Mitosis

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Mitosis

• AP Biology Ch. 12

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Big Picture Terms

• Cell Division
• The reproduction of cells
• Cell Cycle
• Life of a cell from its origin to its own division

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Why Divide?

• Reproduction
• A unicellular organism like an amoeba divides and
  forms duplicate offspring
• These are, in fact, clones

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Why Divide?

• Growth
• To make an organism bigger
• Zygote to embryo to infant to adult

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Why Divide?

• Repair / renewal
• Mitosis allows for the repair of damaged tissues
• Also for the regeneration of cells that have a
  high turnover rate
• Blood
• Skin

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Goal of Cell Division

• Genetically IDENTICAL daughter cells
• In other words, VARIATION is NOT the goal here.
• Cloning is.

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Fundamental Processes of Cell Division

• Genetic material is duplicated
• Genetic material is PRECISELY and EQUALLY divided
  
• the two copies that result from duplication move
  to opposite ends of the cell
• The cytoplasm of the cell itself then splits.

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Genome

• A cells total hereditary endowment
• In plain English this means all the genes of a
  cell genome

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Prokaryotic Genomes

• Single long molecule of DNA
• Usually in a ring

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Eukaryotic Genomes

• Total genome is much longer than prokaryotes
• Made of several DNA molecules that are linear in
  shape (not in a ring)
• In a typical human cell 2 meters of DNA
• Makes copying and distribution of this enormous
  amount of information possible.

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Eukaryotes and Chromosome Number

• Each species has a characteristic number of
  chromosomes in the nuclei of their cells

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Major Eukaryotic Cell Types

• Somatic Cells
• All body cells except reproductive cells
• Humans have 46 chromosomes in all (almost)
  somatic cells
• Gametes
• Sperm and egg
• ½ the chromosome number of somatic cells
• 23 in sperm and egg

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Eukaryotic Chromosome Structure

• Chromatin
• DNA
• VERY long linear DNA molecule
• contains thousands of genes
• Protein
• Maintain the structure of the chromosome
• Histone proteins
• Like wrapping thread (DNA) around a spool
  (histone)
• Help control the activity of the genes on the
  chromosome

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Non-dividing vs. Dividing Chromosomes

• Chromosomes that are NOT dividing appear in the
  form of long, thin chromatin fibers
• They are indistinguishable in the microscope

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Non-dividing vs. Dividing Chromosomes

• Chromosomes that are about to divide get much
  shorter and thicker
• The chromatin that makes up the now
  distinguishable chromosomes, becomes highly
  coiled and folded.

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Eukaryotic Chromosome Structure

• Once a chromosome has been duplicated, it is
  composed of two sister chromatids
• Each chromatid is an EXACT copy of the other.
• They are held together by a centromere
• During cell division, these chromatids separate

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Terms

• Division of the NUCLEUS
• Mitosis
• Division of the CYTOPLASM
• Cytokinesis
• Cell division that results in GAMETES
• Meiosis
• Non identical daughter cells result
• 1 set of chromosomes (1/2 as many as in parent
  cell)
• Only occurs in gonads
• Fertilization restores full chromosome number

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The Cell Cycle

• Mitosis
• Shortest part of the cell cycle
• Includes both mitosis and cytokinesis
• Interphase
• 90 of time in cell cycle

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3 parts of interphase

• G1
• 1st Gap
• Cell growth and production of proteins
• S
• Synthesis
• Chromosomes are duplicated
• Cell growth occurs
• G2
• 2nd Gap
• More cell growth
• Completes preparations for cell division

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Details on Cell Cycle late interphase

• Nuclear envelope present and in tact
• 1 or more nucleoli visible inside nucleus
• Centrosomes found just outside nucleus
• In animals, these contain 2 centrioles
• Asters extend from centrosomes
• Radial array of microtubules
• Individual chromosomes cannot be distinguished
• The chromosomes HAVE been duplicated

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Late Interphase (on far left)
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Details on Cell Cycle - Prophase

