Cytoskeletons

• Cytoskeleton


  • Complex array of: microtubules, microfilaments (actin filaments) and intermediate filaments
  • Can only be seen via electronic microscope
  • Determine shape of cells, play an important role in the movement of organelles and vesicles and even whole cells
• Microtubules


  • Organized in larger, more stable arrays called axonemes in the cytoplasmic extensions called cilia ("hair" almost all cells have one) and flagella
  • Hollow, with an outer diameter of 25nm and a wall 5nm thick, which helps maintain cell shape
  • Length is dynamic and variable and can be many micrometers long
  • Polymers made of α and β tubulins
  • Polymerization (assembly) is directed by microtubule organizing centers (MTOCs)
  • The dominant MTOC in most cells is the centrosome, it is organized around two cylindrical centrioles, each about 0.2μm in diameter and 0.3-0.5μm in length
• Centrioles


  • Each centriole is composed of nine highly organized microtubule triplets
  • Before cell division, centrosomes duplicate and each centrosome has two pairs of centrioles
  • During mitosis centrosome divides in halves and each moves to the opposite end of the cell and become organizing centers for the mitotic spindle
• Microtubule transport


  1. Microtubules form part of the system for intracellular transport of membranous vesicles, macromolecular complexes, and organelles

  2. Kinesins carry material away from the MTOC near the nucleus toward the plus end of microtubules (anterograde transport)

  3. Dyneins carry material along microtubules in the opposite direction (retrograde transport)

  4. This system also extends ER from the nuclear envelope to plasmalemma and moves vesicles to and through the Golgi apparatus
• Microfilaments (Actin Filaments)


  • Abundant in all cells
  • Composed of actin sub-units and allow contraction of cells
  • Associated with myosin protein family
  • 5-7nm in diameter, polarized polymers, shorter and more flexible than microtubules, assemble in presence of K and Mg
  • Monomers are added rapidly at the (+) or barbed end, with ATP hydrolysis at each addition; at the same time monomers dissociate at the (-) or pointed end (treadmilling)
  • Various myosin motors use ATP to transport cargo along actin
  • Actin-myosin interactions are needed for: Transport of organelles, vesicles, and granules in the process of cytoplasmic streaming, Cytokinesis during cell splitting, endocytosis and forceful muscle contractions
• Intermediate Filaments


  • Intermediate in size between the other two
  • More stable and allows for mechanical stability of the cell
  • Some more specialized intermediate filaments include: Keratins, Vimentin, Desmin, Glial fibrillar acidic protein (GFAP), Neurofilament, Lamins
• Nucleus


  Main structure of cell as it contains DNA (a code for all the cell proteins)
• Nuclear envelope


  • Selectively permeable barrier, between the nuclear and cytoplasmic compartments
  • Outer membrane is continuous with RER, while Inner membrane is closely associated with meshwork of proteins - nuclear lamina (for stabilization)
  • Major components of this layer are intermediate filament proteins called lamins
  • Bridged by 3000-4000 nuclear pore complexes made of nucleoporins
  • Proteins have specific import and export sequences that bind to transport proteins (importins, exportins), which in turn interact with pore complex proteins and use GTP
• Chromatin


  • Consists of DNA and all the associated proteins involved in its organization and function
  • Divided into 23 pairs of chromosomes (except gametes), each consisting of 2 identical chromatids connected by cohesin proteins
  • DNA is approx. 2m long with 3.2 billion base pairs and must be packaged in nucleus
  • This is done firstly by wrapping 150bp of DNA around the histone proteins and forming nucleosome, which has 4 histone pair core (H2A,H2B,H3 and H4) and H1 outside
  • Nucleosomes and 50-80bp linker DNA have "beads on a string" appearance
  • DNA in nucleosomes is further coiled with multiple steps and become visible via light microscopy during mitosis
  • Two types of chromatin exist, euchromatin and heterochromatin
• Euchromatin


  Gene rich "open" version of chromatin, that can be easily transcribed
• Heterochromatin


  • Has little to no transcriptional activity and is "gene poor"
  • Constitutive - mainly similar in all cells and contain repetitive sequences (centromeres, telomeres)
  • Facultative - inactivated but can be "opened" and transcribed if necessary
• The ratio of heterochromatin to euchromatin seen with nuclear staining can provide a rough indicator of a cell's metabolic and biosynthetic activity
• Facultative heterochromatin also occurs in small, dense "sex chromatin" or Barr body which is one of two X chromosomes present only in females
• Recent studies have shown that heterochromatin tends to be located near the nuclear lamina, and that different chromosomes occupy different chromosomal territories, with more active domains located deeper
• Karyotype


  Microscopic image of chromosomes that allows to see any numerical abnormalities
• Nucleolus


  • A generally spherical, highly basophilic subdomain of nuclei in cells actively engaged in protein synthesis
  • This basophilic nature comes from high amounts of rRNA that is transcribed, processed, and assembled into ribosomal sub-units
  • The rRNA sub-units are matured and quickly associated with proteins, after which newly organized sub-units are exported in the cytoplasm via nuclear pores