Cytoskeleton n Means cell skeleton Internal framework of

Cytoskeleton n Means “cell skeleton” Internal framework of cell Has many functions Anchoring cell organelles ¨ Provide cell shape ¨ Aids in cell motility ¨ Response to environmental signals ¨ n Comprises Microtubules ¨ Microfilaments ¨ Intermediate filaments ¨

Microtubules n Hollow tubes made of the protein tubulin ¨ n n Alternating dimers of a and b tubulin Largest of cytoskeleton filaments Is used for: Maintenance of cell shape ¨ Motility ¨ n ¨ Movement of organelles through cell n ¨ Flagella or cilia Often involves motor molecule Often originate from centrosome

Table 6 -1 a 10 µm Column of tubulin dimers 25 nm Tubulin dimer

Centrioles n n Located in centrosome of animal cells Occur in perpedicular pair Have 9 triplets of microtubules Facilitate microtubule assembly and chromosome separation in some cells

Fig. 6 -22 Centrosome Microtubule Centrioles 0. 25 µm Longitudinal section Microtubules Cross section of one centriole of the other centriole

Flagellum structure n n n Basal body links flagellum or cilia to cell surface ¨ Basal body looks just like a centriole 9 +2 arrangement of microtubules Radial spokes prevent dramatic sliding and only bending

Fig. 6 -24 Outer microtubule doublet 0. 1 µm Dynein proteins Central microtubule Radial spoke Protein crosslinking outer doublets Microtubules Plasma membrane (b) Cross section of cilium Basal body 0. 5 µm (a) Longitudinal section of cilium 0. 1 µm Triplet (c) Cross section of basal body Plasma membrane

Motor molecules n n Interact with tubulin or actin Are fixed at one end allowed to move freely at the other end ¨ n Undulation-used for flagella and cilia movement ¨ n Movement is directional Two microtubules moving relative to one another Organelle movement is like a ski lift tram or a monorail

Fig. 6 -21 ATP (a) Receptor for motor protein Microtubule (ATP powered) of cytoskeleton Microtubule (b) Vesicles 0. 25 µm

Cell motility n n n Cell movement facilitated by flagella or cilia Unlike in prokaryotes, eukaryotic flagella undulate Cilia are small appendages and they move like a swimmers arm-active stroke and return stroke

How cell movement works n n Dynein is motor molecule that interacts with tubulin Dynein walks along one microtubule, while bound to another This results in bending ¨ If no radial spokes or organelle coat, then microtubules would walk out of cell ¨

Fig. 6 -25 Microtubule doublets ATP Dynein protein (a) Effect of unrestrained dynein movement ATP Cross-linking proteins inside outer doublets Anchorage in cell (b) Effect of cross-linking proteins 1 3 2 (c) Wavelike motion

Microfilaments n n Made of two intertwined strands of actin Helps maintain cell shape Actin rearrangements allow engulfment events ¨ Psuedopod formation in ameoba ¨ n n Promote cytoplasmic streaming in plants Essential for muscle contraction Used by invading bacteria to move around cell Frequently being assembled and disassembled within cell

Table 6 -1 b 10 µm Actin subunit 7 nm

Microfilaments 2 n n Myosin interacts with actin to cause contraction Cytoplasmic streaming and ameoboid motion are similar Cortical cytoplasm around the perimiter of cell contains perpendicular actin (wind fence) ¨ Streaming portion has parallel actin which facilitates cytoplasm movement ¨ Plant cell wall prevents amoeboid movement of plant cell ¨

Intermediate Filaments n n n Resemble cable in structure Are made of protein subunits Help maintain cell shape Are durable and not assembled and disassembled as other cytoskeleton components May help maintain organelle position

Table 6 -1 c 5 µm Keratin proteins Fibrous subunit (keratins coiled together) 8– 12 nm