Cell Reproduction
The Cell Cycle and Mitosis
Previous | Index 
A. The Cell Cycle
B. Phases of Mitosis
C. Prophase
D. Metaphase
 E. Anaphase
 F. Telophase
 G. Cytokinesis
 H. The Mitotic Spindle 
The Cell Cycle
A. The cell cycle is the well ordered sequence of events between the time a cell divides to form two daughter cells and the time those daughter cells divide.
1. Includes a doubling of a cell's cytoplasms, precise duplication of DNA < mitosis and Cytokinesis.
2. Each daughter cell contains a single, intact nucleus and some surrounding cytoplasm.
3. Duration of cell cycle varies with the type of cell. Some cells divide each hour, others take
more than 24 hours.
4. Some cell types, for example, nerve and muscle cells, never or rarely divide once they are formed

5. Two phases
         a. M phase (mitotic phase) = Mitosis + Cytokinesis (usually).
         b. Interphase = Remainder of cell cycle excluding M phase

i. Interphase comprises about 90% of the cell cycle and includes most of a  cells growth and metabolic activities with a very high degree of biochemical activity.
ii. Many components are made continuously throughout Interphase although DNA synthesis occurs only during a limited time.
iii. Interphase includes G 1 phase, S phase (DNA synthesis), and G2 phase.
iv. In brief, what we see in late Interphase is:

    • The nucleus is well defined and bounded by the nuclear envelope.
    • One of more nucleoli are present.
    • .Two pairs of centrioles are adjacent to nucleus (formed earlier by replication of original pairs).
    • Around each pair of centrioles, microtubules form in a radial array, called as aster.
    • Chromosomes are duplicated (occurred in S phase), but cannot be distinguished individually due to loosely packed chromatin fiber arrangement.
B. Phases of Mitosis
    1. Mitosis is unique to eukaryotes and may be an evolutionary adaptation for distributing a
    large amount of genetic material.
    2. Details may vary, but overall process is similar in most eukaryotes.
    3. It is a reliable process with experimental evidence showing only one error per 100,000 cell divisions.
    4. Mitosis and cytokinesis form a continuum, but for ease of description, mitosis is usually divided into four stages: prophase, metaphase, anaphase and telophase. 
C. Prophase
    1. During prophase, changes occur in both the nucleus and cytoplasm.
    2  In the nucleus
      a. Nucleoli disappear. :
      b. Chromatin fibers become tightly coiled and folded into discrete, observable
      c. Each duplicated chromosome is composed of two identical sister chromatids joined at the centromere.
3. In the cytoplasm:
    a. Mitotic spindle forms. It is composed of microtubules and associated proteins which are arranged between the centriole pairs.
    b. Centriole pairs move apart, apparently propelled along the surface of the nucleus by
    lengthening of the microtubule bundles between them.
    c. Nuclear envelope fragments in late prophase.
    d. Absence of nuclear envelope allows microtubules to interact with the more highly
    condensed chromosomes.
    e. Polar fibers (bundles of microtubules) extend from each pole toward the equator of the cell.
4. Each chromatid new has a specialized structure called the kinetochore located at the
centromere region.

5. Bundles of micro tubules (kinetochore fibers) are attached and interact with the polar fibers of the spindle to put the chromosomes into agitated motion.

D. Metaphase

    1. During metaphase:
      a. Centriole pairs are positioned at opposite ends (pole) of the cell
      b. Chromosomes move to the metaphase plate, the plane equidistant between the spindle poles.
      c. Centromeres of all chromosomes are aligned on the metaphase plate. '\
      d. The long axis of each chromosome is roughly at a right angle to the spindle axis.
      e. Kinetochore fibers of sister chromatids face opposite poles, so identical chromatids are attached to kinetochore fibers radiating from opposite ends of the parent cell.
      f. Entire structure formed by polar fibers plus kinetochore fibers is called the spindle.
E. Anaphase
    1. Anaphase begins when paired centromeres of each chromosome move apart.
      a. Sister chromatids separate and are considered chromosomes.
      b. The spindle apparatus starts moving the separate chromosomes (once joined as sister chromatids) toward opposite poles of the cell. Due to the attachment of the kinetochore fibers to the centromeres, the chromosomes move in a "V" shape.
      c. The kinetochore fibers begin to shorten at the chromosomes near the poles.
      d. The poles of the cell start to move farther apart slightly elongating the cell.
    2. At the end of anaphase, the two poles have complete and equivalent collections of
F. Telophase
    1. The main events during telophase:
      a. Polar fibers further elongate the cell.
      b. Daughter nuclei begin to form at the two poles.
      c. Nuclear envelopes are formed around the chromosomes from fragments of the parent cell's nuclear envelope and other portions of the endomembrane system.
      d. Nucleoli reappear.
      e. Chromatin fiber of each chromosome uncoils and the chromosomes become less distinct.
    2. The spindle poles move away from each other as a cell elongates during mitosis, and. different hypotheses have been formulated to explain this:
      a. Some of the movement may be due to addition of subunits to the polar fibers and their resulting elongation.
      b. The poles may be pushed apart by interdigitating polar fibers that slide past each other in the equatorial region of spindle overlap. According to this hypothesis, cell elongation is powered by ATP hydrolysis catalyzed by an ATPase related to dynein.
    3. Sister chromatid separation and movement by kinetochore microtubules during anaphase is not fully understood:
      a. Dynein and ATP do not appear to be involved.
      b. Low-energy dissociation of kinetochore microtubules into their protein subunits may play a role in moving chromosomes.
      c. One hypothesis holds that the polar ends of the microtubules depolymerize. As the tubulin subunits dissociate, the shortening microtubules with their attached chromosomes are thus drawn to the poles.
      d. A second model is based on evidence that dissociation of microtubules occurs at their kinetochore ends. This model suggests that depolymerization of microtubules at this end is an exerbonic reaction that provides the energy for the kinetochore to move poleward along the remaining tip of the microtubule ahead of the depolymerization
G. Cytokinesis
1. Cytokinesis means movement of the cytoplasm.
      a. Important changes can be observed in the cytoplasm during late anaphase or early telophase.
      b. These changes result in division of a cell into two cells.
    2. Cytokinesis occurs as cleavage in animal cells. 
      a. First indication of cleavage is the formation of a cleavage furrow which forms as a shallow groove in the cell surface near the old metaphase plate.
      b. A contractile ring of micro filaments composed on the protein actin forms on the cytoplasmic side of the furrow.
      c. These micro filaments contract, reducing the diameter of the contractile ring.
      I d. This reduction continues, deepening the cleavage furrow, until the parent cell is pinched in two.
      e. The remains of the mitotic spindle, which is the last connection between the daughter cells, breaks and the two cells are completely separate.
    3. Cytokinesis in plant cells is very different due to the presence of cell walls. 
      a. No cleavage furrow forms
      b. Across the midline of the parent cell (old metaphase plate), a structure called the cell plate forms
      c. This cell plate forms from the coalescing of vesicles derived from the Golgi apparatus.
    4. By the end of telophase
      a. Mitosis, the equal division of one nucleus into two genetically identical nuclei, is complete.
      b. Cytokinesis has begun and the appearance of two separate daughter cells occurs shortly after mitosis is completed.
H. The Mitotic Spindle 
1. The mitotic spindle is important to the events occurring in mitosis.
a. It forms in the cytoplasm during prophase and is composed of fibers formed from microtubules and associated proteins.
b. Microtubules of the cytoskeleton are partially disassembled during assembly of the mitotic spindle

Previous | Index