BIOL 2020 > Lecture 14 > Flashcards

Lecture 14 Flashcards

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1
Q

Actin subunits assemble to from what

A

Actin subunits assemble head-to-tail to form a tight, right-handed helix, forming a structure about 8 nm wide called filamentous or F-actin

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2
Q

Describe the composition of a Myosin II molecule.

A

It is composed of two heavy chains (which form the coiled-coil tail and globular heads) and four light chains (attached to the neck/hinge region).

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3
Q

Dimerization occurs when what

A

the two α helices of the heavy chains wrap around each other to form a coiledcoil.

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4
Q

What are the approximate dimensions of a Myosin II molecule?

A

The total length is about 150 nm. The tail is 2 nm wide, and the coiled-coil region is roughly 100 nm long (excluding the head/neck).

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5
Q

Each myosin head does what

A

binds and hydrolyzes ATP, using the energy of ATP hydrolysis to walk toward the plus end of an actin filament

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6
Q

In skeletal muscle, these tail–tail interactions form what

A

large, bipolar thick filaments that have several hundred myosin heads, oriented in opposite directions at the two ends

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7
Q

What is a muscle fiber (skeletal muscle cells)?

A

It is a large, multinucleated cell formed by the fusion of many muscle cell precursors.

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8
Q

What are Myofibrils?

A

Cylindrical structures (1-2 µm diameter) inside the muscle fiber that extend the entire length of the cell. They consist of a long chain of repeated contractile units called sarcomeres.

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9
Q

What is the functional unit of contraction in a muscle cell?

A

The Sarcomere

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10
Q

Each sarcomere is formed from what

A

a miniature, precisely ordered array of parallel and partly overlapping thin and thick filaments

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11
Q

The sliding filament theory

A

a muscle fiber contracts when myosin filaments pull actin filaments closer together and thus shorten sarcomeres within a fiber

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12
Q

Actin filaments slide on what

A

myosin filaments

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13
Q

Define the “A-Band” (Dark Band)

A

The region containing the full length of the thick (myosin) filaments. It appears dark in microscopy.

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14
Q

Define the “I-Band” (Light Band)

A

The region containing only thin (actin) filaments (no overlap with myosin). It appears light in microscopy.

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15
Q

Sarcomere shortening is caused by

A

the myosin filaments sliding past the actin thin filaments, with no change in the length of either type of filament

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16
Q

how are myosin and actin filaments packed together

A

almost crystalline regularity.

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17
Q

Define the “M-line”.

A

The center of the sarcomere where myosin (thick) filaments are anchored

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18
Q

Define the “Z-disc”.

A

The boundary at each end of a sarcomere where actin (thin) filaments are anchored by their plus ends.

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19
Q

According to the Sliding Filament Theory, which bands change length during contraction

A

I-Band: Shortens (or disappears). A-Band: Remains the same length. (The sarcomere as a whole shortens).

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20
Q

The thin filaments are composed of what

A

actin and associated proteins,

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21
Q

What is the function of CapZ in the sarcomere

A

It caps the plus ends of actin filaments, anchoring them into the Z-disc.

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22
Q

What is the function of Titin?

A

acts as a molecular spring, with a long series of immunoglobulin-like domains that can unfold as stress is applied to the protein. Titin keeps the thick filaments poised in the middle of the sarcomere

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23
Q

What is the function of Tropomodulin?

A

It caps the minus ends of the actin filaments (preventing depolymerization).

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24
Q

What is the role of Alpha-Actinin in the Z-disc?

A

It acts as a bundling protein that holds actin filaments together in a regularly spaced bundle at the Z-disc.

