The Cytoskeleton,cillia And Undulipodia

Question 1

What is the cytoskeleton

Answer 1

• Fibrousnetwork–to which organelles are tethered
• Providesstructure& organisation
• Cytosol~55%totalcell volume
• 20% of the cytosol is proteins

Question 2

Proteins in the cytoskeleton

Answer 2

~ microfilaments (actin)
~ myosin (types I & II)
~ microtubules (tubulin)
~ intermediate filaments (cytokeratins)

Question 3

Role of the cytoskeleton

Answer 3

Mechanical support to the cell-shape(continues to remain dynamic)
Cell motility relies on cytoskeletal structures

Question 4

Components of the cytoskeleton

Answer 4

Microfillaments

Interemediate

Microtubules

Question 5

Microfilaments

Answer 5

• Two strands of intertwined actin – 7nm in diameter
• Cell shaper – tension bearing load
• Muscle contraction
• Cytoplasmic streaming •
Cell motility

Question 6

Actin filaments

Answer 6

• G-actin is a 5 nm diameter globular protein that can polymerise to form F-actin which has a diameter of 7 nm
• F-actin has two intertwined polymer chains of G-actin that form a right-handed double helix with 13 actin monomers per turn
• F-actin microfilaments have +ve ends where polymerisation occurs and -ve ends where actin is lost
~ polymerisation requires hydrolysis of ATP
~ polymerisation it is controlled by capping proteins

Question 7

Interactions of the actin filaments

Answer 7

• Fungal cytochalsins bind to +ve ends of F-actin and inhibit
polymerisation leading to cell death
• Fungal Phalloidin (Amanita toxin), from death cap mushrooms binds and stabilises F-actin preventing microfilament disassembly leading to cell death
• Mini-myosin (type I) has a globular head with ATPase activity and aAmanita phalloides short tail which can bind to other proteins
• Mini-myosin (type I) can attach to organelles including endoplasmic reticulum, Golgi and vesicles and ‘walk’ along F-actin microfilaments carrying organelles
~vesicle movements in cells, in synapses and cytoplasmic steaming in plant cells

Question 8

Ameboid movement of actin and myosin

Answer 8

• Cell moves forward due to G-actin flowing into pseudopodia

• Actin interacts with mini-myosin causing contraction of the cell which pulls the cells trailing end

Question 9

Ameboid movement

Answer 9

Crawling like type of movement/cell migration

Question 10

Pseudopodia

Answer 10

Cytoplasmic rich projection used for motility

Question 11

Actin, myosin & and Cytoplasmic Streaming

Answer 11

• A layer of cytoplasm cycles around the plant cell, moving over a carpet of parallel actin filaments (F-actin).
• Mini myosin motor proteins attach to organelles in the cytosol driving the streaming via interactions with F-actin

Question 12

Actin in muscle filaments(contractions)

Answer 12

Search up

Question 13

Intermediate filaments

Answer 13

• Size~10nm
• Structural/mechanical strength of cells and tissues
• Cable-likestructure
Comprised of a variety of proteins
(no single polymers) Found only in vertebrates.

Question 14

How do intermediate filaments assemble

Answer 14

• Singlepolypeptidechains–wind to form tetramers
  Organisedinanti-parallelfashion Filamentiscomprisedofeight
  protofilaments
  IFs more stable – lack dynamic
  movement
  Phosphorylation–regulates function (nuclear envelope during mitosis) - Lamins

Question 15

Intracellu;ar organisation

Answer 15

• Complex network in the cytoplasm
• Extends from plasma membrane to the nucleus
• Keratin/vimentin anchor the nucleus within a cell
• Integrates all aspects of the cytoskeleton (actin/microtubules)

Question 16

Dmd

Answer 16

• Severe muscle weakness
• Loss of muscle mass (atrophy)
and function
• Disability
• Ca2+ mediated damage due to lack of dystrophin

Question 17

What is dystrophin

Answer 17

Acts as a stabiliser during muscle contraction to prevent damage

Question 18

Cell junctions

Answer 18

• Provide the link between neighbouring cells, tissues and organ systems
• Area of direct physical contact • Plasmodesmata in plant cells
• Connection of the cytosol from adjacent cells
• Unifies the plant as one living organism
• Transferofproteins/RNAandsolutes

Question 19

Cell junctions in animals

Answer 19

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Tight Junctions
plasma membranes are tightly bound by proteins that form a continuous seal around cells that is water tight
Desmosomes
act like protein rivets that anchor cells together
desmosomes link to sturdy cytokeratin* fibres that project into the cytoplasm
Gap junctions
provide cytoplasmic channels between cells allowing cell functions to be linked in synchrony such as heart muscle and smooth muscle contractions

