Cell Divisions

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Cell Divisions

• Nancy G. Morris
• Volunteer State Community College

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Cell Division Functions

• asexual reproduction
• growth
• repair

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Bacteria reproduce by binary fission

• Prokaryotes
• contain about 1/1000 of the DNA in a eukaryote
• Division in Half
• single circular chromosome folded in region
  called nucleoid
• the chromosome is attached to the plasma membrane

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Figure 9.1 Bacterial Cell Division
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Genome

• Genome refers to a cells total hereditary
  endowment
• The genome of a eukaryotic cell is organized into
  multiple linear chromosomes

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Genome

• Each species has a specific number of
  chromosomes.
• Human somatic cells have 46.
• Gametes contain half the number of chromosomes of
  somatic cells.
• Human gametes have 23.

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Genome

• Chromatin in nucleus condenses to form visible
  chromosomes
• Chromosomes
• (linear in eukaryotes circular in prokaryote)

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Genome

• Mitosis ( mitos thread)
• nuclear division
• duplicated chromosomes are evenly distributed
  into two daughter cells
• daughter cells are the genetic equivalent of the
  parent cell

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Before Mitosis

• the cell copies every chromosome
• two sister chromatids are formed
• After Mitosis
• cytokinesis division of the cytoplasm

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The Cell Cycleevents prior to division

• a doubling of the cells cytoplasm
• reproduction of cellular organelles
• precise duplication of DNA

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The Cell Cycle

• mitosis
• (karyokinesis division of the nucleus)
• cytokinesis division of the cytoplasm

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The Cell Cycle

• G1 phase - first gap phase
• S phase - synthesis
• G2 phase - second growth phase

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The Cell Cycle

• Mitosis is a reliable process with only one error
  per 100,000 cell divisions !

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The Cell Cycle (Fig. 9.7)
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Stages of Mitosis (Figure 9.3)

• G2 of Interphase
• two pair of centrioles
• centrioles surrounded by Aster rays
  (microtubules)
• duplicated chromosomes cannot be individually
  distinguished

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Stages of Mitosis Prophase

• nucleoli disappear
• chromatin condenses to form chromosomes
• each chromosome contains two sister chromatids
• Mitotic spindle forms

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Stages of Mitosis Metaphase

• Centrioles at opposite ends of the cell
• Chromosomes line up at equator
• Kinetochores attach to the spindle

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Stages of Mitosis Anaphase

• sister chromatids split and move toward opposite
  poles
• the cell elongates moving poles further apart

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Stages of Mitosis Telophase

• Daughter nuclei begin to form at two poles
• Nuclear envelopes form around nuclei
• Nucleoli reappear
• Chromatin uncoils chromosomes become less
  distinct

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Cytokinesis

• Begins at telophase
• In animal cells,
• a cleavage furrow forms
• In plant cells,
• a cell plate forms

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Cytokinesis
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Cues control division

• Normal growth and development depend on timing
  and rate of mitosis
• Human skin cells divide frequently
• Liver cells only divide when induced by
  environmental cues of injury
• Nerve and muscle cells do not divide in mature
  humans

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Cues control division

• In Normal growth and development, cells exhibit
  density-dependent inhibition.
• In culture, cells stop dividing when they form a
  single layer and touch other cells.
• If some cells are removed from the layer, those
  bordering the open space will divide again to
  fill it up.

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Cues control division

• Whether a cells will divide or not is determined
  by the restriction point
• A cell destined to divide will progress beyond
  the restriction point into the S phase
• A cell not destined to divide will exit form the
  cell cycle at the restriction point and switch to
  non-dividing Status, G0 Phase.

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Cues control division

• A cell must add enough cytoplasm to gain a
  certain size before DNA is synthesized. This
  prevents daughter cells from becoming
  progressively smaller.

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Division out of Control

• Some cells escape from the controls on cell
  division
• CANCER cells - Divide excessively, invade other
  tissues, and if unchecked, can kill the organism
• Cancer cells in culture are immortal. They
  continue to divide indefinitely, as long as
  nutrients are available. (Normal mammalian cells
  in culture divide only about 20 - 50 times before
  they stop dividing.)