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THE CELL CYCLE The Key Roles of Cell

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THE CELL CYCLE The Key Roles of Cell Powered By Docstoc
					                CHAPTER 12
              THE CELL CYCLE

    The Key Roles of Cell Division
Cell division results in genetically identical
 daughter cells

The Mitotic phase alternates with interphase
 in the cell cycle
                                    Introduction
•   The continuity of life from one cell to another is
    based on the reproduction of cells via cell division.

•   This division process occurs as part of the cell cycle
    (the life of a cell from its origin in the division of a
                                              two).
    parent cell until its own division into two).
                                                               Fig. 12.1, Page 216
•                                                      (e.g.
    The division of a unicellular ‫ وﺣﯿﺪ اﻟﺨﻠﯿﺔ‬organism (e.g.
    Amoeba)
    Amoeba) reproduces an entire organism, increasing
    the population.

•   Cell division is also central to the development of a
    multicellular ‫ ﻋﺪﯾﺪ اﻟﺨﻠﯿﺔ‬organism that begins as a
    fertilized egg or zygote.
                                                                      12.
                                                                Figs. 12.1, Page 216


                                                                                       2
 Multicellular organisms also use cell division
 to repair and renew cells that die normally or
              (blood cells from bone marrow).
 by accidents (                               ).


Cell division distributes the genetic
material (DNA) to two daughter cells.
                                                               12.
                                                         Figs. 12.1, Page 216




Division is differ among cells:.

  - Skin cells divide frequently.
  - Liver cells divide when needed (damage repair).
  - Nerve cells and muscle cells do not divide at all.



                                                                                3
   Cell division results in genetically
   identical daughter cells
 A cell’s genetic information (genome ‫ )اﻟﺒﻨﻚ اﻟـﭽﯾﻨﻰ‬is packaged as DNA.
 In prokaryotes, the genome is often a single long DNA molecule.
     In eukaryotes, the genome consists of several DNA molecules.
 A human cell must duplicate about 3 m of DNA and separate the two copies
  such that each daughter cell ends up with a complete genome.
 DNA molecules are packaged into chromosomes.
     Every eukaryotic species has a characteristic                     number of
      chromosomes in the nucleus.
     Human somatic cells (body cells) have
                              (body cells)                                   46
      chromosomes.
     Human gametes ‫( أﻣﺸﺎج‬sperm or eggs) have                                      23
      chromosomes, half the number in
                                   .
      a somatic cell ‫.اﻟﺨﻠﯿﺔ اﻟﺠﺴﺪﯾﺔ‬
 Each eukaryotic chromosome consists                                          of a long,
  linear DNA molecule.                                       Fig. 12.2, Page 216         4
 Each chromosome has hundreds or thousands of genes, the units that specify an
  organism’s inherited traits.
 Associated with DNA are proteins that maintain its structure and help control
  gene activity.
 This DNA-protein complex, chromatin, is organized into a long thin fiber.
 After the DNA duplication in preparation for cell division, chromatin condenses,
  coiling and folding to make a smaller package.
 Each duplicated chromosome consists of two sister chromatids which contain
  identical copies of the chromosome’s DNA.
 Asthey       condense,     the
  region where the strands
  connect     shrinks     to   a
  narrow      area,    is    the
  centromere.



                               Fig. 12.3
 Later, the sister chromatids are pulled apart and repackaged into
  two new nuclei at opposite ends of the parent cell during cell
  division.
 The process of the formation of the two daughter nuclei called
  (mitosis) and is usually followed by division of the cytoplasm
  (cytokinesis ‫ .) اﻹﻧﺸﻄﺎر اﻟﺨﻠﻮى‬It occurs in somatic cells ‫اﻟﺨﻼﯾﺎ اﻟﺠﺴﺪﯾﺔ‬
               Chromatid
                Chromatin +
                   DNA
                              Sister chromatid


                                   Homologous
                                   Chromosome
        Centromere



               Chromosome
                                                                         6
                   ‫اﻟﺼﺒﻎ‬
  In the gonads ‫ ,اﻟﻤﻨﺎﺳﻞ‬cells undergo a meiosis division, which yields four daughter
                 ‫,اﻟﻤﻨﺎﺳﻞ‬
cells, each with half the chromosomes number of the parent cell.

                                                                 23.
 In humans, meiosis reduces the number of chromosomes from 46 to 23.

 Each of us inherited 23 chromosomes from each parent: one set in an egg and
one set in a sperm during meiosis.

  gametes ‫(اﻷﻣﺸﺎج‬eggs or sperm) are produced only in gonads ‫( اﻟﻤﻨﺎﺳﻞ‬ovaries or
          ‫(اﻷﻣﺸﺎج‬
testes).

 The fertilized egg undergoes trillions of cycles of mitosis and cytokinesis to
produce a fully developed multicellular human.

 These processes continue every day to replace dead and damaged cell.

