; The Cell Cycle and Mitosis
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The Cell Cycle and Mitosis


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									           The Cell Cycle and Mitosis

A.   The eukaryotic cell cycle
B.   Regulation of the cell cycle
C.   Mitosis: an overview
D.   Stages of mitosis
E.   The spindle apparatus
F.   Cytokinesis
      A. The Eukaryotic Cell Cycle

Eukaryotic cell cycle:
– A series of stages through which most eukaryotic
  cells pass during some time in their development
– In single-celled eukaryotes, the cell cycle often
  represents the major mechanism for asexual
  reproduction of the species
– In multicellular eukaryotes, the cell cycle is used for
  cellular reproduction in embryonic cells and stem
      A. The Eukaryotic Cell Cycle
Key events of the cell cycle
– Interphase: a period of extensive metabolic & synthetic
– During this time the DNA (chromosomes) of the cell are
– Mitosis: the nucleus of the cell divides to form two identical
  nuclei, with each new nucleus having the same number of
  chromosomes as the parent nucleus
– Cytokinesis: in most cases (with some exceptions), the cell’s
  cytoplasm divides to form two cells, each containing one of
  the new nuclei
     A. The Eukaryotic Cell Cycle

Stages of the cell cycle
– G1 (gap-1) is the period before chromosomal
  replication takes place
– S (synthesis) is the period when the chromosomes
  are replicated
– G2 (gap-2) is the period following chromosomal
– M (mitosis) is the period of nuclear division
     A. The Eukaryotic Cell Cycle

G1, S, and G2 are collectively called “interphase,”
characterized by:
– The presence of a well-defined nucleus
– Individual chromosomes are not visible
– Chromatin: chromosomes are dispersed in an
  uncondensed state called chromatin; “Grainy” or
  “sandy ” appearance
– Nucleolus: a dark-staining spot sometimes (but not
  always) seen in the interphase nucleus; The site of
  ribosome synthesis
    B. Regulation of the Cell Cycle

In multicellular eukaryotes, dividing cells are
– In embryonic tissue;
– In certain tissues that are constantly renewed, such
  as bone marrow stem cells or plant meristems;
– In damaged tissue, such as cell division in damaged
    B. Regulation of the Cell Cycle

Go (stationary) cells: cells that have reached
terminal differentiation and no longer divide
Cells stop dividing before they enter “S” phase;
Therefore, Go is similar in some respects to G1
except that the cell does not enter “S” phase
The cell cycle is regulated by several types of
regulatory proteins that act at two major points:
the G1 – S junction, and the G2 – M junction
          C. Mitosis: An Overview

a) During G1 each chromosome is uncondensed
   and in an unreplicated state.
           C. Mitosis: An Overview

b) In the S phase, each chromosome is replicated.
   The replicated chromosome consists of two
   sister chromatids attached at the centromere.
            C. Mitosis: An Overview

c) At the beginning of mitosis, the chromosomes
   condense and become visible. Note that the
   chromosome is still in a replicated state, with a
   pair of chromatids attached at the centromere.
          C. Mitosis: An Overview

d) During mitosis: the centromere splits, the
   chromatids separate and become daughter
   chromosomes, and the daughter chromosomes
   move to opposite sides of the cell.
           C. Mitosis: An Overview

e) At the end of mitosis, the chromosomes on each
   side of the cell decondense and form two new,
   identical nuclei.
            D. Stages of Mitosis

– The nuclear membrane and nucleolus disperse
– The chromatin condenses to form visible
– The mitotic spindle apparatus begins to form
            D. Stages of Mitosis

– The chromosomes line up along the equator of the
  spindle apparatus
– Some writers use the term “prometaphase” to
  represent the transitional state between prophase &
– The centromeres split and the daughter
  chromosomes move to opposite poles of the spindle
            D. Stages of Mitosis

– At each pole of the spindle, the chromosomes
  decondense and return to the state of chromatin
– New nuclear membranes and nucleoli form at each
  spindle pole, completing the formation of two new
  identical nuclei
       E. The Spindle Apparatus

Composed of microtubules: fibers that are
dynamically assembled from the protein tubulin
– Found at the poles of the spindle in animal cells
– Each centrosome consists of two centrioles: short
  tubulin structures that help to regulate tubulin
– In plant cells: plant cells have “microtubule-
  organizing centers” (instead of centrosomes) at each
       E. The Spindle Apparatus

Some fibers are attached to chromatids
– Each attached fiber is connected to the kinetochore
  of the chromatid: a region near (or at) the
The movement of chromatids is accomplished by
shortening of the kinetochore fibers
– This is done by depolymerization of the microtubules
  at the spindle poles
       E. The Spindle Apparatus

Chemicals that inhibit microtubule formation
(such as colchicine) cause mitosis to stop at
Colchicine and similar chemicals are used to
arrest mitosis at metaphase for the purpose of
chromosomal analysis (karyotyping)
                   F. Cytokinesis

In animal cells:
– Cytokinesis is accomplished by the formation of the
  cleavage furrow
– A contractile ring or “girdle” of actin fibers and
  myosin (a transient muscle-like assembly) forms just
  inside the plasma membrane, around the equatorial
– The contractile ring pulls the plasma membrane
  together to pinch the cell into two cells
                  F. Cytokinesis

In plant cells:
– Cytokinesis is accomplished by the formation of the
  cell plate
– Cell plate: A new cell wall forming at the equatorial
– The cell plate enlarges until it completely separates
  the cell into two cells

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