cell bio exam 4

cytoskeleton functions

cell shape — especially in cells w/o cell wall

internal organization — organelle position, shape of nucleus

cell movement

3 types of filaments

microtubules, actin, intermediate filaments

assign actin vs microtubules to the following statements:

• moves chromosomes in mitosis

• phagocytosis

• microvilli (intestines)

• cleavage furrow (animal cytokinesis)

• flagella

• amoeboid movement (white blood cells, fibroblasts)

intermediate filaments

most diverse

only found in some animals

provide mechanical strength

intermediate filaments structure

bundles of fibers — like a rope

bend + stretch w/o breaking

very stable

types of int filament: keratins

skin, hair, nails, claws, scales, horns

found in epithelial cells

provide mechanical strength

many different types

mutation: rare genetic disease (epidermolysis bullosa simplex) disrupts keratin filament formation

types of int filaments: neurofilaments

strength and stability along axon

types of int filament: nuclear lamina

lines inside of nuclear envelope

lamin protein

provides mechanical stability and may be involved in chromosome positioning

lamin mutation — rare genetic disorder (type of progeria). signs of aging begin in childhood; mechanism unclear

microtubules image

intermediate filaments image

actin image

microtubule assembly and structure

made of repeating subunits of alpha and beta tubuliin proteins — always oriented in same direction and polar: (+) vs (-) end

repeating alpha beta subunits create protofilament

lateral association of 13 parallel protofilaments creates hollow tube -- can spontaneously assemble in vitro. filament has a consistent structure — always assembles this way

physical properties — compare/contrast microtubules and int fil

microtubules: small globular proteins. multiple contacts. very stiff

int filaments: elongated proteins. fewer contacts. very flexible

microtubule organization

cellular arrangement is controlled by an organizing center. centrosome — common in animal cells near nucleus

concentration of alpha and beta tubulin is too low in cells to spontaneously assemble. gamma tubulin nucleates microtubule assembly. filaments grow from gamma tubulin rings towards the (+) end

organization of microtubules can vary depending on species and cell type

microtubule dynamic instability

each filament can grow/shrink

microtubule growth

tubulin is a GTPase. free subunits are GTP bound.

GTP tubulin interactions are more stable. tubulin eventually hydrolyzes to GDP

rapid growth — GTP cap — stabilizes microtubule

shrinking microtubule

if end hydrolyzes to GDP, cap is lost

tubulin GDP interactions are less stable

rapid disassembly occurs

cap is dynamic — maintained by the rate of growth

microtubule dynamic instability is regulated by ___

other proteins

microtubule function

intracellular transport, organelle positioning, mitosis, flagella and cilia

spindle microtubules in mitosis

move chromosomes to metaphase plate

move chromosomes to opposite poles in anaphase

taxol (paclitaxel) is a chemotherapy drug. it stabilizes microtubules. its primary effect is thought to be during mitosis. why would stable microtubules prevent mitosis? how might taxol affect tumors vs how it might affect other body tissues?

**add more if you can

tumors are defined by uncontrolled mitosis; by preventing them from dividing/replicating/etc the cancer does not grow

intracellular transport

motor proteins move along microtubules and carry cargoes

kinesins move towards plus end (outward); dyneins move towards minus end (inward)

microtubule gliding assay

used to study motor proteins

which direction does kinesin go? where could a vesicle be going?

kinesin goes from (-) to (+)

a vesicle could go from the ER to the golgi, the golgi to PM, etc

microtubules also help position organelles

ER attaches to microtubules — tubes extend to periphery

• kinesin is the motor protein (remember — moves out to periphery)

golgi attaches to microtubules — keeps it “collapsed”/pulled in so it remains near nucleus

• dynein is the motor protein (remember — pulls inward)

flagella and cilia

same structure. 

lung cells, sperm cells, single-celled organisms like paramecium, etc

one long flagella vs many tiny cilia

same organization — arrangement of microtubules.