• Chromatin fibers become more highly coiled
  discrete chromosomes are visible
• 2 sister chromatids are visible
• Nucleoli disappear
• Mitotic spindle begins to form
• Microtubules radiate from the two centrosomes
• Asters are the shorter microtubules that extend
  from the centrosomes
• Centrosomes move away from each other
• Lengthening microtubles carry them

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Prophase (center)
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Details on Cell Cycle Prometaphase

• Nuclear envelope completely fragments
• Microtubules extend to middle of cell
• Microtubles attach to kinetochores
• Structure protruding off of the centromeres of
  the chromosomes
• Microtubules jerk the chromsomes back and forth

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Prometaphase (right)
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Details on Cell Cycle metaphase

• Presence of metaphase plate becomes apparent
• Imaginary line between the cells two poles
• Becomes apparent due to lining up of chromsomes
• Centromeres of chromsomes line up along the
  metaphase plate
• Spindle is completely formed

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Metaphase (left)
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Details on Cell Cycle Anaphase

• Begins as two sister chromatids separate
• Each chromatid is then considered a full-fleged
  chromsome
• Each c-some moves to opposite ends of the cell
• They move because the microtubles attached to the
  kinetichores shorten.

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Anaphase (center)
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Details on Cell Cycle Telophase

• 2 daughter nuclei form inside the cell
• Chromosomes become less condensed
• Cytoplasm divides
• Cleavage furrow forms to cut cell in half
• This is animal cell structure in plants its a
  bit different.

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Telophase (right)
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Cleavage Furrow

• Shallow groove in cell surface near old metaphase
  plate
• Formed by actin microfilaments that are literally
  pinching the cell in two.
• Not present in plants
• Cell plate is in its place

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Cell Plate

• Forms as new cell wall is laid down between new
  plant cells

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Binary Fission

• Cell division in bacteria
• Mitosis does NOT occur in bacteria
• Mitosis may have evolved from binary fission

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Binary Fission

• Replication of the single circular chromosome
  begins at an origin point
• The replication runs in opposite directions
  (beginning at the origin) along the DNA molecule
• The origins of the two newly forming DNA
  molecules move to opposite ends of the cell
• No mitotic spindles No microtubules
• When replication is done, the cell splits

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Binary Fission
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Frequency of Cell Division

• Varies with type of cell
• Skin cells
• Frequent and throughout life
• Liver cells
• Maintain ability to divide, but only do this when
  needed
• Nerve and muscle cells
• Do NOT divide in a mature adult

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What Controls the Cell Cycle?

• Important to researchers due to implications in
  cancer

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Cancer Cells

• Divide excessively and invade other tissues
• Do not respond to the bodys control mechanisms

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Cancer Cells Grown in Culture Do NOT

• Stop dividing when growth factors are depleted
• Do not stop dividing at normal points, but at
  random points
• Can do on dividing indefinitely
• Normal cells can divide 20-30 times in culture
• Cancer cells from one woman continue to grow in
  culture since 1951

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Transformation

• When a normal cell becomes cancerous
• Normally these cells are destroyed by the immune
  response
• A cell that evades destruction may become a tumor

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Tumor

• Mass of abnormal cells within normal tissue
• Benign
• Remains in a single site
• Malignant
• Invasive spreads to other organs

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What makes malignant cells abnormal?

• In addition to increased proliferation
• Unusual number of c-somes
• Metabolism disabled
• Secret signal molecules that cause blood vessels
  to form
• Lose attachments to neighboring cells and break
  away

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Metastasis

• Spread of cancer to distant locations

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Treatments

• Radiation
• Localized tumor
• Damage DNA in cancer cells

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Treatments

• Chemotherapy
• Metastatic tumors
• Toxic to dividing cells
• Administered through circulatory system
• Interfere with steps in cell cycle

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Transformation

• Always involves alteration of genes that
  influence the cell cycle control system
• Causes are diverse