Cell Division

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Cell Division

• Mitosis and Meiosis

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Cell Cycle

• Encompasses the time between the creation of a
  new cell and that cells division.
• Cell Division the splitting of one cell into
  two.
• The process that makes growth and reproduction
  possible for any organism.
• Each division different depended upon if the cell
  is eukaryotic or prokaryotic

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• Two major phases
• 1- Interphase Preparation for cell division.
• Three phases
• G1 phase (Growth 1) Cell growth Cell
  organelles are formed within the cell.
• S phase (Synthesis) DNA is synthesized
• G2 phase (Growth 2) Second period of cell
  growth, during which the cell prepares for the
  division.
• Example Some cells, including many nerve cells,
  are programmed never divide. These cells are said
  to be in G0 or resting phase.
• 2- Mitosis Division of Nucleus
• Four phases of mitosis (Prophase, Metaphase,
  Anaphase, Telophase)
• Cytokinesis division of cytoplasm and cell
  membrane

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Cell Cycle Control

• All of the cell cycle are controlled by
  checkpoints
• There are three checkpoints

Checkpoints Occurs at Details
G1 The end of the phase If conditions are not suitable for replication, the cell will not proceed to S phase but will instead enter a resting phase G0
G2 The end of the phase If conditions are not suitable, transition to the M phase will be delayed. If DNA is damaged, cell division will be delayed to allow time for DNA repair
M Between metaphase and anaphase stages of mitosis If the chromosomes are aligned properly and ready for division, the cell will proceed from metaphase to anaphase, during which it will divide. If the chromosome are not aligned properly, the anaphase stage will be delayed
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• Triggers at each checkpoint assess the cells
  readiness to proceed to the next stage.
• Checkpoints makes sure proper number of
  chromosomes and type of chromosomes organelles

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Example of Checkpoint

• Example Malignant cancer are deadly, in part,
  because they undergo unregulated cell division,
  which enables them to spread rapidly throughout
  the body. Scientists have discovered one reason
  behind this uncontrolled growth a defective p53
  gene. Proteins produced by the p53 gene assess
  the cells DNA for damage at the G1 checkpoint.
  If the DNA is intact, cell division proceeds. If
  the DNA is damaged, however, the p53 proteins
  halt cell division until the DNA is repaired or
  the cell is destroyed. If the p53 gene itself has
  been damaged, as in the case of cells that are
  cancerous the G1 checkpoint will fail and a
  malignant cancer cell may develop.

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Other Cell Division Controls

• Density-dependent inhibition When a certain
  density of cells is reached, growth of the cells
  will slow or stop because there are not enough
  raw materials for the growth and survival of more
  cells.
• Example Cancer cells can lose this inhibition
  and grow out of control.
• Growth Factors Some cells will not divide if
  certain factors are absent.

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Continue with Other Cell Division Control

• Cyclins is a protein that acccumlates during G1,
  S, G2, of the cell cycle
• Protein Kinase is a protein that control other
  proteins through the addition of phosphate groups.

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Chromosomes

• In eukaryotic cells, DNA and associated proteins
  are wrapped together in packages called
  chromosomes.
• DNA in eukaryotic cells is wrapped around the
  proteins to form a complex called chromatin
• Throughout the cells life, the chromatin becomes
  is loosely packed within the nucleus.
• Chromatin can not been seen by humans.
• Think a rubber band ball.

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• During cell division, however, the chromatin
  becomes highly condensed and folds up to form
  condensed chromosomes. (This is when we can see
  it).
• DNA is always replicated, or copied before
  becoming condensed .
• The x shape associated with chromosomes actually
  represents a replicated chromosome consisting of
  two identical sister chromatids joined at the
  centromere

Example Prokaryotes do not have chromosomes.
Prokaryotic DNA exist in a single loop
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Chromosome Number

• Refers to the number of chromosomes within each
  cell of an organism.
• Most animals possess two nonidentical version of
  every chromosome. These are known as homologous
  chromosomes.
• Homologous chromosomes have the same size, shape,
  and function but may have slightly different
  versions of most genes, the basic unit of
  hereditary information.