flagella and cilia movement

dynein movement produces bending. bending on alternate sides of flagella creates whip-like, back and forth motion

actin organization

typically most dense at cell cortex

creates cell shape

bundles — parallel fibers

webs — branching fibers

arrangements vary depending on cell and function

actin assembly/structure

made of repeating subunits of actin monomer. pointed in same direction = polarized

forms helix

actin physical properties

more flexible than microtubules (which are very stiff)

strength+flexibility incr with bundling and branching

actin filament assembly

actin monomer-ATP binds to + end

soon hydrolyzes to ADP

actin ADP dissociates from - end

regulation of actin network

regulated by other proteins

myosin

motor protein

muscle contraction

myofibrils fill most of cytoplasm of muscle cell

cells merge (syncytia) to create large continuous bundles

cell migration — actin function

cell migration = crawling along surface

continuous reshaping of the front and back of the cell

fish scale keratocyte is a model for actin based movement. fluid continuous movement

3 processes that req actin: protusion (cell extends @ front end), attachment to surface, contraction of rear end

migration — protusion (relevant structures)

lamellipodium — flat sheet

filopodium — strands sticking out from cell

lamellipodia

sheets. continuous polymerization at + end pushes plasma membrane

actin branching creates flat sheet — ARP complexes

suppose that actin molecules in a cultured cell have been labeled so that 1 in every 10k actin molecules has a fluorescent label

how would a fluorescent molecule appear @ leading edge? a dot or a line?

what would the fluorescent look like as the cell moves? would it move with the cell?

filopodia

bundles of actin — formins

thought to function in sensing, attachment and guidance during movement

migration — attachment

actin attaches to integrin proteins which span the plasma membrane and attach to smth extracellular

adhesion matters

why is this necessary for movement?

migration — contraction of rear end

actin forms bundles @ rear end

myosin motor proteins create contraction of cytoplasm

cell migration summary

actin networks assemble and disassemble continuously

structure and proteins involved depends on region of cell

rho family proteins regulation of actin dynamics

related GTPases are regulators of actin shapes

these proteins can be active in different parts of the cell to create directional movement

these are effectors of various signaling cascades

many of these GTPases are upregulated in cancer. how might this contribute to cancer progression?

the only way to get new cells is by ___

division of existing cells

functions of cell division

development, reproduction, wound healing, continual cell division of skin, etc

phases of euk cell cycle — control system

processes happen in the correct order

each phase is finished before the next begins

diagram cell division vs cell growth vs cell growth+divison

how long is the cell cycle? what proportion of time will a cell spend in each phase?

during early development?

a skin cell?

cell cycle control — asks 2 questions

how do cells control coordinated progression thru cell cycle

how do cells ensure each daughter cell gets a complete genome + necessary cytoplasm

cell cycle checkpoints

cells pause until ready for next phase

how is cell cycle control coordinated?

early studies used frog eggs — easy to see/manipulate

suggests a maturation-promoting factor (cyclin): can trigger mitosis, activity is cyclical

cyclins

proteins that increase and decrease throughout cell cycle

bind and activate cyclin-dependent kinases (Cdk)

Cdk regulate cell cycle activites. phosphorylate effector proteins, levels stay constant, only active when bound to cyclin partner

Cdk inactive vs active

inactive: not bound to cyclin. always present

active: bound to cyclin. adds phosphates to target protein → activate cell cycle functions

cyclin-cdk complexes

specific pairs function in phases of cell cycle

G1-cdk complex: early G1

G1/S-cdk complex: late G1

S-cdk complex: DNA synthesis

M-cdk complex: mitosis

cyclin vs cdk: what cell processes might affect cyclin levels vs cdks?

cyclin level fluctuates (goes up during transcription/translation, then down during degradation) while cdk is always around

skipped these slides in class i think but i wasnt paying that much attention so maybe

cell cycle control review

where does cell cycle begin + why does it matter?

G1 — a reset of the control system

reset necessary bc during mitosis: cyclins are degraded, synthesis of new cyclins is blocked, cdk inhibitors are active

in mammalian cells, arrest is the default

mammalian cells require mitogens (growth factors) to progress thru G1

mitogen signaling activates synthesis of G1 cyclins (RTK - MAP kinase cascade)

G1-cdk: activates sequential production of cyclins

sequential cdk activity leads to DNA replication