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Cell Biology > The Cytoskeleton > Flashcards

The Cytoskeleton Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

What is the definition of the cytoskeleton?

A

“The cytoskeleton is a dynamic structure made up of proteins which can self assemble into long polymers of repeating subunits”

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2
Q

What cellular processes/features is the cytoskeleton critical for?

A
  • Cell Morphology
  • Cell Migration
  • Vesicle Transport
  • Cell Division
  • Cytokinesis
  • Chromosome separation
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3
Q

What three networks of filaments comprise the cytoskeleton?

A
  1. Microtubules
  2. Microfilaments
  3. Intermediate Filaments
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4
Q

Rank the cytoskeletal filaments in order of size (smallest to largest)

A
  1. Microfilaments - smallest diameter at 7-9nm
  2. Intermediate Filaments - 10nm diameter
  3. Microtubules - 25nm - largest diameter.
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5
Q

What are fibroblasts?

A

Fibroblasts are a type of cell found in connective tissues. They secrete collagen and other components of the extra cellular matrix.

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6
Q

Describe the structure of microfilaments.

A

G-actin monomers self-assemble into linear polymers of F-actin. F-actin comprises or ~14 repeating G-actin subunits and is in a tight helix. F-actin polymers form the microfilament structure.

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7
Q

What does the ‘F’ in F-actin mean?

A

Filamentous

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8
Q

What does the ‘G’ in G-actin mean?

A

Globular

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9
Q

“G-actin is ubiquitously expressed in eukaryotes and has a highly conserved identity (~90%) across species.” True or False?

A

True

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10
Q

G-actin exists in multiple isoforms with slight sequence variations allowing for differences in function and tissue specificity. Give 3 examples.

A

Alpha-G-actin is expressed in vertebrate contractile muscles.
Beta-G-actin and Gamma-G-actin are expressed in non-muscle cells.

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11
Q

Describe the structure of G-actin with reference to its lobes and domains and state the functional importance of these structures in terms of actin polymerisation.

A

G-actin is structured in 2 lobes, with a total of 4 domains, I-IV. A hinge between domains I and III allows the two lobes to move relative each other, forming a nucleotide binding cleft.

This nucleotide binding domain, NBD, permits ATP binding. When ATP binds, the NBD folds, stabilising it’s structure. This allows self-assembly of G-actin into F-actin.

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12
Q

G-actin is able to self-assemble to F-actin freely, without the use of ATP. True or False?

A

False. G-actin is an ATPase and requires binding of ATP to induce a stabilising conformational change initiating actin polymerisation.

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13
Q

An actin trimer forms, serving as the centre for actin assembly, to which continuous addition of monomers occurs. What is the name of the process forming an actin trimer?

A

Nucleation - three monomers of G-actin forming an actin trimer nucleus.

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14
Q

The process of actin polymerisation is dependant on the concentration of G-actin. True or false?

A

True. Actin polymerisation will only occur if [G-actin] is at its threshold.

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15
Q

What is the meaning of “critical concentration” in actin assembly?

A

The critical concentration is the threshold concentration of G-actin at which actin polymerisation begins. Below the threshold, no actin polymerisation occurs.

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16
Q

The rate of filament assembly increases exponentially with increasing free subunit concentration. True or false?

A

False. Filament assembly rate increases linearly with [G-actin]

17
Q

Actin polymers are polarised, with polymerisation occurring at different rates at either end.

Describe what is occurring at each end and explain the difference in rates.

A
  • Affinity of ATP-actin to associate with other ATP-actin monomers is greater than affinity for ADP-actin.
  • Higher rate of polymerisation of ATP actin at + end as associates more readily with neighbouring subunits.
  • Lower rate of polymerisation of ADP actin at - end as dissociates more readily from neighbouring subunits.
18
Q

What causes the difference in affinities in actin monomers at the + end versus actin monomers at the - end?

A

Actin at the + end is ATP-bound, which has a different confirmation from actin at the - end which is ADP-bound. The difference in conformation results in different binding affinities.

19
Q

What is treadmilling?

A

The phenomenon where free G-actin/ATP-actin is being polymerised at the + end at a rate that is equal to the dissociation of ADP-actin at the - end.
One dissociated, ADP is exchanged for ATP which can then depolymerise start the + end and the cycle continues.

There is no net gain or loss in the length of the filament but a force is generated allowing movement.

20
Q

G-actin is an ATPase with relatively high enzymatic activity. True or false?

A

False. It is a ATPase but has very low activity. The rate of ATP hydrolysis, and the release of the inorganic phosphate, is quite slow.

The longer it has been since a G-actin monomer was polymerised onto the filament, the more likely it is to have hydrolysed it’s ATP and have ADP bound

21
Q

What are the critical concentration values for actin polymerisation?

A

[G-actin] > 1.0uM
polymerisation occurs at both ends but faster at + than -

0.1uM < [G-actin] < 1.0uM polymerisation only occurs at + end.

[G-actin] < 0.1uM
polymerisation does not occur.

Filament extension actively reduces the concentration of free G-actin

22
Q

What is the leading edge?

A

Actin bundles form at the leading edge of the cell, and is in the direction the cell is moving in.

23
Q

Where is the tail in relation to the leading edge?

A

The tail is present on the opposite side of the leading edge.

24
Q

Stress fibres stretch from actin bundles at the leading edge to the tail. True or False?

A

True.

25
Q

What are filopodia?

A

Fine cytoplasmic extensions/ruffles formed at the leading edge that act as an antennae for the cell to probe the environment.

They are typical of slower moving cells.

26
Q

What are lamellipodia?

A

Broad membrane protrusions formed at the leading edge, typical of rapidly migrating cells.

27
Q

What are ruffles?

A

Highly dynamic assemblies of micro-filamentous structures that do not form tight adhesions with the substrate.

Ruffles include filopodia and lamellipodia.

28
Q

What is the substrate in terms of cell motion?

A

The surface/media that the cell is moving on/through.

29
Q

What is the role of focal adhesions in cell migration?

A

Focal adhesions provide a mechanical link between the intracellular actin filaments and the extracellular substrate.

Cell Biology (9 decks)

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