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• Cells with two sets of every chromosomes between
  their homologous chromosomes are diploid (2n),
  while cells with one set of every chromosome are
  haploid (1n)
• Diploid Somatic (Body) Cells
• Haploid Gamete (Sex) Cells

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Human Chromosome Number

• Humans has 46 chromosomes or 23 pairs
• 2n indicates diploid
• 2n 46 (2 sets of 23 chromosomes)
• 1n indicates haploid
• 1n23 (1 set of 23 chromosomes)
• Egg and sperms are haploid
• The union of sperm and egg that occurs during
  fertilization restores the chromosomes number of
  the resulting embryo to 2n 46

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Mitosis

• Is the method of eukaryotic cell division that
  produces two genetically identical cells.
• All cells in an organisms, except for sperm and
  eggs, are produced by the process of mitosis.
• Mitosis progresses along five stages Prophase,
  Metaphase, Anaphase, Telophase, and Cytokinesis

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Prophase

• Duplicated chromosomes condense and become
  visible as distinct sister chromatids.
• Nuclear envelope breaks down
• Centromeres move toward the poles of the cell.
• The mitotic spindle, which is made of
  microtubules attaches to a specialized structure
  called the kinetochore, located at the centromere
  of each replicated chromosomes.

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Metaphase

• Replicated chromosomes align at the equator, or
  metaphase plate, of the cell.
• MMs (Metaphase Middle)

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Anaphase

• The sister chromatids separate and are moved
  toward opposite poles of the cell by the spindle.
• As this happens, the cell begins to elongate
  toward the poles.

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Telophase

• Mitotic spindle breaks down.
• Nuclear envelope forms at each end of the cell,
  and the chromosomes within begin to unfold into
  chromatin.

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Cytokinesis

• The cytoplasm and organelles are evenly divided
  between the two new cells during cytokinesis,
  completing the process of cells division.
• Plants and animals cells differ in cytokinesis
• Plants, a cell plate is formed as vesicles
  containing cell membrane materials fuse together
  along the equator of the cell.
• Animals, a ring of microfilaments contracts in
  the center of the elongated cell, producing a
  cleavage furrow that eventually pinches off the
  two cells.

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Example of cytokinesis
Plant Cell
Animal Cell
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Cell Cycle
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Binary Fission

• Occurs in prokaryotes because have a single
  double-stranded loop of DNA.
• Occurs in four steps
• 1. DNA is replicated
• 2. Cell doubles in size
• 3. Cell membrane grows into the center of the
  cell, between the two circles of DNA, dividing
  the cell in two.
• 4. Two cell seperate, and a cell wall forms
  around each new cell.

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MeiosisMe likes Sex (Cells)

• The method of cell division that takes place in
  sexually reproducing organisms specifically for
  the creation of gametes sperm and egg cells.
• Production four haploid cells, each genetically
  different
• Meiosis requires two rounds of cell division.

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• Meiosis I Homologous pairs of each chromosome
  join and might exchange genetic material. The
  homologous chromosomes are pulled to opposite
  poles in the cell, at which point the cell
  separates, resulting in two cells. Each cell
  contains half the chromosome number of the
  original diploid cell. Each chromosome remains in
  the duplicated state and is made up of two sister
  chromatids

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• Meiosis II The second stage of meiosis follows
  similar steps as mitosis in the creation of two
  more cells. Chromosomes do not replicate between
  Meiosis I and Meiosis II.
• The result is four haploid cells genetically
  different from one another.

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Prophase I

• The most important events in prophase I are
  synapsis and crossing over
• Synapsis occurs when the two homologous
  chromosomes condense and combine to form
  complexes called tetrads
• Crossing over is the exchange of genetic material
  that takes place between these homologous
  chromosomes along several junctions known as
  chiasmata (place where crossing over occurs)

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Metaphase I

• The tetrads align along the metaphase plate of
  the cell.

