The Cell Cycle
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Interphase: main part of the cycle
- G1: first gap
- S: DNA synthesis (duplication)
- G2: second gap
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Mitotic phase
- Mitosis: division of the nucleus
- Cytokinesis: division of the cytoplasm
Chromosome Glossary
- DNA
- Basic genetic material
- Chromosome
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- DNA + proteins
- Prokaryotic: circular
- Eukaryotic: linear
- Chromatin
- Eukaryotic chromosome
- Visible chromosome
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- Highly coiled and packed together
- Only seen during mitosis or meiosis
- Sister chromatids
- Duplicated chromosomes joined at their centromeres to each other
Look at this graphic to see how DNA builds up to create chromosomes.
Before Mitosis: G2 of Interphase
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Chromosomes (chromatin) have (has) already been duplicated.
- They are not visible, however, since they have not yet condensed.
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Centrosome duplicates
- Spindle microtubule assembly begins in the centrosomes (a.k.a. “microtubule organizing center”).
- Spindle microtubules will grow out from the centrosome.
- Animal cells: centrioles present inside.
- Plant cells: centrioles absent inside.
Mitosis
Mitosis occurs in six parts:
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis
1. Prophase
- Chromosomes condense: two sister chromatids are now visible.
- Mitotic spindle begins to form.
- Centrosomes move away from one another.
- Nucleoli disappear.
- Nuclear envelope fragments.
- Spindle microtubules invade what was once the nuclear region.
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Kinetochore appears.
- One for each sister chromatid.
- Located at centromere region.
- Nonkinetochore microtubules interact.
- Chromosomes begin to move.
- Centrosomes are now at opposite ends of the cell.
- Chromosomes are at the metaphase plate.
- Centromeres are aligned with sister chromatids straddling the metaphase plate.
- The microtubule arrangement is called the spindle.
4. Anaphase
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Paired centromeres of each chromosome separate.
- Sister chromatids leave each other.
- Kinetochore microtubules shorten.
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Poles of the cell move apart
- Nonkinetochore microtubules lengthen.
5. Telophase
- Microtubules disappear.
- Daughter nuclei form.
- Chromatin fibers uncoil and begin to disappear.
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Cytokinesis begins:
- Animal cells: the cleavage furrow begins to form.
- Plant cells: the cell plate begins to form.
6. Cytokinesis
- Cytokinesis is division of the cytoplasm.
- In animal cells: the cleavage furrow forms.
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In plant cells:
- Vesicles from the Golgi apparatus fuse at equator of the cell, forming a cell plate.
- This cell plate eventually forms a new cell wall.
How Chromosomes Move
- Question: does the chromosome move along the microtubule, or does the microtubule pull the chromosome towards the centrosome?
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Answer:
- Laser experiments show that the chromosome moves along the microtubule.
- It chops up the microtubule as it moves along it, so the microtubule becomes shorter during the process.