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microtubules and microfilaments

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helpful in understanding cytoskeleton of the cell

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microtubules and microfilaments

  1. 1. The eukaryotic cells possess a skeletal system called cytoskeleton that has got analogous function. The cytoskeleton is composed of 3 well defined filamentous structurs – microtubules, microfilaments and intermediate filaments with distinct functions . Each filaments are made of protein subunits held together by weak non covalent bonds.  This type of construction allows rapid assembly and disasembly contolled by cell regulation
  2. 2. MICROTUBULES They are components of a diverse array of substances including the mitotic spindles of dividing cells and core of flagella and cilia STRUCTURE AND COMPOSITION Have an outer diameter of 25nm and a wall thickness of 4nm and may extend across the length and breadth of the cell The wall of microtubule is composed of globular proteins arranged in longitudinal rows called protofilaments that are allinged parallel to the long axis of the tubule .
  3. 3. When veiwed in cross section they are seen to have 13 protofilaments allinged side by side in a circular pattern Each protofilament is assembled from dimeric building blocks consisting of one alpha and one beta subunits The protofilament is asymmetric with alpha subunit on one end and beta on the other One end of the protofilament is known as the plus end is terminated by a row of beta tubulin units and the mnus end is terminated by the alpha tubulin units
  4. 4. 1. ACT AS STRUCTURAL SUPPORT AND ORGANIZERS  They are stiff enough to resist the forces that can bend or compress the fibre  The distribution of microtubules through the cytoplasm of a cell determines the shape of a cell eg: in coloumnar epithelial cells the microtubules are alligned along the axis of the cell  Maintain a key role in the internal organization of a cell
  5. 5. 2.ACT AS AGENTS OF INTRACELLULAR MOTILITY Involved in the movement of vesicles,proteins,organellsetc across the cytoplasm throught the cell Eg: AXONAL TRANSPORT: proteins such as neurotransmittors are secreated and packed in membranous vesicles by golgi body and endoplasmic reticulum of the cell body are transported through the axon which consists of a number of of microtubules and motor proteins which takes it down the axon
  6. 6. MOTOR PROTEINS :  they convert chemical energy into mechanical energy that is used to generate force or move the Types of cargo include vesicles,chromosomes , mitochondria, proteins etc They can be classified into 3 types mainly : kinesins and dyneins that move along the microtubules and myosin that move along microfilaments The binding of ATP and its hydrolysis provides energy to them to travel cargo attached to the motor
  7. 7. KINESINS: is a tetramer constructed by 2 identical heavy chains and 2 identical light chains, has a globular head that binds ATP ,a neck a stalk and a fan shaped tail that binds to the cargo to be transported
  8. 8. DYENEINS: it is a huge protein composed of two identical heavy chains and a variety of intermediate and light chains. They are responsible for the movement of cilia and flagella .move towards the minus end of the microtubule. They act as: As a force-generating agent in positioning the spindle and moving chromosomes during mitosis As a minus end–directed microtubular motor with a role in positioning the centrosome and Golgi complex and moving organelles,vesicles,and particles through the cytoplasm
  9. 9. MICROTUBULE ORGANIZING CENTRE 1. CENTROSOME: In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring. • the centroles are surrounded by an electron rich pericentriolar matrix
  10. 10. Microtubules are major components of spindle fibre used to pull apart chromosomes during cell division
  11. 11. 2. BASAL BODIES the outer microtubules in a cillia or flagella arise from basal bodies attached to the base of cillia or flagella
  12. 12. Microtubules are the central structural supports in cilia and flagella. Both can move unicellular and small multicellular organisms by propelling water past the organism. If these structures are anchored in a large structure, they move fluid over a surface.  For example, cilia sweep mucus carrying trapped debris from the lungs.
  13. 13. A flagellum has an undulatory movement. Force is generated parallel to the flagellum’s axis. Fig. 7.23a
  14. 14. Fig. 7.23b Cilia move more like oars with alternating power and recovery strokes. They generate force perpendicular to the cilia’s axis.
  15. 15. In spite of their differences, both cilia and flagella have the same ultrastructure. Both have a core of microtubules sheathed by the plasma membrane. Nine doublets of microtubules arranged around a pair at the center, the “9 + 2” pattern. Flexible “wheels” of proteins connect outer doublets to each other and to the core. The outer doublets are also connected by motor proteins. The cilium or flagellum is anchored in the cell by a basal body, whose structure is identical to a centriole.
  16. 16. The bending of cilia and flagella is driven by the arms of a motor protein, dynein. Addition to dynein of a phosphate group from ATP and its removal causes conformation changes in the protein. Dynein arms alternately grab, move, and release the outer microtubules. Protein cross-links limit sliding and the force is expressed as bending. Fig. 7.25
  17. 17. microfilaments  the thinnest class of the cytoskeletal fibers, are solid rods of the globular protein actin. An actin microfilament consists of a twisted double chain of actin subunits. Microfilaments are designed to resist tension. With other proteins, they form a three- dimensional network just inside the plasma membrane.
  18. 18. In muscle cells, thousands of actin filaments are arranged parallel to one another. Thicker filaments, composed of a motor protein, myosin, interdigitate with the thinner actin fibers. Myosin molecules walk along the actin filament, pulling stacks of actin fibers together and shortening the cell. Fig. 7.21a
  19. 19. In other cells, these actin-myosin aggregates are less organized but still cause localized contraction. A contracting belt of microfilaments divides the cytoplasm of animals cells during cell division. Localized contraction also drives amoeboid movement.  Pseudopodia, cellular extensions, extend and contract through the reversible assembly and contraction of actin subunits into microfilaments. Fig. 7.21b
  20. 20. In plant cells (and others), actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming. This creates a circular flow of cytoplasm in the cell. This speeds the distribution of materials within the cell. Fig. 7.21c

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