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How and why do cytokinesis and mitosis occur?
Essential Question: How do cells maintain homeostasis?
Focus Question:
1
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Why do cells divide?
The Cell Cycle

Eukaryotic Cell Cycle
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The cell cycle is a repeating sequence of cellular growth and division during the life of a cell
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The first three phases together are known as interphase
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Interphase
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During interphase, the cell is not dividing
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Different types of cells spend different amounts of time in interphase
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Cells that divide seldom, such as
nerve cells, spend most of their time in interphase
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What are the stages of the cell cycle? What happens in each stage?
2
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For most
organisms, this phase occupies the major portion of the cell’s life
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At the end of the S phase, the cell’s nucleus has
twice as much DNA as it did in the G1 phase
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Hollow protein
fibers called microtubules are organized in the cytoplasm during G2
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The process of dividing the nucleus into two daughter nuclei is called mitosis
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During mitosis, the nucleus divides to form two nuclei
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The nuclear membrane breaks down briefly
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As the nucleus divides, the cytoplasm also begins to divide
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Remember, some proteins are
used to build cell parts
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Stages of Mitosis
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Although mitosis is a continuous process, biologists traditionally divide it into four stages
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Review/practice: What exactly is mitosis? Why is it important?

Stages of Mitosis
Stage 1 Prophase
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1
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3
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These are formed at a right angle to the cells center
4
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Think: Why do these steps occur?

Stage 2 Metaphase
 There are 3 steps to metaphase
1
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Chromosomes move to the center of the cell and line up along the equator
3
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Once all of the chromosomes are lined up, the spindle fibers shorten
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Sister chromatids move toward opposite poles as the spindle fibers that are attached continue to
shorten
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Each pole now has a full set of chromosomes
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Mitosis is complete
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Describe Prophase and Metaphase
2
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3
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The cytoplasm is separated, and two cells are formed
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Each daughter cell has about half of the parent’s cytoplasm and organelles
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5

Separating the Cytoplasm
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In animal cells and other cells that lack cell walls, the cell is pinched in half by a belt of protein threads
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Plant Cell Cytokinesis
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Vesicles holding cell wall material line up across the middle of the cell
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When it is completely formed, the cell plate separates the plant cell into two new plant cells
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The activity of each cell continues because each has its own DNA and organelles
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Summary
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The life of a eukaryotic cell cycles through phases of growth, DNA replication, preparation for cell
division, and division of the nucleus and cytoplasm
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During cytokinesis, the cell membrane grows into the center of the cell and divides it into two daughter
cells of equal size
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