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What are the 4 phases in the cell cycle?
- -G1 phase
- -S phase
- -G2 phase
- -M phase
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Explain G1 phase
- First gap
- The time cell grows in size and carries out normal functions.
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Explain S phase
- DNA synthesis phase
- All the DNA in the nucleus are duplicated, so that all chromosomes have replica of themselves.
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Explain G2 phase
- Second gap phase
- The cell prepares to divide by checking for errors in DNA replication, and multiplying important organelles like mitochondria.
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Explain M phase
- Mitosis, or the division phase
- The nucleus divides, and the cell divides to make two new cells.
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What are the two ways cells in eukaryotic organisms divide?
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Mitosis
- This is the most common type of cell division, by which cells are added to the growing body, to replace lost cells, to heal wounds etc.
- The purpose of mitosis is to divide a cell in to two new cells that are identical to each other and to the original cell.
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Meiosis
- This happens only during the production of gametes.
- The purpose of meiosis is to divide the hereditary material in the nucleus by half, and to produce unique gametes.
- Important for evolution because it adds to the variations among individuals in a population.
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What are the stages of Mitosis?
- IPPMAT
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
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Prophase
- chromosomes condense, and sister chromatids become visible.
- Centrosomes divide and migrate to opposite poles of the cells, forming a spindle of microtubles.
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Prometaphase
nuclear envelope disappears, spindle fibers from opposite poles attach to kinetochores of sister chromatids
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Metaphase
Chromosomes align at the center of the cell, sister chromatids attached to spindle fibers.
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Anaphase
Spindle fibers shrink, pulling sister chromatids of each chromosome apart from each other, towards opposite poles
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Telophase
- new nuclear envelopes form at opposite ends of the cell.
- Chromosomes stretch again to threadlike form.
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What follows nuclear division?
- Cytokinesis.
- Dividing the cell into two new cells that carry exactly the same hereditary materials in both.
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Homologous Chromosomes
They are the matching pairs of chromosomes in all diploid (2n) organisms like humans.
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What are the stages in Meiosis I?
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
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Prophase I
- Most unique stage of meiosis.
- Homologous chromosomes come together (synapsis), and exhange parts from each other's sister chromatids.
- Crossing over
- The crossing over pts look like an "X"
- Called chiasma
- Spindle fibers from opposite poles of the cell attach to each homolog.
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Metaphase I
The homologs align at the cell center
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Anaphase I
- Homologous chromosomes are pulled by spindle to the opposite poles of the cell.
- Sister chromatids are still attached to each other at centromeres.
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Telophase I
Nuclear envelopes may reform in some species, the spindle disappears.
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What follows meiosis I?
- Cytokinesis.
- New cells are haploid (1n)
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The purpose of meiosis II?
To separate sister chromatids that are still attached. Therefore, meiosis II can ve compared to mitosis.
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Prophase II
Spindle reappears, and spindle fibers from opposite poles connect to kinetochoes of sister chromatids.
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Metaphase II
Chromosomes are lines up at the cell center
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Anaphase II
Sister chromatids are separated from each other, and are pulled to the opposite poles of the cell due to shrinking of spindle fibers.
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Telophase II
Nuclear envelope reforms, chromosomes stretch.
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What is Meiosis II followed by?
- -Second cytokinesis.
- -Resulting in 4 haploid cells.
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