CELL DIVISION

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• CELL DIVISION
• Between fertilization and physical
• maturity, a human being goes from a
• single cell to roughly 75 trillion cells.
• This amazing increase in number
• occurs through a form of cellular
• reproduction called what?

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• Cell division- is the increase in number of cells
  during cellular reproduction.
• Even when development has been completed, cell
  division continues to be essential to survival as
  it replaces old and damaged cells.
• cell division is the accurate duplication of the
  cells genetic material and its distribution to
  the two new daughter cells formed by division.

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WHY A CELL DIVIDES

• Cell division is the principal means of
  reproduction
• In Eukaryotes a cell divides
• When it has grown to a certain maximum size which
  disturbs its surface volume ratio
• Certain mitogens also trigger cell division
• Plant mitogen is hormone cytokinin
• Animal mitogens are lymphokines, epidermal growth
  factor, platlet derived growth factor
• Fleming described Mitosis in Plants

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• Despite differences between prokaryotes and
  eukaryotes, there are several common features in
  their cell division processes
• Replication of the DNA must occur.
• Segregation of the "original" and its "replica"
  follow.
• Cytokinesis ends the cell division process.

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CELL DIVISION IN PROKARYOTES

• DNA replication starts from a single point as it
  is circular
• It proceeds in both the directions and ends at
  opposite side

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• Some Eukaryotes also show such type of division
  like Yeast, old and diseased cells, Protozoans,
  Cartilage of mammals.
• This division does not involve spindle formation
  and is also known as AMITOSIS or DIRECT NUCLEAR
  DIVISION

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CELL DIVISION IN EUKARYOTES

• The Cell Cycle is the sequence of growth, DNA
  replication, growth and cell division that all
  cells go through.
• Period between two cell divisions is called
  Generation Time
• Cell cycle includes Interphase and Division Phase

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• Interphase- is the time between cell division,
  and is divided into three phases.
• Interphase stages
• 1. G1 phase-
• a) the offspring cells grow to mature size.
• b) rapid synthesis of rRNA, mRNA and Ribosomes
• c) cell division promoters or inhibitors are
  produced.

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• 2. S phase-
• a) the cell copies its DNA. DNA contents doubles
  
• b) each chromosome has two chromatids.
• Next the cell enters the third stage.
• 3. G2 phase-
• a) cell prepares for cell division at this time.
• b) number of macro molecules required for
  multiplication of cell organelles and spindle
  formation are produced.

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• Cells can also exit cell cycle at G1 and enter
  into a state called the G0 Phase.
• G0 Phase- is the stage that the cell leaves the
  cell cycle, cells do not copy their DNA, do not
  prepare for cell division, EX nerve cells stop
  dividing at maturity, and normally never divide
  again.

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MITOSIS

• Mitosis- is the process that separates and
  encloses the duplicated chromosomes of the
  original cell into two identical nuclei.
• Separation of the cytoplasm to form two separate
  and distinct cells.
• It is common method of cell division in somatic
  cells so is known as Somatic cell division.
• It is indirect division because it involves an
  elaborate process of spindle formation.

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• The two daughter cells formed are qualitatively
  and quantitatively identical. So it is also
  called equational division.
• Because of spindle or mitotic apparatus
  chromosomes and organelles divides equally
  between two daughter cells.
• Mitosis has two steps Karyokinesis and
  Cytokinesis

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KARYOKINESIS

• Four stages of Karyokinesis
• 1. Prophase- is the first phase.
• DNA shortens and coils into chromosomes, forming
  two chromatids and connected by centromeres.
• The nucleolus and nuclear membrane disappears and
  spindle fibers form.

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• Centrioles- is a pair of small cylindrical bodies
  that appear in animal cells, organize the
  formation of spindles and is called Amphiastral
  spindle.
• Spindle fibers- made of microtubules, are
  produced by the centrosomes, are the fibers that
  pulls the chromatids apart.
• In plant cells spindle is formed without
  centrioles so is called Anastral spindle

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KINETOCHORE
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METAPHASE ANAPHASE
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• Metaphase- chromosomes line up in the middle of
  the cell and become visible.
• Maximum condensation of chromosomes occurs in
  this phase.
• It is the best time to observe chromosome
  morphology.

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• Anaphase
• it is a rapid phase
• centromere splits into two so that each chromatid
  has its own centromere.
• centromere lead the path while chromatids trail
  behind.
• chromosomes appears V,L,J and I shaped
• by the end of anaphase daughter chromosomes are
  able to reach the opposite poles.

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TELOPHASE
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• Telophase
• Telophase marks the end of mitosis but the
  daughter cells have yet to complete their
  physical separation.

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CYTOKINESIS

• Cytokinesis- begins with the pinching in of the
  cell membrane between the two poles, the membrane
  keeps pinching until it divides into two cells
  and all the cytoplasm and organelles are divided
  equally.

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PLANT CYTOKINESIS

• Small vesicles containing polysaccharide material
  from golgi complex collect at the equator
• Membranes of the vesicles fuse and get solidify
  to form cell plate or middle lamellae
• It grows centrifugally