  Fertilization fuses two gametes together and doubles the number of
chromosomes to 46 again.
                               Definitions
Genes:
Genes: Small segments of DNA that specify an organism’s inherited characters.
                                                                  characters.
Chromatin: DNA-
Chromatin: A DNA-protein complex which is organized into a long thin fiber

 Chromosome:In the nucleus of each cell, the DNA molecule is packaged into
 Chromosome:
 thread-like structures called chromosomes. Each chromosome is made up of
 DNA tightly coiled many times around proteins called histones that support its
 structure.
 Chromatids:
 Chromatids: Two sister arms (chromatids) formed in each duplicated
        chromosome. They contain identical copies of the chromosome’s DNA
Centromere: The narrow region at which the chromatids are attached to each
Centromere:
other
Mitosis:
Mitosis:Mitosis is a process of cell division which results in the production of
two identical daughter cells from a single parent cell.
    okinesis:
Cyt okinesis: ‫ اﻹﻧﺸﻄﺎر اﻟﺨﻠﻮى‬Is the division of the cytoplasm which
              usually follows mitosis.
 Meiosis:
 Meiosis: Cell division that occurs In the gonads ‫ ,اﻟﻤﻨﺎﺳﻞ‬and yields
                                                  ‫,اﻟﻤﻨﺎﺳﻞ‬
     four daughter cells, each with half the number of chromosomes of the          8
 parent cell.
The mitotic phase alternates with interphase
        in the cell cycle: an overview
Phases of the Cell Cycle
• The mitotic (M) phase of the cell cycle alternates
  with the much longer interphase.
  – The M phase includes mitosis and cytokinesis.
  – Interphase accounts
    for 90% of the cell
    cycle.


                  Fig. 12.5
A. Interphase has three subphases:
  A. the G1 phase (“first gap”) centered on growth,
  – the S phase (“synthesis”) when the chromosomes are
    copied,
  – the G2 phase (“second gap”) where the cell completes
    preparations for cell division,
  – and divides (M).
• The daughter cells may then repeat the cycle.
B. Mitosis: is usually include five sub-phases ‫:ﻣﺮاﺣﻞ ﻓﺮﻋﯿﺔ‬
                                    sub-
   A.   Prophase, ‫اﻟﺘﻤﮭﯿﺪﯾﺔ‬
   B.   Prometaphase,
        Prometaphase, ‫ﻗﺒﻞ اﻹﺳﺘﻮاﺋﯿﺔ‬
   C.   Metaphase, ‫اﻹﺳﺘﻮاﺋﯿﺔ‬
   D.   Anaphase, ‫اﻹﻧﻔﺼﺎﻟﯿﺔ‬
   E.   Telophase. ‫اﻹﻧﺘﮭﺎﺋﯿﺔ‬


                     (G2
  By late interphase (G2), the
  chromosomes have been
  duplicated ‫ ﺗﻀﺎﻋﻔﺖ‬but are
  loosely packed.

  The centrosomes have
  been duplicated and begin
  to organize microtubules
  into an aster (“star”).                         12.
                                             Fig. 12.5a, 218   11
1)   Prophase, ‫ اﻟﺘﻤﮭﯿﺪﯾﺔ‬the chromosomes are
     tightly coiled, with sister chromatids
                                   disappear.
     joined together, The nucleoli disappear.
     The mitotic spindle begins to form and
     appears to push the centrosomes away
     from each other towards opposite ends
                    cell.
     (poles) of the cell.

2)   Prometaphase, ‫ ﻗﺒﻞ اﻹﺳﺘﻮاﺋﯿﺔ‬the nuclear
     envelope fragments and microtubules
     from one pole attach to one of two
     kinetochores (special regions of the
     centromere)
     centromere) while microtubules from
     the other pole attach to the other
     kinetochore.
     kinetochore.

3)   Metaphase, ‫ اﻹﺳﺘﻮاﺋﯿﺔ‬the spindle fibers
     push the sister chromatids until they are
     all arranged at the imaginary plane
     equidistant between the poles, defining
                                      12.
                                 Fig. 12.6a, b,c,d
Anaphase,
Anaphase, ‫ اﻹﻧﻔﺼﺎﻟﯿﺔ‬the centromeres
    divide, result in separating the
            chromatids.
    sister chromatids. Each is then
    pulled toward the pole to which it
                           fibers.
    is attached by spindle fibers. By the
    end, the two poles have equivalent
                   chromosomes.
    collections of chromosomes.

Telophase,
Telophase, ‫ اﻹﻧﺘﮭﺎﺋﯿﺔ‬the cell continues to
     elongate as free spindle fibers from
     each centrosome push off each
     other.
     other.
    1)   Two nuclei begin to form,
         surrounded by the fragments of the
                          envelope.
         parent’s nuclear envelope.
    2)   Chromatin becomes less tightly
         coiled.
         coiled.
Cytokinesis,
Cytokinesis, begins as the
division of the cytoplasm
occurs.
                                                   12.
                                              Fig. 12.6 e and f
Fig. 12.6
Fig. 12.6
               cytokinesis:
        B. The cytokinesis: ‫ اﻹﻧﺸﻄﺎر اﻟﺨﻠﻮى‬divides the cytoplasm:




• Cytokinesis (division of the
  cytoplasm)
  cytoplasm) typically follows
  mitosis.

• Contraction ‫ إﻧﻘﺒﺎض‬of the cell
  pinches the cell into two
  new cells


                                                                    16
Fig. 12.8, Page 222
                                     Cell Cycle


     Interphase                               Division process


                                             Mitosis                Cytokinesis

G1        S        G2




      Prophase          Prometaphase      Metaphase    Anaphase      Telophase
       ‫اﻟﺘﻤﮭﯿﺪﯾﺔ‬          ‫ﻗﺒﻞ اﻹﺳﺘﻮاﺋﯿﺔ‬    ‫اﻹﺳﺘﻮاﺋﯿﺔ‬    ‫اﻹﻧﻔﺼﺎﻟﯿﺔ‬     ‫اﻹﻧﺘﮭﺎﺋﯿﺔ‬


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