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How do little elephants grow up to be BIG elephants?
How do little elephants grow up to be BIG elephants?
Why do animals shed their skin?
Three reasons why cells divide :(multi-cellular organisms)
1. Growth2. Repair
3. ReplacementGRR…
SOME single-celled organisms use cell division to reproduce ASEXUALLY!
Cell Division = Asexual Reproduction = Natural Cloning
ALL organisms have cells that go through MITOSIS.
Only SOME cells in SOME organisms undergo MEIOSIS!
Outline of the Cell Cycle Interphase:
G1 (and G0), S, G2 Mitotic Phase: Division of the cell
Mitosis: Division of the nucleus (1st Stage of Mitotic Phase)1. Prophase2. Pro-Metaphase3. Metaphase4. Anaphase5. Telophase
Cytokinesis: Division of cytoplasm (2nd Stage of Mitotic Phase)
Cell Cycle ≠ Mitosis Cell Cycle = Interphase + Mitotic Phase
Cell Division ≠ Mitosis Cell Division = Mitosis + Cytokinesis
Cell Division = Asexual Reproduction = Natural Cloning
(for SOME single-celled organisms)
Interphase 1st Phase of Cell Cycle
• During INTERPHASE, the cell is engaged in normal metabolic activity. This is when the cell is most active, and genes are transcribed to make proteins.
• G1 is the main growth stage when a cell grows from a small daughter cell into a full-sized cell.
• S-Phase starts if/when the cell receives a signal that it may need to divide at some point in the future.
• During G1 and S-phases, chromosomes appear as threadlike or spaghetti-like strands called chromatin.
• During the S-phase, the DNA in the nucleus is replicated (to produce sister chromatids.• The two identical sister chromatids joined at the centromere.
• By the END of G2, each chromosome and its copy (the sister chromatids) will become condensed and appear as thick, worm-like chromosomes!
• During G2, the mitochondria in plant and animal cells and the chloroplasts in plant cells replicate.
CELL MEMBRANE
Nucleus Cytoplasm
• G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct “quiet” stage that occurs outside of the cell cycle. • On occasion, a distinction in terms is made between a G0 cell that is only
temporarily “resting” (eventually may re-enter the cell cycle) versus a cell that has become “quiet” and will never re-enter the cell cycle (e.g., heart muscle cells and neurons).
• Some types of cells, such as nerve and heart muscle cells, become “quiet” when they reach maturity (i.e., when they are fully differentiated).• Quiet cells continue to perform their main functions for the rest of the
organism's life, but do not divide and cannot be replaced.• Multi-nucleated muscle cells are basically muscle cells that never fully finished cell
division (did not complete cytokinesis) and therefore have multiple nuclei and continuous cytoplasm.• Because these cells do not undergo cytokinesis, they are also often considered
to be in the G0 stage.
InterphaseAnimal Cell Plant Cell
Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Prophase 1st step in Mitosis (1st Step in Mitotic Phase)
• Mitosis begins (cell begins to divide)• DNA appears as condensed, worm-like chromosomes (not spaghetti-
like chromatin seen in Interphase).• Chromosomes consist of two identical sister chromatids joined at
the centromere.• Two pairs of centrioles appear and begin to move to opposite ends
(poles) of the cell. • Centrioles occur in pairs; each centriole in a pair is oriented at a right
angle to its partner.• Spindle fibers form between the two pairs of centrioles.
Centrioles
Sister chromatids
Spindle fibers
ProphaseAnimal Cell Plant Cell
Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Pro-Metaphase• The NEW step in the cycle that occurs between
Prophase and Metaphase.• Activities formerly considered to mark the last part of
Prophase: Nuclear envelope disintegrates. Centrioles arrive at the poles of the cell.
• Activities formerly considered to indicate the beginning of Metaphase: Spindle fibers attach to the centromeres linking
two sister chromatids together. Chromosomes (sister chromatids) begin
migrating/moving towards center of cell.
