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Section 1 - Cytoskeleton - 9.docx

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Biology 2382B
Robert Cumming

Section 1 – Cytoskeleton How is cellular movement achieved Cellular trafficking of organelles Cell migration So far talked about how to look at cells, structures inside the cells – elements of the cell, we know that things move around. These vsicles aren’t just floating around though, they’re being moved around by way of the cytoskeleton Intricate network of protein filaments that extend throughout the cytoplasm  Microtubules 25nm  Microfilaments 7-9 nm  Intermediate filaments 10nm  Cytoseklaeton are proteins found throughout all the cells  These filments and tubules represent the components of the cytoskeleton  Functions often require energy – not always but sometimes Cytoskeleton Roles  Organelle/protein trafficking  Cilia/flagella  Mitosis/cytokensis  Muscle contraction  Cell aDhesion  Cell Migration  Extravasation o Moving from one part of the body to another part (like white blood cells/ immune system) Earlier we’ve talked about a lot of regulatory functions but haven’t talked about how these things are actually moving about Microtobubules and microfilaments are really big  Often span the length of the cell and provide tracks for things to move There are tracks inside the cell, but these tracks aren’t in a vacuum – you have microtubules coming uot from the centrosome, but these microtubules that aren’ in a vacuum so there are liquids in between Things actual When talking about cell surfaces – they are surrounded by tonnes of other molecules Microtubules : Structure  Polymer of Alpha and Beta Tubulin o A repeating structure of tubulin and is a polymer of the above (alpha and beta tubulin)  Alpha and beta area each individual monomers Section 1 – Cytoskeleton  Monomers 55kDa each  Alpha Beta dimer is basic subunit o Most simple subunit – this is what you generally find int eh cell, you never find alpha or beta alone  These alpha beta dimers can polymerize to form the protofilament o Putting Alpha betamolecules end to end you’ll eventually form the protofilament o Have polarity  Protofilament has polarity – this is to say that one end is different from the other o One end will be an apha and one end will be a beta  13 of these protofilaments come together to form the microtubule  Beucase the protofilametns have polarity the microtubules also have polarity o One side is also called the minus and one end is called the +  Under EM these mictorbules have striations, these striations are the protofilaments  Hollow tubes  25nm diameter, up to 100s of um long  Knowing the size of the dimer can tell you how things are moving along it Dimeric Tubulin Subunit  A bit more about the dimer  Dimer is VERY stable  Alpha binds permenatnly toGTP, Beta can hydrolyze GTP so can be bound to GTP or GDP - Beta dimer also binds to GTP but can hydrolyze it o When microtubules come apart they don’t break apart to the monomers but to the dimers because so stable  Tubulin is very abundant protein to isolate and characterize so we know a lot about it  As the polymer protofilament grows, Betas GTP is hydrolyzed  When growing a protofilament the Beta portion of the dimer hydrolyzes the GTP to GDP Arrangement of MT protofilaments  The protofilaments can arrange themselves in different ways  Singlets – Most common – 13 protofilaments forming a single tube of 25 nm diameter o There are exceptions though, 13 protoflimanets doesn’t always happen, can be different from it o Can be polymerized or depolymerized – can be grown or shrunk  Doublets and Triplits (Stable) can also be formed o Made of 13 protofilament tubule with one or two 10 protofilament tubes attached o THEY DO NOT POYMERIZE OR DEPOLYMIERZE since they are stable o Doublets found inCilia Flagella Section 1 – Cytoskeleton o Tirplits found in Basal bodies, cnetrioles o Looking at their organization theya re mad eup of different rings o A double t has a ring of 13 protofilanets (the A ring) and then another ring called the B ring that will have 10 protofilaments o A triplit is similar – An A ring of 13, a B ring of 10 and a C ring of 10  THE SINGLETS ARE MOST COMMON  Microtubles organize the interior of the cell  2 “types” – Cytoplasmic and axonemal (cilia etc) o If a microtubule is in the cytoplasm it is cytoplasmic tubule o There are mirotuble structures called axonemes, axonemal microtubules are found in cilia and flagelia  ONLY REFERS TO CILIA AND FLAGELA  Axonemal mictorbutles are NOT FOUND IN AXONS!  Microtulbes found in the axon are found found in the axon cytoplasm and thus are cytoplasmic microtubules Microtbules organization  Microtubles are often organized in certain ways and seem to be found coming from a center (origin point where most microtubules ar emminating from)  The Centrosome is the Major microtubules organizeing Center (MTOC) in animal cells  MTOC in animal cells is called the cnetrosomes  Cnetorsomes are not found in plants o Plants do have MTOC! But they are called something different  Centrosomes contain centrioles o In animal cells you have these MTOC called cnetrosomes, if you look at them at High maginification you can see entrioles made up of centrioles o They are small barrel strucutures that lie in 90 degrees to each other o AEch dot represents microtubules and together form the centrole o The whole thing is MTOC – The microtubules do not grow from the centrioles, they don’t touch the centrioles!  Instead the Microtubules are coming from the stuff around the centrioles, the pericentriolar matrix – pericentriolar just means “stuff that’s around the centriole”  Matrix around the protein that allows for the polymerization of the mictrotubules here  What the centrioles are doing here we don’t know  We kno stuff flike: o Gama tubulin, augmin complex etc. are proteins that ARE important that allow the microtubules to grow Centriole Details  Centrioles are made up of triplit microtubules that form a barrel shape Section 1 – Cytoskeleton  The centrioles lie 90 degrees to each other  There is a mother and a daughter centriole  THEY ARE NOT DIRECTLY RESPONIBLE FOR POLYMERIZATION OF NEW MICRTOBULES  IT’s the stuff that’s around it  The mother and daughter centriole are different from each other but we don’t talk about it  Centrioles aroe found INSIDE the centrosome Microtbule organization  The MTOC functions to nucleate the assembly of mictrotubules  Dynamic process  Nucleate – to act as a starting point  Nucleus in this context is something to start to grow on  The MTOC provide a nucleus from which the micortutbules can grow  The centrosome – there are others however  Spindle poles – The mitotic apparatus that is formed, the spindle poles in this are examples of MTOC  In Cilia and Flagela – The basal body is the MTOC for cilia and flagella  When looking at the MTOC, the – end of the mictorbuels is what is associated the the MTOC  The – end is ALWAYS in the MTOC!  The – end is INSIDe theMTOC – IT is the + end that is growing away from the MTOC (That’s why it is called the + end)  Nerve cells are a little bit different – nerve cells have axons, have the minus ends that are point towards the MTOC but the + ends are moving towards the Cell body  All of the microtubules are parallel and normal in the Axons  In dendrites you have some that are running rom – to positive and from positive to minus  So this is to say that – end is NOT ALWAYS in the MTOC  But in general the – end is ni the MTOC and it is the + end that grows away from the MTOC Gama Tubulin Nucleates Polymerization
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