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Cytoskeleton: • Three Main Structures of Cytoskeleton: • Each with their own unique functions and protein subunits which assemble during polymerization. • All contains different monomers — basic subunits. • Intermediate filaments, microtubules and actin filaments. • Intermediate Filaments: Extended in the cytosol, found around the nucleus and plasma membrane. • Extend to other cells forming cell to cell junctions — means of communication. • Strongest cytoskeleton structure, can withstand extreme conditions. • Structure: • Made up of alpha helix — thicker than actin but thinner than meiosis filaments. • Has a “ropelike” structure — each alpha subunits produces a coiled coil dimer. Two dimers bind together to form a tetramer — the final shape contains eight tetradimers in a coiled coil structure. • Four Classes: • In cytoplasm the three classes are found in: •Keratin — In epithelial cells (cell to cell junction is important here). •vimentin — In connective tissue, muscle cells and glial cells. •Neurofilaments — In nerve cells • In nucleus it is found in: •Nuclear Lamins — In all animal cells. • Microtubules: Extend from centromeres and are involved in the cell cycle by segregated chromosomes during anaphase of mitosis. • Organizing roles — extend in different directions in the cell to form rows. • Organize the movement of vesicles and other molecules — use microtubules as a method of transport and direction. • Form flagella/cilia structures — moment depends on the microtubules that they have. • Structure: • Composed of alpha and beta tubulin. • Hollow cylinder made up of thirteen “protofilaments” to make one cylinder of microfilament. Bono, 1
• Have a plus and minus end — beta is on the plus end, alpha is on the minus end. • Plus and minus end is also present in actin filaments. • Grow from gama tubuline rings found on centrosomes — all begin growth on the plus end. • Microtubules also shrink from the same end because the plus end is attached to the centrosome, therefore they shrink from the plus end. • Outside the cell they shrink on the minus end. • Centrioles give direction and orientation to the cell, but main function is unknown. • Plant cells do not have centrioles, only found in animal cells. • Growth: • Begin to grow out of the centromeres and at the same time they grow and shrink. • Grow and shrink independent of one another — all grow towards positive end and shrink towards the minus end if they bind to structures, if they don't bind to a structure random polymerization continues.• Dynamic Instability: • If they are able to reach another structure they become stabilized, however if they do not theycontinue depolymerization — Fast process which destroys the microtubule. • How Does this Work: • Begins at the gama — minus section. • During polymerization when it is growing tubular carrying GTP on the growing microtubule.• Newly added tubulines can only add to GTP not GDP. • GTP carrying microtubule is more stable than GDP microtubule. • Hydrolization normally occurs faster than production of GDP. •When other microtubules are added, GTP becomes hydrolyzed into GDP. • If hydrolization takes more time than a new GTP being formed, polymerization occurs — the production of a fast growing microtubule. • If hydrolysis is faster than addition of a new tubulin with GTP the molecule will be destroyed containing free tubulin’s within the cell. • As hydrolysis catches up the tubuline contains mostly GDP molecules and can no longer bind to another GTP molecule which leads to a decrease in stability and destruction of the microtubule.• Polymerization and depolymerization occurs on the positive end. • Normally a result of having a lac of tubuline containing GTP in the environment. • Importance: • Important in the cell cycle — results in attachment of microtubules to specific kinetochores. • Segregation of the chromosomes depends on this. • Cancer treatment work to stabilize the microtubules or destabalize microtubules which results in cell death of cancerous cells. •Taxol — stabilizes microtubules • Bind to microtubules and wont allow them to shrink — does not allow recycling of tubulins (old parts can not make new parts). •Colchicine —Destabilizes microtubules. Bono, 2
• Bind to tubulin and wont allow them to attach to the growing microtubules — this prevents them from entering the cell cycle because they cannot attach to kinetochores• Motor Protein Families — Dyneins and Kinesins: • Can get motor proteins from microtubules. Specific to microtubules are Dyneins and Kinesin. • Specific In Movement:• Dynein will always go towards the minus end — centromeres. • Kinesins will always go towards the plus end. • Similar In Structure and Process: • Both contain a globular head with a tail that holds the cargo. • Movement is based on globular heads— walk with the head portion. • Globular head interacts with the microtubule through ATP hydrolysis. • Each cycle uses one ATP — equivalent to one step, the tail binds to the cargo (ie. transport vesicle). • Transport Different Cargo: • Based on their direction they are responsible to move different organelles — either away or towards the nucleus. • They organize the cell because of this function. •If the movement is on the plus end — towards the plasma membrane, away from the nucleus. • Kinesin takes vesicles or organelles away from the nucleus towards the plasma membrane. •If the movement is on the minus end — towards the nucleus. • Dynein takes vesicles or organelles towards the nucleus and away from the plasma membrane. • Actin Filaments:• Present in all eukaryotic cells. • Involved in strength, shape and structure and are found under the plasma membrane. • Contain a specific polymerization and depolymerization and associated motor proteins. • Function:• Provide structure to the cell (bones and muscles of the cell), motility and muscle movement.contractions.• Involved in the cell cycle — cytokensis occurs by actin filaments facilitating the dividing. • Movement — phagocytosis is carried out by actin filaments. • Can not occur in isolation — groups of actin at the same location provides function. • Structure: • Made up of actin monomers — contain a minus and plus end. • Actin monomers carry ATP. • Treadmilling — Polymerization/depolymerization Process: • New actin are added to the plus end and are released from the minus end during function — this occurs when their is normal concentration in the cell. Bono, 3
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School: Carleton University
Course: Cell Biology & Biochemistry
Professor: Mohsen Hooshyar
Term: Fall 2017
Name: Week 10, BIOL2201
Description: This is everything that we have covered so far for the material that will be on our final exam.