Download Why do cells divide?

Why do cells divide?
Mar 23­10:39 AM
Questions
• Read pages 241 to 243.
• Answer the questions on the worksheet.
• On a separate piece of paper:
1. Explain in detail the two reasons why cells divide.
2. Explain how the function of a chloroplast and a mitchondria may be impaired if a cell were to become too large.
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Mar 23­10:33 AM
The Cell Cycle
http://www.youtube.com/watch?v=4W8kupKFCPk
Mar 22­1:57 PM
The Cell Cycle
• During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells. These two daughter cells will begin the cycle again.
• Consists of 4 phases:
G1 phase
S phase
G2 phase
M phase
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The Cell Cycle
M phase
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Interphase
• Longest event in a cell’s life.
• Period of great metabolic activity.
• When a cell can no longer maintain homeostasis it must divide.
G1 phase: The cell increases in size and synthesize new proteins and organelles.
S phase: DNA replication.
G2 phase: Various organelles and molecules required for cell division are created at this time.
Mar 22­1:57 PM
DNA Replication: (S-Phase)
"You are what you eat" ????
An enzyme called DNA Helicase causes the base pairs of DNA to split down the middle forming 2 single strands.
The nucleus contains free floating nucleotides from the food you eat.
These nucleotides are attached to each single strand of DNA by another enzyme DNA Polymerase to form 2 identical double stranded DNA molecules.
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A
T
T
A
C
G
Helicase
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A
T
T
A
C
G
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A
T
T
A
C
G
Draw a diagram to represent the replication of a DNA molecule with the letters A,C,T down the left hand side.
step 1: show original molecule
step 2: show split molecule
step 3: (in a different colour) fill in your missing nucleotides to your drawing from step 2
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Interphase
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M phase: 1. Mitosis (division of the nucleus) 2. Cytokinesis (division of the cytoplasm)
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What is Mitosis?
Mitosis is the division of the cell nucleus.
There are 4 main events or phases make up mitosis:
Prophase
Metaphase
Anaphase
Telophase
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Prophase
• Replicated chromatin from interphase begin to shorten , thicken , and coil up to form Chromatid Pairs.
• Nucleolus & nuclear envelope disappears.
• Centrioles split and move to opposite sides of the cell (moves to the poles). Protein fibers begin to radiate from each. These structures are called Asters.
• When asters reach opposite sides of cell they form a Spindle.
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Prophase
• Note: Plant Cells do not have centrioles or asters but the spindles still form
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Metaphase
• Shortest stage of mitosis.
• Chromatid pairs align at the equator of the spindle.
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Anaphase
• Centromere splits and chromatid pairs separate.
• Spindle fibers begin to contract and drag the Chromosomes to opposite sides of the cell.
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Telophase
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Telophase
• Final stage of mitosis , chromosomes reach the poles and begin to uncoil to form chromatin.
• Spindle fibers disappear , nuclear envelope & nucleolus begin to re­appear. • In early telophase the cell membrane begins to indent at the equator to form a Cleavage Furrow (start of actual cytoplasm division called Cytokinesis). • In late telophase cleavage furrows have indented further.
• MITOSIS IS NOW OVER !!
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Cytokinesis
• Refers to the division of the cytoplasm to produce 2 genetically identical daughter cells.
• Animal & Plant cell’s achieve cytokinesis differently.
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Animal Cell • Animal cell divides from the outside in by forming these cleavage furrows. The cleavage furrows eventually pinches off the middle.
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Plant Cell Cytokinesis
• Plant cells divide from inside out through the formation of a Cell Plate.
Mar 22­1:57 PM
Mitosis Animation
http://phschool.com/webcodes10/index.cfm
?wcprefix=cbp&wcsuffix=3102
&area=view&x=0&y=0
Mar 27­1:01 PM
INTERPHASE
(G1, S, G2)
M­PHASE
(Mitosis)
­Prophase
­Metaphase
­Anaphase
­Telophase
(Cytokinesis)
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