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25
What is the function of Nebulin?
maintains the length and the stability of the thin filament and consists almost entirely of a repeating 35-amino-acid actin-binding motif..
26
A Sudden Rise in Cytosolic Ca2+ Concentration Initiates what
Muscle Contraction
27
The signal from the nerve triggers what
an action potential in the muscle cell plasma membrane, and this electrical excitation spreads swiftly into a series of transverse tubules
28
What are T-tubules (Transverse tubules)?
Invaginations of the plasma membrane that extend inward around each myofibril, allowing electrical signals (action potentials) to penetrate deep into the muscle cell.
29
What is the Sarcoplasmic Reticulum (SR)?
A specialized ER that surrounds myofibrils and stores high concentrations of Calcium (Ca^{2+})
30
Describe the sequence of events from nerve signal to Calcium release
Action potential arrives at the neuromuscular junction. Acetylcholine (ACh) release opens Na+ channels, triggering an action potential in the muscle membrane (sarcolemma). Action potential travels down T-tubules. Activation of voltage-gated channels in T-tubules opens $Ca^{2+}$ release channels in the adjacent SR membrane. $Ca^{2+}$ floods the cytosol. 20202020
31
What is the Troponin complex
A complex of three polypeptides (Troponin T, I, and C) associated with tropomyosin.
32
What is Tropomyosin and what does it do in a resting muscle?
It is an elongated protein that binds along the groove of the actin helix. In a resting state, it blocks (interferes with) the binding of myosin heads to actin.
33
Ca2+ flooding into the cytosol then initiates
the contraction of myofibrils through the Troponin complex and Tropomyosin
34
How does Calcium initiate contraction
$Ca^{2+}$ binds to Troponin C. This causes a conformational change that releases Troponin I's hold on actin. The Troponin complex pulls Tropomyosin away from the myosin-binding sites on actin. Myosin can now bind actin and contraction proceeds. 23232323
35
How is muscle contraction stopped (Relaxation)
Calcium is rapidly pumped back into the Sarcoplasmic Reticulum by an ATP-dependent calcium pump called SERCA
36
The increase in Ca2+ concentration is what
transient
37
What is "Stretch Activation" in insect flight muscles
An intrinsic length-sensing mechanism where muscles activate due to stretching (bypassing the calcium cycle for every beat). This allows for extremely fast wing beats (asynchronous muscle).
38
What is a syncytium?
A single cell that contains multiple nuclei. Muscle fibers are syncytia formed by the fusion of many muscle cell precursors.
39
How do skeletal muscle cells grow and acquire multiple nuclei?
Through cell fusion. A single-nucleated cell migrates to and fuses with the muscle fiber, donating its nucleus.
40
What is the sarcoplasmic reticulum (SR) and its specific function in muscle?
It is a specialized version of the smooth ER found only in muscle cells. Its primary function is to store calcium.
41
What connects muscles to bones?
Tendons, which are elastic tissue structures.
42
Why do muscle cells contain mitochondria situated directly next to myofibrils?
To provide immediate ATP for contraction, as there is very little space for other organelles packed between the myofibrils.
43
What happens if calcium is NOT pumped back into the sarcoplasmic reticulum?
The muscle would remain permanently contracted (hypercontracted), preventing relaxation.
44
Besides insect flight muscles, where does stretch activation occur in humans?
In the heart.
45
What is the primary characteristic of epithelial tissues?
Cells are tightly bound together into sheets called epithelia (e.g., gut lining, skin epidermis)
46
What are the two components of the Basement membrane?
The basal lamina and the reticular lamina
47
In intestinal epithelial cells, what structural feature increases surface area on the apical region?
Microvilli
48
Define apical-basal polarity in epithelial cells.
It is the result of differential distribution of phospholipids, protein complexes, and cytoskeletal components, creating distinct Apical (top) and Basolateral (side/bottom) regions in epithelial cells
49
Where are cell junctions typically located in polarized epithelial cells?
They connect cells at the lateral region (Basolateral region)
50
How are mechanical stresses transmitted across epithelial tissue?
Stresses are transmitted from cell to cell via cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites.
51
Within the epithelium, cells are attached to each other directly by what
cell–cell junctions
52
Within the epithelium, cells are attached to each other directly by
cell–cell junctions, where cytoskeletal filaments are anchored, transmitting stresses across the interiors of the cells
53
List the three VERY important types of cell junctions mentioned in the lecture
Tight junctions Adherens junctions Gap junctions
54
What is the main function of Adherens Junctions?
They provide strong cell-cell adhesion and associate with the actin cytoskeleton
55
What is the main protein complex component of Adherens Junctions?
The Cadherin/Catenin complex
56
What are the specific names for different types of Adherens Junctions mentioned?
Zonula adherens (ZA) (belt-like) and Punctum adherens (PA) (spot-like).
57
Explain the structural role of Calcium in Cadherin domains
Ca^{2+} ions bind to the hinge regions between cadherin domains, preventing them from flexing (making the structure rigid). Without calcium, the structure becomes floppy.
58
Which ion is essential for Cadherin-Cadherin binding?
Calcium
59
Catenins link what to what
Catenins link cadherins to the actin cytoskeleton
60
Cadherins are what
homophilic (bind to each other) Cadherin
61
