The Cytoskeleton

1. The Cytoskeleton • Cells contain elaborate arrays of protein fibres called the cyctoskelton that serve such functions as: • Gives the cell shape and allows it to change shape when required • providing mechanical strength • locomotion • chromosome separation in mitosis and meiosis • intracellular transport of organelles • Allow cells to form tough layers or ‘tissues’ • The cytoskeleton is made up of three kinds of protein filaments ranging in size from relatively large to small: • Microtubules • Intermediate filaments and • Actin filaments (also called microfilaments)

3. Microtubules Microtubules are conveyer belts inside the cells. They move vesicles, granules, organelles like mitochondria, and chromosomes via special attachment proteins. They also serve a cytoskeletal role. Structurally, they are polymers of tubulin which is a globular protein.. The tubulin molecules are the bead like structures.  A protofilament is a linear row of tubulin dimers.Microtubules may work alone, or join with other proteins to form more complex structures called cilia, flagella or centrioles . • A single microtubule is made up of 13 protofilaments • Each protofilament is made up of tubulin dimers • Microtubules can build and disassemble rapidly by a process known as dynamic instability • Dynamic instability allows microtubules to move organelles around inside a cell and also allows the cell to change shape rapidly

4. Microtubules are produced by the MTOC or Microtuble Organising Center • The MTOC can change the length of the microtubules by adding or removing tubulindimers. This is important for processes such as:- • Changing the cell shape – Microtubules attached to the cell membrane lengthen or shorten pushing or pulling the membrane into different shapes e.g. During phagocytosis • Moving organelles – Organelles and structures such as chromosomes can be moved around inside the cell again by anchoring microtubules to them and changing the length of the microtubules by addition or removal of tubulindimers by the MTOC

5. Intermediate filaments Intermediate filaments work together with Microtubules and actin filaments to give the cell added strength, help define the cell shape, and aid cell and organelle motility.  Intermediate filaments are stable, durable. They range in diameter from 8-10 nm (intermediate in size compared with thin filaments and microtubules). They are prominent in cells that withstand mechanical stress and are the most insoluble part of the cell.    • Functions of Intermediate filaments • help stabilise the inner membrane of the nucleus • help form ‘junctions’ between cells

6. Actin Filaments • Actin Filaments • Monomers of the protein actin polymerize to form long, thin fibers. These are about 8 nm in diameter and, being the thinnest of the cytoskeletal filaments, are also called microfilaments. • Functions • form a band just beneath the plasma membrane that provides mechanical strength to • the cell • links transmembrane proteins (e.g., cell surface receptors) to cytoplasmic proteins • anchors the centrosomes at opposite poles of the cell during mitosis • pinches dividing animal cells apart during cytokinesis • generate cytoplasmic streaming in some cells • generate locomotion in cells such as white blood cells and the amoeba

7. Did you get the functions of each of the following? • establishing cell shape = Intermediate Filaments • providing mechanical strength - Intermediate Filaments • locomotion - Actin Fillaments • chromosome separation in mitosis and meiosis -microtubules • intracellular transport of organelles -microtubules • Allow cells to form tough layers or ‘tissues’-microtubules