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cellbio exam 2
part 2
42
Biology
Undergraduate 2
11/28/2012

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Term
three major roles of actin
Definition

- cytokinesis

- cell motility

- cell adhesion

Term
where are actin genes expressed in mammals?
Definition
4 in muscle cells; 2 in other cells
Term
how does the actin of primitive eukaryotes differ from that of complex modern eukaryotes?
Definition
it's almost entirely identical - HIGHLY CONSERVED
Term
G-actin vs. F-actin
Definition
G-actin: globular actin - two fibers wrapped around each other in a double helix. These TRIMERIZE and then interact head-to-tail with other G-actins to form F-actin, which has POLARITY because all the monomers are oriented head-to-tail in the same direction. We call one the PLUS or BARBED end and the other the MINUS or POINTED end. Additional G-actins add more easily to the plus end.
Term
structural polarity of actin
Definition
refers to the fact that one end is structurally distinct from the other - plus and minus/barbed and pointy ends
Term
actin nucleation
Definition
G-actin forms into trimers and adds onto the end of F-actin, growing the polymer
Term
stages of actin polymerization in vitro
Definition

1. lag phase - G-actin aggregates into trimers

 

2. elongation phase - rapid growth on both ends

 

3. steady state - filament retains the same net length because the rate of gain and loss stays equal

 

 

Term
critical concentration (Cc) of actin monomers
Definition
concentration of actin monomers when the filament is in STEADY STATE
Term
what happens to monomeric G-actin once it bonds to the F-actin filament?
Definition
the ATP bound to it becomes hydrolyzed to ADP, which gives it a lower affinity for other subunits. This makes it more likely to dissociate at the pointed end - the critical concentration for the pointed end is higher than that of the barbed end.
Term
in conditons of Ccminus > [G-actin] > Ccplus, what is taking place?
Definition
TREADMILLING - the plus end is polymerizing and the minus end is DEpolymerizing
Term
cytochalasians and phalloiden
Definition

cytochalasians: bind to the barbed end of an actin filament and prevent elongation

 

phalloiden: bind to the pointed end of an actin filament and prevent dissociation; these can be labelled with colored dye to help visualize actin filaments

Term
an actin-binding protein that slowed depolymerization would most likely work by....
Definition
preventing the ATP from being hydrolyzed to ADP because ADP dissociates more easily
Term
Formins vs. Arp2/3
Definition

both direct the growth of actin filaments by controlling where nucleation happens. 

 

Formins: make unbranched filaments

 

Arp2/3: make BRANCHED filaments

Term
orginization of actin filaments
Definition

1. actin bundles - filaments are cross-linked into closely packed parallel bunches.

 

2. actin networks - cross-linked into networks with gel-like behavior

Term
cell cortex
Definition

actin filaments in a mesh lining the inner membrane of the cell

- also associated proteins!

- determines cell shape

- helps with cell movement

Term
how do cells stick to things?
Definition

- transmembrane INTEGRIN proteins bind to whatever the cell needs to bind to - these places are called FOCAL ADHESIONS

- these are also attachment sites for actin bundles called STRESS FIBERS, which are cross-linked with alpha actinin.

Term
adherens junctions
Definition

cell-cell contact (in epithelial cells all touch all and form adhesion belt)

 

- transmembrane proteins called CADHERINS bond to the other cell on the outside and CATENINS on the inside, which in turn bind to actin filaments

Term
microvilli
Definition
fingerlike extensions on cells involved in absorption; made of actin filaments
Term
myosin
Definition

cross-linking ATP-powered motor protein that has two regions; the GLOBAL HEAD region interacts with actin fibers.

 

- these actin muscle fibers are attached on the barbed end to Z-DISCS; myosin uses ATP hydrolysis to contract and pull the Z-discs toward each other.

 

- they let go to relax the muscle.

 

SWINGING CROSS-BRIDGE MODEL

Term
skeletal muscle contraction
Definition

normally, the troponin complex binds to actin filaments; this keeps myosin from binding.

 

however, when nerve impulses release Ca2+ from the sacroplasmic reticulum, it shifts the complex to reveal the myosin-binding sites, and contraction can proceed.

Term
contrast intermediate filaments with actin
Definition

- ends are not structurally polar

- they stay the same length (are very stable) - good at providing structure

- composed of many different filamentous proteins (actin is only composed of actin dimers-->trimers); in general, its compositional proteins have long a-helical regions that twist together and then join with other dimers, etc. 

