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ANAT 262 Lecture Notes - Spindle Apparatus, Intermediate Filament, Ultimate Tensile Strength

Anatomy & Cell Biology
Course Code
ANAT 262
John Presley

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Cytoskeleton is made of three components:
1) microtubules
2) actin
3) intermediate filaments
Main functions of the cytoskeleton are:
1) structure and support
-determination of cell shape
-establishing intracellular contacts
-actin contraction allows the cell to move away from a repulsive cue
2) intracellular transport (highway to move vesicles)
-intracellular transport nerve cell sends out growth cone to sense the environ-
3) contraction and motility (ex: muscle cells)
-cell division
-centrioles organize microtubules (negative at centrioles and positive end out)
4) spatial organization
-signal transduction
-tensile strength - cell is squishy, it has structure and resistance (tensile strength)
The term cytoskeleton may in one sense have an unfortunate connotation since it im-
plies that the various structural elements making up the cytoskeleton form a static struc-
tural framework similar to the bone skeleton that frames the body
-well this only partly is true because certain types of cytoskeleton elements are more or
less permanent features of the cell
-examples of such cytoskeleton structures include the actin and myosin filament bun-
dles in muscle cells and the microtubules arrays in cilia and flagella
Other cytoskeleton structures are very dynamic, continuously assembling and disas-
sembling as part of their functional cycle for use in various cellular processes
-perhaps the most dramatic example of such a dynamic process is the complete re-
modeling of the microtubule array of a cell during mitosis - when it changes from a net-
work radiating throughout the cell to a compact, bipolar, mitotic spindle
-at interphase, the cell needs bipolar mitotic spindle in order to divide

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F-actin: 8 nm in diameter and is highly concentrated at the cell cortex and is associated
with the plasma membrane
-looks like 2 chains twisted together, but this is just the way actin polymerizes
Microtubules: 25 nm in diameter and is a hollow tube made of tubulin
-there are 13 laterally associated protofilaments each being a linear polymer
-one end of the microtubule is attached to the centrosome
Intermediate filament: 10 nm in diameter, a complex assembly of tetramers made from
-various types of IF subunits - over 50 cytoplasmic IFs and nuclear lamins
-ex: desmosomes in epithelial cells
The basic structural elements of the cytoskeleton do not act alone
-there is a multitude of cytoskeleton associated proteins that anchor, cross-link and oth-
erwise regulate these major elements
-the ability of actin, tubulin, and IF proteins to form polymers, to depolarize, to interact,
elongate and contract in conjugation with subtle cell movements and shape changes
gives the cytoplasm its unique properties of life
The organized arrays of fibrous elements interact to form a highly integrated structural
network called the cytoskeleton
-this cytoskeleton gives the cytoplasm its unique properties
Actin and microtubules are found in all eukaryotic cells
-tubulin is generally present in smaller amounts than actin, except in neurons where it is
very abundant
-prokaryotes have analogous proteins (FtsZ protein is bacteria tubulin and MreB protein
is the bacteria’s F-actin)
-cytoplasmic intermediate filaments are only in multi-cellular organisms
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