Metaphase 2nd (or 3rd) step in Mitosis
• Spindle Fibers attach to the chromatids (sister pairs of chromosomes) at the centromere.• Spindle fibers are usually visible under a microscope in this stage.
• Sister Chromatids migrate until they are all lined up at the cell “equator” along the metaphase plate.• The location of the equator or metaphase plate is determined by WHERE the
chromosomes line up.• Meta = Middle• Organization of sister chromatids along metaphase plate helps
ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.
Centrioles
Spindle fibers
MetaphaseAnimal Cell Plant Cell
Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Comparing Prophase, Prometaphase, and Metaphase
Animal Cell: comparison of prophase, prometaphase, and metaphase.
Plant Cell:comparison of prophase,
prometaphase,and metaphase.
Anaphase 3rd (or 4th) step in Mitosis
• The paired chromosomes (sister chromatids) separate and begin to move/migrate towards the opposite ends (poles) of the cell.
• “Ana” = opposite• The migrating chromatids may appear V-like
as they are pulled towards opposite poles.Centrioles
Spindle fibers
AnaphaseAnimal Cell Plant Cell
Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Telophase 4th (or 5th) step in Mitosis
(Last step in mitosis, but NOT the last step in the Mitotic Phase)
• The chromatids arrive at the poles of the cell.• A new nuclear envelope begins to form around each of the
two clumps of chromatids, eventually forming two new nuclei.
• Chromosomes gradually become uncondensed and appear as chromatin (threads rather than rods).
• A cleavage furrow or cell plate may become visible towards the end of telophase.
• This is the end of Mitosis.NucleiNuclei
Chromatin
TelophaseAnimal Cell Plant Cell
Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
• Cytokinesis: the division of the cytoplasm (and everything in it).
• Cytokinesis usually begins during Telophase (occurs concurrently).
• In animal cells, the cleavage furrow pinches inward.• In plant cells, a cell plate forms along the metaphase plate
region and grows towards the cell walls.• Cytokinesis results in two daughter cells – each with its own
nucleus with identical chromosomes.• Each daughter cell will end up with half of the cytoplasm and
about half of the organelles—there are usually enough in the original cell that each daughter cell will end up with some.
Cytokinesis2nd “Stage” in the Mitotic Phase
Animal Mitosis -- ReviewInterphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Plant Mitosis -- ReviewInterphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
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MitosisDiscussion Question:Mitosis is the process in
which the nucleus divides to form two new nuclei. How does mitosis differ in plants and animals?
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Cell Cycle
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- Cell Division
The Cell Cycle6
1
2
3
4
5
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From double helix… to chromatin… to chromosome…. The Many Faces of DNADuring interphase,
DNA that is actively being transcribed to make proteins is generally loosely wrapped around special proteins called histones. The DNA + histone
structure is called a nucleosome.
A strand of loosely wrapped, spaced out nucleosomes is called “beads on a string.”
Chromatin is actually a spaghetti-like fiber made of hundreds of thousands of tightly wrapped and closely packed nucleosomes. Chromatin is the way DNA is stored during interphase if it is NOT
being actively transcribed.
At the end of G2 and into the beginning of prophase (the mitotic phase), the chromatin fiber will begin folding up along a protein scaffold (chromosome loops), which twist up and wrap in on itself again (forming condensed chromosome loops), and then coils up on itself to create the highly condensed chromosomes that we observe during the mitotic phase.
Beads on a String: Active DNA being transcribed to make proteins during INTERPHASE.
Chromatin Fiber: Inactive DNA being stored/not transcribed during INTERPHASE.
Condensed Chromatin: Chromatin begins to fold up at the end of G2.
Condensed Chromosome: In PROPHASE, condensed chromatin coils in on itself to form the highly condensed chromosome.
CLICK HERE for animation of DNA double helix wrapping up around histones to form nucleosomes and the “beads on a string” structure, nucleosomes packing up tight to create chromatin, and chromatin coiling up and condensing to form chromosomes.
The process of asexual reproduction begins
after a sperm fertilizes an egg.
