spindle checkpoint

Examples of spindle checkpoint in the following topics:

• Regulation of the Cell Cycle at Internal Checkpoints

  • 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.
  • The cell cycle is controlled at three checkpoints.
  • The integrity of the DNA is assessed at the G1 checkpoint.
  • Attachment of each kinetochore to a spindle fiber is assessed at the M checkpoint.

• 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.
  • If tone decreases and the muscle stretches the spindle, an impulse results in a muscle contraction.
  • With this contraction, the spindle is no longer stretched.
  • Smooth and cardiac muscles do not have specialized muscle spindles.

• Regulator Molecules of the Cell Cycle

  • In addition to the internally controlled checkpoints, there are two groups of intracellular molecules that regulate the cell cycle.
  • Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are responsible for the progress of the cell through the various checkpoints.
  • The different cyclins and Cdks bind at specific points in the cell cycle and thus regulate different checkpoints.
  • For the cell to move past each of the checkpoints, all positive regulators must be "turned on" and all negative regulators must be "turned off."
  • There is a direct correlation between cyclin accumulation and the three major cell cycle checkpoints.

• 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.

• Centrosome

  • 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.
  • They are also defective in spindle positioning and in ability to establish a central localization site in cytokinesis.
  • The mitotic spindle then forms between the two centrosomes.

• Proto-oncogenes

  • For example, a mutation that allows the Cdk gene to be activated without being partnered with cyclin could push the cell cycle past a checkpoint before all of the required conditions are met.
  • In addition to the cell cycle regulatory proteins, any protein that influences the cycle can be altered in such a way as to override cell cycle checkpoints.

• Gas Vesicles

  • Gas vesicles are spindle-shaped structures that provide buoyancy to cells by decreasing their overall cell density.
  • Gas vesicles are spindle-shaped structures found in some planktonic bacteria that provides buoyancy to these cells by decreasing their overall cell density .

• 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.

• 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.