Question 20

Extracellular matrix

Answer 20

• Glycoprotein rich network of molecules
• Collagen is the main ECM component in animal cells
• Fibrillar/non-fibrillaryformsof collagen
• Multiple types of collagen, however type I is most abundant in humans
• ECM provides a link between intra/extracellular environment

Question 21

Properties of the extra cellular matrix

Answer 21

> Regulation
• polarity
• cell division
• adhesion
• motility
Structural support
development

Question 22

Composition of the ECM

Answer 22

• Proteins
• Almost all of the proteins are glycoproteins
• A wide variety of collagens. Laminins. Abundant in the basal lamina of epithelia.
• Fibronectin: Binds cells to the ECM.
• Elastins: Provide flexibility to skin, arteries, and lungs. (These are not glycosylated.)
• Proteoglycans
• Proteoglycans are glycoproteins but consist of much more carbohydrate than protein
• The protein backbone of proteoglycans is synthesized, like other secreted proteins, in the endoplasmic reticulum.
• Several sugars are incorporated in proteoglycans. The most abundant one is N- acetylglucosamine (NAG)
• This glycosylation occurs in the Golgi apparatus.

Question 23

ECM and stem cells

Answer 23

• ECManchorsstemcellniches
• Anchoring important for mitotic spindle orientation
• Essential in stem cell self-renewal
• PropertiesoftheECM–stiffness, may play a role in cell-fate determination

Question 24

Mictrotubule structure

Answer 24

• Alphatubulin(purple)
• Betatubulin(blue)
• Thirteenpro-filaments, comprised of tubulin dimers, arrange to form cylindrical microtubule
• Dynamicprocessofgrowth & shrinkage
• Mediated by GTP hydrolysis

Question 25

Microtubule structure

Answer 25

• Microtubules can have three structures:
O singlets in cytoplasm & mitotic spindles
OO doublets in cilia & flagella
OOO triplets in centrioles & basal bodies

Question 26

Centrioles and the Centrosome

Answer 26

• In many cells, microtubules grow out from a centrosome near the nucleus
• The centrosome is a “microtubule- organizing center”
• In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring

Question 27

Role of mixctrotubules in cell cycle

Answer 27

• Mitosis–partitionofreplicated
chromosomes
• Involvestheassemblyand disassembly of a key microtubule structure – mitotic apparatus or mitotic spindle

Question 28

Centrioles

Answer 28

• Centrioles have 27 stable microtubules organised into 9 + 0 triplets surrounded by a protein matrix
• Centriole pairs are organising centres that form microtubule spindles during mitosis

Question 29

Microtubule Arrangement

Answer 29

Do it

Question 30

Mitotic Spindle

Answer 30

-an apparatus of microtubules that controls chromosome movement during mitosis
-• During prophase, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center
• The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them
• An aster (a radial array of short microtubules) extends from each centrosome
• The spindle includes the centrosomes, the spindle microtubules, and the asters

• During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes
• In metaphase, the chromosomes are all lined up at the metaphase plate, the midway point between the spindle’s two poles

• In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell
• The microtubules shorten by depolymerising at their kinetochore ends
• Microtubules are dismantled by the kinetochore to release tubulin subunits

Question 31

Microtubules – organelle movement

Answer 31

• Axonal transport of vesicles along microtubules in nerve axons involves kinesin and cytosolic dynein motor proteins:
➢ vesicle can be transported 250 – 400 mm/day
• Kinesin /dynein head proteins have ATPase activity and tails that bind to vesicles:
➢ kinesin mediates anterograde movement of vesicles towards synapses along singlet microtubules
➢ cytosolic dynein mediates retrograde movement to the cell body for recycling along singlet microtubules
• In synaptic endings motor proteins can transport neurotransmitter vesicles along actin microfilaments

Question 32

Kinesin

Answer 32

anterograde (forwards)
• Spindle length, movement during mitosis & depolymerisation

Question 33

Dynein

Answer 33

retrograde (backwards)
• Transport of cellular cargo
• Cytoplasmic dynein: organelle function & integration
• Axonemal dynein: cilia/flagella movement

Question 34

Cilia

Answer 34

-short hair like appendages extending from the surface of a living cell
-short
-rotational motion like a motor and are very fast moving
-found in eukaryotes

Question 35

Flagella

Answer 35

-long threadlike appendages on the surface of a living cell
-longer than cillia
-wave like motion
-found in eukaryotic and prokaryotic cells

Question 36

Similarities of flagella and cillia

Answer 36

• A core of microtubules sheathed by the plasma membrane
• A basal body that anchors the cilium or flagellum
• A motor protein called dynein, which drives the bending movements of a cilium or flagellum

Question 37

Dynein walking and its contribution to cilia and flagella walking

Answer 37

−Dynein arms alternately grab, move, and release the outer microtubules
• Protein cross-links limit sliding
• Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum
• Dynein arms can ‘walk’ along microtubules towards the basal body (-ve end) bending and rotating the cilia / flagella