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Anaphase I

• The homologous chromosome of each tetrad and are
  pulled toward opposite poles of the cell by the
  spindle.
• The side of the cell toward which a homologous
  chromosome is pulled a random, depending only on
  the orientation of the tetrad.
• The independent assortment of chromosome for each
  cell is result of this random mix of chromosomes
  derived from that organisms parent

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Telophase I

• Identical to telophase in mitosis.
• The cell continues to elongate, and the mitotic
  spindle breaks down.
• A new nuclear envelope forms at each end of the
  cell the chromosomes within unfold into chromatin

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Example

• Crossing over and the independent assortment of
  chromosomes during meiosis are two forces that
  help to produce genetic variation . By
  independent assortment alone, a single human can
  produce more than 8 million genetically different
  gametes. When crossing over is also considered,
  the possible number of genetically different is
  nearly limitless.

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Cytokinesis I

• Cytokinesis is very similar to mitosis divide
  cytoplasm and organelles.
• Two genetically different haploid cells have been
  produced.
• Each chromosome is still in the duplicated state
  and is made up of two sister chromatids.
• Because crossing over during prophase I, the
  sister chromatids are no longer identical.

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Meiosis II

• Meiosis II occurs right after Cytokinesis I
• There is no Interphase (therefore No DNA
  Replication)

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Prophase II

• Chromosome condense within haploid cell condense,
  and the spindle attaches to the kinetochore of
  each chromosome.
• The nuclear envelope breaks down and the
  centrosomes move toward the poles of the cell.

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Metaphase II

• Chromosomes align along the center of the
  metaphase plate

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Anaphase II

• The sister chromatids separate and are moved
  toward opposite poles of the cell by the spindle.
  
• The cell begins to elongate toward the poles

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Telophase II

• The cell continues to elongate and the mitotic
  spindle breaks down.
• A new nuclear envelope forms at each end of the
  cell and the chromosomes within may unfold into
  chromatin.

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Cytokinesis II

• The cytoplasm and organelles are divided between
  the two cells, completing the process of cell
  division.
• By the end of this stage, four genetically
  different haploid cells have been produced.

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Sex Cells (Gametes)

• Meiosis produces four genetically different
  haploid cells.
• Males haploid cells are sperms
• All four sperm can be used in sexual
  reproduction.
• Females haploid cells are 1 egg and 3 polar
  bodies
• Only the 1 egg can be used in sexual reproduction
• The three polar bodies will be recycled back into
  the body.

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Example

• The process of meiosis results in four
  genetically different haploid cells. In animals,
  these haploid cells develop into gametes, a sperm
  in males and an egg in females. Fertilization is
  the process by which a sperm and egg fuse
  together. The resulting zygote is diploid, with
  half the chromosomes coming from the mother and
  other half coming from the father. The processes
  of meiosis and fertilization both account for the
  genetic variation found in animals of the same
  species. Meiosis is responsible for creating
  gametes whose genetic material varies from that
  of the parent. Fertilization then combines the
  genetic material of the two parents to produce
  the genetic material of the offspring

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Life Cycles

• Life Cycle is the sequence of events that make up
  the reproductive cycle of an organisms.
• Alternation of generations Plants sometimes
  exist as a diploid organism and other times as a
  haploid cell.
• Two haploid gametes combine to form a diploid
  zygote, which divides mitotically to produce.
• Sporophyte undergoes under meiosis to produce a
  haploid spore
• Gametophytes Mitotic division leads to
  production of haploid multicellular organisms.
• Produces haploid gametes, which form diploid
  zygotes

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Diagram of Alternation of Generation
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Human Life Cycle

• The only haploid cells present in this life cycle
  are gametes formed during meiosis.
• Two haploid gametes combine during fertilization
  to produce a diploid zygote.
• Mitotic division then leads to formation of the
  diploid multicellular organisms.
• Meiotic division later produces haploid gametes.

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Example of Human Life Cycle
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Life Cycle of a Fungi

• Fungi are haploid organisms with the zygote being
  the only diploid form.
• Like humans, the gametes for fungi are haploid
  (n), and fertilization yield a diploid zygote.
• Instead of dividing by mitosis, the zygote
  divides by meiosis to form a haploid organisms.
• Gametes are formed by mitosis, not meiosisthe
  organism is already haploid, before forming the
  gametes.

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Life Cycle of a Fungi
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• We will discuss more about the life cycles as we
  get into the individual kingdoms.