Many other types of what cadherins
E-cadherin = epithelial N-Cadherin = neuronal
62
Cadherin is what
transmembrane protein
63
Catenin is what
cytoplasmic
64
What is the difference between Homophilic and Heterophilic binding in Cadherins?
Homophilic: Typical cadherins (e.g., E-cadherin) bind to the same type of cadherin on the adjacent cell. Heterophilic: Atypical cadherins (e.g., Fat and Dachsous) bind to different types of cadherins
65
The cadherin superfamily members all have what
extracellular portions containing multiple copies of the extracellular cadherin domain
66
Each cadherin domain forms what
a more-or-less rigid unit, joined to the next cadherin domain by a hinge.
67
Ca2+ ions bind to each what
hinge and prevent it from flexing.
68
When Ca2+ is removed, the hinges what
flex, and the structure becomes floppy
69
Cadherins mediate what
highly selective recognition, enabling cells of a similar type to stick together and to stay segregated from other types of cells
70
Cells are sorted by what
the type of cadherin and amounts of cadherins
71
Assembly of Strong Cell–Cell Adhesions Requires what
Changes in the Actin Cytoskeleton
72
Which GTPases are involved in the assembly of strong cell-cell adhesions
Cadherins generate signals to inhibit Rho and activate Rac.
73
Heterophilic cadherins are called
atypical
74
What are Fat (Ft) and Dachsous (Ds)?
They are atypical cadherins that exhibit heterophilic binding (Ft binds Ds).
75
Evidence of heterophilic binding
cells with Fat bind cells with Ds and experimental evidence: clumping assays below
76
What role do Fat and Dachsous play in tissue organization?
They play a role in Planar Cell Polarity (the collective alignment of cell polarity across the tissue plane)
77
What is the main function of Tight Junctions?
They act as selective permeability barriers, sealing adjacent cells together to prevent molecules from leaking freely across the cell sheet.
78
tight junctions are visualized by what
freeze-fracture electron microscopy, they are seen as a branching network of sealing strands that completely encircles the apical end of each cell
79
Which transmembrane proteins form the sealing strands of Tight Junctions?
Claudins and the related proteins Occludins
80
What structural feature of claudins allows them to function as channels?
They form selective pores (e.g., 35 Å entrance, 14 Å innermost diameter) that allow specific ions to cross
81
Claudins form what
selective channels allowing specific ions to cross the tight-junctional barrier, from one extracellular space to another.
82
What is the insect equivalent of Tight Junctions?
Septate junctions. They are organized differently but have the same function.
83
Septate junctions are made of what
Claudins
84
Which specific claudin is mentioned in the structure of septate junctions?
Sinuous (analogous to hClaudin 1).
85
What experiment demonstrated the barrier function of Septate Junctions in flies?
A blue dye tracer was fed to flies. In normal flies, the dye stayed in the gut lumen. In mutant flies without septate junctions, the dye diffused through the epithelium into the body.
86
Diffusion between cells is mediated by what
Gap junctions
87
A fluorescent dye diffuses from what
one cell to the others through Gap junctions
88
What is the main function of Adherens Junctions?
They provide strong cell-cell adhesion and associate with the actin cytoskeleton.
89
Gap junctions
are clusters of channels that join two cells together and consist of building blocks of two connexons or hemichannels, one contributed by each of the communicating cells
90
What is the pore size of a Gap Junction channel?
Approximately 1.4 nm
91
What are the protein building blocks of Gap Junctions in vertebrates vs. invertebrates?
Vertebrates: Connexins (form Connexons). Invertebrates: Innexins
92
Each connexon or hemichannel is formed of a complex of what
six connexin proteins.
93
What is the main function of Gap Junctions?
They allow the exchange of inorganic ions and small molecules (connect cytoplasms) between adjacent cells.
94
Both connexin and innexin form what
similar sized pores
95
What forms a complete Gap Junction channel?
Two connexons (hemichannels) in register, one from each adjacent cell.
96
What is the difference between Homomeric and Heteromeric connexons?
Homomeric: Formed by six identical connexin proteins. Heteromeric: Formed by a mix of different connexin proteins
97
In an epithelial sheet, what is a "Tricellular Junction"?
A point where three cells meet. It is the strongest point of adhesion due to the accumulation of junctional proteins.
98
How does the connection between Cadherins and the Actin cytoskeleton depend on Catenins?
Experiments show that without catenins (alpha, beta, p120), there is no connection; cadherins alone cannot bind to actin.
99
Describe the structural change in Cadherin domains when Calcium binds.
Without Calcium, the domains are twisted/floppy. When Calcium binds to the hinges, the domains straighten up (unfold) to form a rigid structure capable of reaching the adjacent cell.
100
: How are Tight Junctions visualized using Freeze-Fracture microscopy
The cell is frozen and broken with a "micro-hammer." The break occurs at lipid/water transitions, revealing ridges where the membranes of two cells are fused/embedded into each other.
101
Structurally, how do Insect "Septate Junctions" differ from Vertebrate "Tight Junctions" under an electron microscope?
Tight junctions look like "zippers," whereas Septate junctions look like highly organized "stairs."
102
Why are Gap Junction channels described as "hydrophilic"
Because they allow water and water-soluble molecules (ions, small molecules) to pass through the lipid membrane.
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