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biol2200 end of sem > Module 3 > Flashcards

Module 3 Flashcards

1
Q

Cytoskeleton

A
  1. provide cells with structure (so they keep their shape, internal organisation and polarity)
  2. helps cells move and change shape
  3. needs to “strike a balance” between the two above
  4. needs to be dynamic
  5. provides ‘tracks’ for motor proteins to transport stuff from one part of the cell to another
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2
Q

Main components of cytoskeleton

A

microfilaments
microtubules
intermediate filaments

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3
Q

Microfilaments

A
  • Polymers of actin
  • Extend throughout the cytoplasm
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4
Q

Microtubules

A
  • Polymers of tubulin
  • Radiate out from the centrosome
    towards the cell periphery
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5
Q

Intermediate filaments

A
  • Made from a myriad of units
    depending on the cell type
  • Extend through the cytoplasm
    or nucleus (depending on the
    type)
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6
Q

Examples of actin structures

A

Microvilli
Cell cortex
Contractile ring
stress fibres

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7
Q

Stiffness of cytoskeleton components

A

highest to lowest:
- microtubules
- intermediate filaments
- microfilaments (actin)

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8
Q

Actin polymerisation: the steps

A

Nucleation
- g-actin
- nucleus

Elongation
- f-actin

Steady State
- - and + end

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9
Q

Rate-limiting step in actin polymerisation

A

nucleation

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10
Q

Formins

A

Regulation of actin filament assembly

assemble unbranched filaments

  • 1 Formin dimer binds to two G-actin subunits (early stage of nucleation)
  • By rocking back and forth, additional subunits are added
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11
Q

Arp2/3 complex

A

nucleate branched filaments

  • 1 Arp2/3 complex binds to the
    side of actin filaments (F actin)
    and generates a branch
  • Arp2/3 needs an additional
    protein: WASp (verprolin in
    yeast) - results in nucleation of
    actin filaments.
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12
Q

Actin at steady state

A

Treadmilling

  • Actin is constantly polymerising
    and depolymerising within the
    cell.
  • ATP-G actin subunits are always
    added to the + end. After
    binding ATP is hydrolysed to ADP
    and subunits are lost at the -
    end of the growing filaments -
    treadmilling
  • Addition at the + end is faster
    than the – end, resulting in
    treadmilling
  • relative rate a + end is much greater than at neg end.
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13
Q

Profilin

A

controls treadmilling

  • Catalyses the exchange of ADP/ATP i.e. promotes the formation of ATP-G-Actin, providing a greater supply for binding to (+) end
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14
Q

Cofilin

A

controls treadmilling

  • Binds along the filament, destabilises ADP-actin in filaments, enhancing disassembly at (-) end
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15
Q

Thymosin B4

A

controls treadmilling

  • Sequesters away ATP-G-actin. Acts as a buffer for supply of ATP-G-Actin to (+) end
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16
Q

Actin microfilament motor proteins

A

Myosins

17
Q

Myosins

A

Myosin I found in the cell periphery

Myosin II, found in muscle and
non-muscle cells - required for
cytokinesis and focal adhesion
(in non-muscle cells)

Myosin V, required for organelle transport (e.g. melanosomes are transported from melanocytes to
neighbouring keratinocytes.)

18
Q

Myosin II motor protein (power stroke)

A

myosin head contains motor. binds ATP

  1. myosin head binds ATP, releases actin
  2. head hydrolyses ATP to ADP +P, rotates to ‘cocked’ state
  3. cocked state binds actin
  4. P is released, myosin head moves along filament: the power stroke
  5. head remains bound to actin while ADP is present. when ADP is exchanged for ATP, myosin head is released and cycle starts again.
19
Q

The sarcomere

A
  • The sarcomere is a unitary, contractile unit
  • Each sarcomere contains two types of filaments: thick filaments of Myosin II and thin filaments of actin
  • Z Disc – proteins that anchor thin filaments
  • I band – thin filaments only
  • A band – overlapping thin and thick filaments
  • H zone – thick filaments only
20
Q

The sliding filament model

A
  1. The decrease in sarcomere length is due to decreases in the width of the I band and H zone, with no change in the width of the A band.
  2. The thick and thin filaments slide along one another, increasing their overlap and so pulling the Z discs
    closer together.
  3. Release of Ca2+ from the Sarcoplasmic reticulum* triggers contraction
  4. Reuptake of Ca2+ into Sarcoplasmic reticulum relaxes muscle
21
Q

Microfilament motors: Regulation by calcium

A

Tropomyosin (TM) and troponin (TN) are accessory proteins bound to actin thin filaments.

