mitotic spindle

Examples of mitotic spindle in the following topics:

• Binary Fission

  • The precise timing and formation of the mitotic spindle is critical to the success of eukaryotic cell division.
  • Prokaryotic cells, on the other hand, do not undergo karyokinesis and, therefore, have no need for a mitotic spindle.
  • However, the FtsZ protein that plays such a vital role in prokaryotic cytokinesis is structurally and functionally very similar to tubulin, the building block of the microtubules that make up the mitotic spindle fibers that are necessary for eukaryotes.
  • A survey of mitotic assembly components found in present-day unicellular eukaryotes reveals crucial intermediary steps to the complex membrane-enclosed genomes of multicellular eukaryotes.

• The Mitotic Phase and the G0 Phase

  • Microtubules that will eventually form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen.
  • The mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area.
  • The proteins of the kinetochore attract and bind mitotic spindle microtubules.
  • The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell.
  • During prometaphase, mitotic spindle microtubules from opposite poles attach to each sister chromatid at the kinetochore.

• Centrosome

  • In mitosis the nuclear membrane breaks down and the centrosome nucleated microtubules (parts of the cytoskeleton) can interact with the chromosomes to build the mitotic spindle .
  • The mitotic spindle then forms between the two centrosomes.
  • When the centrioles are irradiated by a laser, mitosis proceeds normally with a morphologically normal spindle.
  • In the absence of the centrioles the microtubules of the spindle are focused by motors allowing the formation of a bipolar spindle.
  • The mitotic spindle then forms between the two centrosomes.

• Interphase

  • In order for a cell to move from interphase into the mitotic phase, many internal and external conditions must be met.
  • The two centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis.
  • The cell cycle consists of interphase and the mitotic phase.
  • Interphase is followed by the mitotic phase.
  • During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei.

• Regulation of the Cell Cycle at Internal Checkpoints

  • The G2 checkpoint bars entry into the mitotic phase if certain conditions are not met.
  • If the DNA has been correctly replicated, cyclin dependent kinases (CDKs) signal the beginning of mitotic cell division.
  • The M checkpoint is also known as the spindle checkpoint because it determines whether all the sister chromatids are correctly attached to the spindle microtubules.
  • Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to at least two spindle fibers arising from opposite poles of the cell.
  • Attachment of each kinetochore to a spindle fiber is assessed at the M checkpoint.

• Comparing Meiosis and Mitosis

  • The nuclei resulting from a mitotic division are genetically identical to the original nucleus.
  • Chiasmata develop and crossover occurs between homologous chromosomes, which then line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad.
  • Meiosis II is much more similar to a mitotic division.
  • During anaphase II and mitotic anaphase, the kinetochores divide and sister chromatids, now referred to as chromosomes, are pulled to opposite poles.

• Muscle Tone

  • Connecting to the endomysium of a muscle fiber, muscle spindles are composed of nuclear bag fibers and nuclear chain fibers.
  • Both cells of the muscle spindle contain sensory neurons and when stretched muscle spindles become activated triggering impulses to the spinal cord which can generate an immediate reflex.
  • Spindles can also trigger impulses to the cerebral cortex providing information about the degree of stretch within the muscle.
  • If tone decreases and the muscle stretches the spindles trigger an impulse resulting in the contraction of the muscle.
  • With this contraction the spindle is no longer stretched and so stops triggering further contraction.

• The Role of the Cell Cycle

  • The cell cycle has two major phases: interphase and the mitotic phase .
  • During the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides.
  • The cell cycle consists of interphase and the mitotic phase.
  • Interphase is followed by the mitotic phase.
  • During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei.

• Classification of Receptors by Location

  • While the cutaneous touch receptors found in the dermis and epidermis of our skin and the muscle spindles which detect stretch in muscles in skeletal muscle are both mechanoreceptors, they serve discrete functions.
  • While it is true that, in both cases, the mechanoreceptors detect physical forces that result from the movement of the local tissue, cutaneous touch receptors provide information to our brain about the external environment while muscle spindle receptors provide information about our internal environment.
  • Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle), and expanded schematic (right).
  • The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers.

• Proprioceptive Sensations

  • Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of a muscle.
  • The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles.
  • The Golgi organ (also called Golgi tendon organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle fibers onto the tendons of skeletal muscle.
  • Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle), and expanded schematic (right).
  • The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers.