Lecture 1 - Cytoskeleton Flashcards

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1
Q

What is the cytoskeleton?

A

The cytoskeleton is a highly ordered, dynamic network of filaments that extend throughout the cell and collectively responsible for cell shape, motility (movement) of the cell as a whole and positioning of internal structures

The cytoskeleton is very dynamic in nature, with its filaments constantly forming and breaking down.

The cytoskeleton is a dynamic structure made up of proteins that self assemble into long polymers of repeating subunits.

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2
Q

What processes is the cytoskeleton involved in?

A
Cell morphology 
Cell migration
Vesicle transport
Cell division
cytokinesis 
chromosome separation
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3
Q

What are microfilaments?

A
  • Polymers of actin
  • carry out cellular movements– and also, play a critical role in muscle contraction and cytokinesis (cell division). Actin cytoskeleton plays important in final step of cytokinesis
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4
Q

What are microtubules?

A
  • Polymers of tubulin
  • act as a scaffold to determine cell shape, and provide a set of “tracks” for cell organelles and vesicles to move along. - also form spindle fibers for separating chromosomes during mitosis.
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5
Q

What are intermediate filaments?

A
  • Polymers of various intermediate filament proteins e.g. keratins
  • Intermediate filaments provide tensile strength to the cell.
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6
Q

How does eukaryotic cell movement take place?

A

Dynamic actin cytoskeleton (microfilaments) drives eukaryotic cell movement
Eukaryotic cell migration involves dramatic changes in cell shape, driven by the cytoskeleton - particularly the actin cytoskeleton.

As the cell moves the leading edge forms ruffles, in between processes called lamellipodia and filopodia .

Actin bundles extend from an actin arc into the leading edge while longer fibers (called stress fibers) extend from the arc in the opposite direction into the cell tail.

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7
Q

What are lamellipodia?

A

Broad membrane extension that moves forward. Typical of rapidly migrating cells

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8
Q

What are filopodia?

A

Finer cytoplasmic extensions, typical of slower moving cells

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9
Q

What are focal adhesions?

A

Structures that form mechanical links between intracellular actin filaments and the extracellular substrate

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10
Q

What are ruffles?

A

Assemblies that do not form tight adhesions with the substrate

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11
Q

What do microfilaments consist of?

A

globular G-actin monomers (single actin subunit) assembled into filamentous polymers (F-actin, actin filament consisting of multiple actin subunits).
F-actin adopts a tight helical structure with 14 actin subunits being required to make a complete turn of the helix.

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12
Q

Describe the structure of G actin

A

•globular protein, 2 lobes separated by a large cleft – each lobe consists of 2 domains (4 domains numbered on this slide as I – IV).
•hinge between domains I and III allows the two lobes to move relative to each other forming a nucleotide binding cleft
•nucleotide binding is required for G-actin stability (G-actin is unfolded in the absence of nucleotide)
•structure of ATP- and ADP- bound actin is identical everywhere EXCEPT in subdomain 2 (this is important for actin function – in the process of treadmilling).
•actin monomers have polarity – a plus and a minus end –also called the barbed and pointed end –this polarity is important for actin structure and function.
G-actin monomers have the ability to self assemble into F-actin. This starts with the formation of an actin trimer that serves as a nucleus for filament extension.

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13
Q

How do actin filaments assemble?

A

Nucleation – 3 monomers form a trimer
Elongation of filament by monomers adding at both ends (in a polarised manner)
Only occurs when the concentration of G-actin monomers is high enough – called the Critical concentration (Cc) for assembly (disassembly occurs if a filament is in a solution containing a concentration of G-actin below this Cc).

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14
Q

What is treadmilling?

A
  • Monomers add to (+) end, dissociate from (-) end at same rate
  • Filament length remains constant but moves across surface
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