In the absence of calcium, TM and
TN molecules block the interaction
of myosin with F actin

In the presence of calcium, TN
induces TM to move to a new site,
exposing the myosin-binding sites
on actin

22
Q

Actin filaments vs microtubules

A

Similarities
* Both actin filaments and microtubules are polymers of a single type of monomer
* Both have a distinct (+) and (-) end
* Both are continuously assembled and disassembled (polymerisation and depolymerisation)
* Both are involved in maintaining cell shape and in cell movements

Differences
* Microtubules are assembled at a specific site in the cell
* Microtubules are larger in diameter than actin microfilaments
* Microtubules are more rigid than actin microfilaments
* Microtubules can grow very long to 100s of µm or even mm (nerve cell axons)

23
Q

`Microtubule assembly

A
  • Assembly of the α/β-tubulin dimer happens at the (+) ends of microtubules.
  • GTP/GDP regulates assembly and disassembly
  • Both α- and β-tubulin dimers can bind GTP but
    hydrolysis only happens in the β-tubulin
  • GTP-to-GDP hydrolysis only happens once the
    tubulin is bound to the microtubule
  • The ring of γ -tubulin at the PCM stabilises the (-) end and prevents disassembly
  • GTP-bound β-tubulins promote assembly
  • GDP-bound β-tubulins promote disassembly
  • The GTP cycle is essential for the dynamic instability of the microtubule (switching between catastrophe aka rapid shrinkage and rescue aka growth)
24
Q

centrosome

A

a complex structure - generally lies next to the nucleus
- consists of two centrioles surrounded by pericentriolar material - PCM
- centrioles are always found in pairs and at right angles to each other

25
Q

microtubule dynamics

A
  • accumulation of GT bound primers called GTP cap
  • the GTP cap stabilises the growing microtubule, preventing disassembly starts to occur

cargo vesicles can be:
- transported towards + end or cell periphery
- transported from periphery to cell body toward - end

26
Q

microtubule motors

A

Kinesin = for anterograde transport (transported towards + end or cell periphery)

PLUS end directed motors that move away from MTOC
- movement regulated by ATP hydrolysis
- movement highly processive, molecular choreography

Dynein = for retrograde transport (transport towards the - end or cell centre)

MINUS-end directed motors that move from the + end towards MTOC
- movement regulated by ATP hydrolysis

27
Q

microtubules in mitosis

A
  • rearrange to from bipolar spindle
    -chromosomes line along midpoint, then pulled apart - achieved by combination of sliding and treadmilling

Astral microtubules
- extend from spindle poles to cell cortex. they orient spindles along axis of cell division

Kinetochore microtubules
- link spindle poles to kinetochore, they transport the newly separated chromosomes to their respective poles

Polar microtubules
- extend from each spindle pole towards opposite pole. antiparallel - critical

28
Q

Microtubules as a cancer treatment

A

vinblastin
- prevents assembly of microtubules

paclitaxol
- prevents disassembly of microtubules

29
Q

Intermediate filament structures

A
  • assembly does not require ATP or GDP hydrolysis
  • disassembly is controlled by phosphorylation of specific residues
  • IFs are polymers of rod-shaped proteins
  • IFs don’t have polarities
  • subunits have common structure: central, helical rod domain flanked by non-helical head an tail domains
30
Q

Keratins

A

Type 1 = acidic, tend to be smaller
Type 2 = basic (or neutral), tend to be longer than type 1

31
Q

nuclear lamina

A

Lains form as a network of filaments that stabilise the nuclear envelope

32
Q

Heptad repeat

A

structural motif that consists of a repeating pattern of seven amino acids

hydrophobic amino acid is repeated every seven (heptad) residues

33
Q

Mutations in IFs are associated with various diseases

A
  • Emery-Dreifuss Muscular Dystrophy - the first laminopathy described
  • Mutations in the gene that encode Lamina A or C protein (types of lamins)
  • Onset at approximately 10 years of age (ranges from infancy to teenage year)
  • Symptoms include joint deformities, muscle weakness, in some cases cardiopathy

biol2200 end of sem (5 decks)

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