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Cell Biology.docx

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Michael Jonz

Cell Biology 07/09/2012 7:41:00 AM Cell Biology -3 Types of filaments (AFs or microfilaments) Found in the periphery, occupy the cortex of the cell and provide structure to that cell. Ex. RBC - larger than some capillaries but can squeeze into them -Microtubules (MTs) Important for transportation, run the entire length of a nerve terminal to the nerve cell body carrying nutrients -Intermediate filaments (Ifs) -Read page 968 Filament Construction: -Smallest subunit of an actin filament is a single actin protein -Composed of small subunits -Subunits that help the cortex to be not static -Actin subunits can form Actin filaments -Soluble subunit: Smallest element of these structures that will not break down or degrade -Filaments break into subunits, contribute to the pool of subunits, and then build on the other side of the cell, a common pool of subunits is always maintained -filaments are constructed form protofilaments Ex. Microtubules -13 protofilaments form a mature microtubule, formed into sheets to increase their stability -(thermally stable) -Non-covalent bonds found between subunits of microtubules, interactions are lengthwise and adjacent, adds more stability -Less stability on the end, not as many bonds as the subunits in the middle, because of this, addition and removal of subunits can occur Nucleation: -Formation of a seed A molecule that is going to start the process A collection of a mass of proteins -Increase the salinity of the solution you can increase nucleation -Y axis has the percent of actin subunits in filaments -X axis is the time after addition of salt -Goes from a finite common pool to a signal received to produce actin filament -Lag phase: takes a little longer than other phase, takes time to nucleate to form the oligomer -Elongation: occurs faster -Steady state equilibrium: Treadmilling dynamic with actin filaments with subunits coming on and off at the ends Tubulin: -Soluble subunit that forms microtubules -Two proteins, beta and alpha tubulin together is a subunit called a heterodimer -Two GTP are there which will bind to any tubulin subunit -Hydrolysis will only occur from the PLUS end, where most polymerization takes place -GTP will become a part of the structure of the protein, two of the binds with any subunit Actin: -Theres a plus end and a minus end -Instead of GTP this utilizes ATP -ATP binding site is in the middleish and exposed, hydrolyzes to ADP, only source of extracting energy -Twisted formation Treadmilling and Dynamic Instability: -Both can happen to both types of protein filaments -Ways in which both shrinkage and growth can be maintained -Actin and Tubulin are protein enzymes that catalyze hydrolysis of nucleotides -ATP/GTP caps: Build-up of ATP or GTP -Loss of phosphate: Reduced to GDP or ADP, cap is lost, hydrolysis has lagged behind -Hydrolysis always lags behind the addition of subunits, concentration of actin subunits decrease, and the process becomes slower, and hydrolysis overcomes the building, and you lose the ATP caps -Critical concentration: When subunits added equals the subunits lost, after common pool is reduced it happens when addition slows down and equals the rate of hydrolysis Pg. 978-979 Treadmilling: -Usually related to actin filaments -The length of the filament is not changing, but appears to be moving -When filaments are seeds in a concentrated actin subunit containing cytosol, it builds faster than the rate of hydrolysis. -As this increases, the concentration in the common pool goes down, BECOMES MORE LIMITING -Minus end and plus end eventually equal, when were losing mostly in the minus end and gaining mostly from the plus end -Can add to the minus end but its VERY UNLIEKLY and not thermally stable Treadmilling: Microtubules have the same thing -As subunit concentration goes up and as that happens elongation happens as long as the linear relation of adding subunits is higher than the minus end and its difficult to add more filaments (less dynamic end) -Critical concentration: Concentration of subunits when they dont change their net length, treadmilling range -Dynamic Instability MTs -Related to the building of mitotic filaments -Minus end is locked in with another structure with the addition and subtraction of subunits on the plus end -If concentration of tubulin is within critical values, we will have stability, if above we have the build-up of a GTP cap -As the concentration decreases, hydrolysis catches up, GTP -> GDP, lose the GTP cap and its called catastrophe, huge concentration of tubulin subunits back in the pool, then goes into rescue, which goes into catastrophe and its a continuation -The concentration of the sub-unit can affect the rate of the growth, higher concentrations make a sharper growth rate of the plus end of a microtubule -Pointy shape happens with higher concentrations Dynamic instability: Microtubules are growing and extending on the plus end, the minus end is attached to something like the MTOC, some microtubules suddenly shrink What is the purpose? -Treadmilling there is use of ATP -Dynamic Instability has GTP -Both systems can provide special and temporal flexibility with a high turnover: Increases turnover, constant addition and loss of subunits, if the filaments were static, it would be difficult to modify the elements of the cytoskeleton. The constant turn over helps them maintain a flexibility. -Microtubules send out their plus ends to find attachment sites, there are two centrosomes, send each of their plus ends of their microtubules to find each other Intermediate Filaments:
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