tetrads become visible during prophase.
Metaphase I
It is during prophase I and metaphase I that genetic recombination is occurring.
Anaphase I
Tetrads pull apart and chromosomes with two chromatids move toward the poles.
Telophase I
Chromosomes with two chromatids decondense and a nuclear envelope reforms around them. Each nucleus is now haploid.
MEIOSIS 2:
At the end of meiosis I, each chromosome still had two chromatids. That is double the amount of DNA that a cell should have. So, the entire reason to go through meiosis II is to reduce the amount of DNA back to normal, to split the chromosomes so that each daughter cell has only one chromatid per chromosome (the normal genetic content).
Phases:
Prophase II
Chromosomes with two chromatids become visible as they condense (and the nuclear envelope and nucleoli disappear, and the spindle is forming).
Metaphase II
Chromosomes with two chromatids line up at the equator. The spindle is fully formed.
Anaphase II
Chromosomes split, so that a chromosome with only one chromatid heads toward each pole.
Telophase II
Chromosomes with only one chromatid decondense and get surrounded by new nuclear envelopes. The four daughter cells are now all haploid and have the right amount of DNA. They are ready to develop into sperm or eggs now.
The diagram b
References:
www.tokresource.org
http://www.cliffsnotes.com/sciences/biology/plant-biology/cell-division/sexual-reproduction-meiosis
http://faculty.stcc.edu/BIOL102/Lectures/lesson9/stepbystep.htm
www.tokresource.org
http://www.cliffsnotes.com/sciences/biology/plant-biology/cell-division/sexual-reproduction-meiosis
http://faculty.stcc.edu/BIOL102/Lectures/lesson9/stepbystep.htm