- two of these dimers wind to make a tetramer; eight of these wind to make a filament

Term
adherens junctions vs. desmosomes
Definition

adherens junctions: joins ACTIN bundles to more actin bundles in the adjacent cell, joining them

 

desmosomes: same thing, but with KERATIN (intermediate filaments) joined to protein plaques on the outside of the cell

Term
hemidesmosomes
Definition
linking epithelial cells to the basal laminal
Term
microtubules
Definition

- rigid hollow rods; DYNAMIC (assembly/dissembly)

- made of atubulin and btubulin HETERODIMERS

- atubulin binds ATP always; btubulin can HYDROLIZE it to ADP

- 13 protofilaments (head-to-tail heterodimers) combine to make one microtubule; therefore it is POLARIZED - btubulin is the plus unit and atubulin is the minus subunit

 

Term
what is the key difference between microfilaments (actin)  polymerization/depolymerization and that of microtubules (tubulin)?
Definition
microtubule assembly is TEMPERATURE DEPENDANT 
Term
dynamic instability 
Definition
at [dimer] close to Cc, microtubules will grow and shrink independant of each other, depending on whether GTP is hydrolized faster than dimers are added
Term
how could microtubule inhibition be used to fight cancer?
Definition
microtubule growth is involved in cell splitting; stopping growth will stop rapid cell division
Term
centrioles
Definition
contained in the centrosomes; cylinder of NINE triplet microtubules
Term
centrioles are necessary for ________ and not found in ___________
Definition

necessary for:

1. flagella

2. cillia 

3. basal bodies

 

NOT found in:

1. most meiotic cells

2. plant cells

3. many unicellular eukaryotes

Term
mitotic spindles
Definition
microtubules that grow out of the centrosomes (minus end anchored to them) during mitosis
Term
COLCEMID is used to dissemble mitotic spindle microtubles. When the drug is removed, what could you observe?
Definition
the spindles will regrow - their minus ends are anchored in the centrosomes. 
Term
MAPs
Definition

microtubule-associated proteins

 

- can promote dissociation or growth

- some attach to the plus end and direct the growth

Term
kynesins and dyneins
Definition

motor proteins that carry things along the microtubules; kynesin goes toward the plus end, dynein toward the minus end.

 

- both have polar head domains that grab on to the microtubles

- their tail domains grab organelles, etc. and carry them where they have to be

Term
structure of cilia, flagella, and kinetochore
Definition

cilia and flagella: one dimer of tubulin in the middle with nine around it (9+2)

 

kinetochore: the same, but with TRIPLETS instead of doublets

 

in cilia and flagella: 

- nexin links the dimers to each other

- two arms of dynein go from the center dimer to each atubulin

 

 

Term
descripe microtubular structural progression in mitosis
Definition

1. microtubule array dissembles; free dimers are made into the MITOTIC SPINDLE, an array of microtubules that pushes the centrosomes apart, stretches the cell, and grabs the chromosomes.

Term
four kinds of mitotic microtubules
Definition

1. kinetochore: attach to the chromosomes via the centromeres

 

2. chromosomal: attach to the chromosomes via the arms (chromoskin)

 

3. polar: push on each other in the middle

 

4. astral: expand out freely from the centrosomes - these have EXPOSED PLUS ENDS

Term
microtubule activity in anaphaseA vs. anaphaseB
Definition

anaphase A: kinetochore microtubules dissemble on their plus end, drawing the chromosomes toward the centrosomes; also, motor proteins carry the chromosomes closer (minus-end directed)

 

anaphase B: polar microtubules are seperated by plus-end directed kinesins (sliding past each other)

Term
how are spindle molecules pulled apart during anaphase B?
Definition

1. they slide past each other, and kinesins move toward the plus ends, pushing them apart

 

2. the astral microtubules have dyneins moving from plus to minus, anchored to the cell cortex, and depolymerizing the plus end at the same time, dragging the polar microtubules back towards their centrosomes

Term
describe microtubule growth and motor protein activity during prophase
Definition

- polar microtubules grow, and align with minus-directed dyneins

 

- then are pushed apart with plus-directed kinesins

 

- astral microtubules are pulled towards the cell walls by dynein minus-directed microtubules

Term
kinetochore tubulin is tagged with fluorescent dye and a region between the chromosome and the centrosome is bleached. As anaphase progresses, the region between the bleached zone and the chromosome shortens. what does this tell us?
Definition
there's dissembly at the plus end
Term
tight junctions
Definition

cells are held tightly together with no gaps in between

 

- composed mainly of CLAUDINS and OCCLUDINS (transmembrane proteins)

Term
anchoring junctions
Definition

- desmosomes: intermediate filaments; cell to cell

- hemidesmosomes: intermediate filaments; cell to underlying tissue

 

- adherens junctions: actin filaments (cadherin proteins)

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