Cytoskeleton and Microtubules Quiz

Cytoskeleton

• Microtubules are long, hollow cylinders made of the protein tubulin, they are more rigid than actin filaments
• Microfilaments (actin filaments) are two-stranded helical polymers of the protein actin, they are less rigid than MT
• Intermediate filaments are ropelike fibers made of intermediate filament proteins, they do not participate in cell motility

Microtubules

• Microtubules are long, hollow cylinders made of the protein tubulin (alpha tubulin and beta tubulin), with an outer diameter of 25 nm
• Microtubules have one end attached to a single microtubule-organizing center (MTOC) called a centrosome
• Microtubules are involved in cell division, organization of intracellular structure, and intracellular transport, as well as ciliary and flagellar motility
• Centrosomes organize microtubules in cytoplasm
• Centrioles are made up of tubulin, two centrioles form a centrosome, which is the major microtubule-organizing center (MTOC) in human cells
• Centriole consist of 9 sets of 3 attached microtubules arranged to form a hollow cylinder
• Microtubules form cilia and flagella
• Cilia are short, hair-like structures that beat rhythmically to move fluid or particles past the cell
• Flagella are long, whip-like structures that propel cells through a fluid environment
• Microtubules are constantly adding and subtracting tubulin dimers at both ends of the filament, this is known as dynamic instability

Microfilaments

• Actin filaments are highly concentrated in the cortex, just beneath the plasma membrane
• Actin filaments are involved in muscle contraction, cell motility, cell division and cytokinesis, vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape
• Actin filament and myosin work together for muscle movement

Intermediate Filaments

• Intermediate filaments are ropelike fibers with a diameter of around 10 nm, they are made of intermediate filament proteins, they do not participate in cell motility
• One type of intermediate filament forms a meshwork called the nuclear lamina just beneath the inner nuclear membrane
• Other types extend across the cytoplasm, giving cells mechanical strength
• In an epithelial tissue, they span the cytoplasm from one cell-cell junction to another, thereby strengthening the entire epithelium

Specialized Junctions

• Tight junctions produce impermeable or semipermeable barriers between cells, especially in epithelial membranes
• Gap junctions allow electrolytes and other molecules to pass between cells, also known as communicating junctions, found in epithelial tissues, cardiac and smooth muscle, nervous and certain connective tissues
• Adherens junctions provide strong mechanical attachment between adjacent cells, holding cardiac muscle cells tightly together as the heart expands and contracts, and holding epithelial cells together
• Desmosomes are like spot welds which hold cells together and help prevent lateral tearing of tissues

Adherens Junction

• Major transmembrane protein: cadherin
• Major peripheral proteins: catenin
• Cytoskeletal anchor: actin

Cadherins

• Cadherins are a group of membrane proteins, the extracellular segments with 5 domains that form dimers in the same cell in a Ca2+-dependent manner, which interacts with cadherin dimers from another cell
• The intracellular domain of cadherin interacts with catenin which adhere the cadherin to intracellular cytoskeleton

Gap Junctions

• Gap junctions allow the transfer of small molecules and ions between adjacent cells
• Major protein component: connexin
• Connexon is made up of 6 connexins

Tight Junctions

• Tight junctions prevent the passage of materials through spaces between cells
• Major transmembrane proteins: occludin, claudin
• Major peripheral protein for interaction with cytoskeleton: ZO-1

True or False

• Actin filament is more rigid than microtubules ( False )
• Cilia are made of microtubules ( True )
• Calcium ion controls the interaction between cells via adherens junctions. ( True )
• Major transmembrane protein in gap junction is claudin. ( False)
• Gap junction permits the free passage of small proteins of ~10kDa between the cells ( False )