Biology 353 Exam 3 Cytoskeletal System

Which of the following structures is made from microfilaments? Possible Answers: Flagella Mitotic spindle Cilia Actin

Actin Explanation: The three major components of the cytoskeleton in cells are microtubules, intermediate filaments, and microfilaments. Microtubules are the larger filaments and make up the mitotic spindle, as well as flagella and cilia. Intermediate filaments are used in structural maintenance. Microfilaments are the smaller filaments and make up the polymerized actin filament in muscle fibers. Microfilaments and microtubules are both polarized, and can be used in vesicular transport. Intermediate filaments lack polarity and serve only structural functions.

__________ is a protein/complex that helps nucleate G-actin. Arp2/3 Myosin F-actin Cofilin

Arp2/3 Explanation: On its own, G-actin (globular actin) is not likely to nucleate and begin to form chains of F-actin (fibrous actin); therefore, it is useful to have proteins to help the nucleation process. One of these protein complexes is Arp2/3. Arp2/3 is especially known for its function of nucleating actin chains that branch off of previously established actin chains. Myosins are motor proteins that interact with actin chains to perform various functions, such as muscle contraction and transporting vesicles. Cofilin is a protein that binds G-actin monomers and helps them dissociate from F-actin.

A genetic defect involving the tubulin molecules will affect which of the following processes? Possible Answers: None of these will be affected Mitosis Both of these will be affected Axonal transport in neurons

Both of these will be affected Explanation: Tubulin is a key protein found in microtubules. Microtubules are large cytoskeletal elements that have several functions inside a cell. One of the biggest use of microtubules arises during mitosis. Recall that chromosomes are pulled apart to opposite ends during the anaphase of mitosis. The rope-like substances that pull these chromosomes to opposite ends of cell are microtubule segments. In addition, microtubules are also involved in axonal transport in neurons. Some nerves (such as sciatic nerve) can have extremely long axons. To maintain the integrity of these distal ends of the axons, nutrients and other important molecules from the cell body of a neuron are carried, along microtubule segments, to axon terminals; therefore, microtubules are also essential for axonal transport.

Microtubules are present in which of the following structures? I. Cilia II. Flagella III. Microvilli Possible Answers: II only I, II, and III II and III I and II

I and II Explanation: Cilia, microvilli, and flagella are all protrusions from the cell body. Cilia and flagella have functions in cell motility and contain organized systems of microtubules and motor proteins. Microvilli are often used as mechanosensors and are primarily made of cross-linked actin filaments. Microtubules are, therefore, found primarily in cilia and flagella, but no in microvilli.

In which of the following processes is the cytoskeleton involved? I. Contractile ring formed during cytokinesisII. Anchoring the extracellular matrixIII. Post-translational modification of proteinsIV. Post-transcriptional modification of mRNA Possible Answers: I, II, and III II and IV I and II I only

I and II Explanation: The contractile ring formed during cytokinesis is composed of actin microfilaments (a component of the cytoskeleton) and the motor protein myosin. The extracellular matrix can be anchored to the cell via the actin cytoskeleton or the intermediate filament cytoskeleton, via adherens junctions or desmosomes. Post-translational modifications of proteins commonly occur in the endoplasmic reticulum or the Golgi apparatus and do not involve the cytoskeleton. The same logic applies for post-transcriptional modifications of mRNA, except these occur in the nucleus.

A mutation in which of the following proteins would affect the functionality of eukaryotic flagella? I. Dynein II. Kinesin III. Myosin Possible Answers: III only I only I and II I, II, and III

I only Explanation: Eukaryotic flagella are primarily composed of microtubules and the motor protein dynein. Hydrolysis of ATP by dynein produces a sliding movement of the microtubule filaments that produces movement. Kinesin is a motor protein that associates with microtubules, but it is not present in flagella. Myosin is a motor protein that associates with actin microfilaments.

Which of the following are functions of the cytoskeleton? I. Support organelles II. Form motile structures III. Create cell junctions IV. Vesicle trafficking Possible Answers: I, II, III, and IV III and IV II and III I, II, and IV

I, II, III, and IV Explanation: Each choice describes a distinct function of the cytoskeleton. The cytoskeleton is involved in supporting various organelles, helping to anchor them in various locations around the cell and maintaining their shape and integrity. It also has the important function of helping with vesicle trafficking by associating with various motor proteins that carry vesicles from one part of the cell to another. The cytoskeleton is also a part of several different types of cell junctions (e.g. adherens junctions). Finally, the cytoskeleton is also an important part of various motile structures, such as cilia and flagella.

Which of the following is a function of intermediate filaments? Possible Answers: Cause movement of cilia Allow contractions in ciliated muscle Form the mitotic spindle Brace cells against surfaces Line the nucleus, forming a veil

Line the nucleus, forming a veil Explanation: Intermediate filaments form a veil right next to the nuclear membrane, are of intermediate thickness with respect to the other two cytoskeletal filaments, and they almost exclusively play structural roles. Actin filaments brace cells against surfaces and allow contractions in striated muscle. Also, actin filaments provide structural support and have a role in determining cell shape. Microtubules form the mitotic spindle and comprise cilia and flagella. They are also the "freeways" on which motor proteins move and transport vesicles throughout the cell.

Which of the following cytoskeletal components is comprised of G-actin subunits organized in polymers? Possible Answers: Intermediate filaments Lamin Microfilaments Microtubules Septins

Microfilaments Explanation: G-actin subunits that polymerize form microfilaments, also known as actin filaments. The other cytoskeletal components listed are formed from different proteins, but only microfilaments are actin-based structures.

Which cellular filament is found in the flagella of eukaryotic cells? Possible Answers: Intermediate filaments Microfilaments Microtubules Collagen

Microtubules Explanation: Eukaryotic flagella are composed of nine pairs of microtubules. In contrast, prokaryotic flagella are composed of the protein flagellin.

Which of the following cytoskeletal components undergo dynamic polymerization and depolymerization through GDP-GTP cycling, and are typically comprised of alpha and beta subunits? Possible Answers: Crescentin Actin Microtubules Microfilaments Intermediate filaments

Microtubules Explanation: GDP-GTP cycling can regulate a number of these cytoskeletal components, although GTP-binding is especially crucial to microtubule polymerization. In addition, microtubule polymers are comprised of alpha and beta tubulin, while this is not the case for the other components listed. Microtubules are the only choice that fit all of the criteria posed in the question.

Which of the following might explain why no known motor proteins have been found to utilize intermediate filaments? Possible Answers: Actin and microtubules are non-polar, while intermediate filaments are polar Microtubules and actin are polarized, while intermediate filaments are not Intermediate filaments are too small for motor proteins Intermediate filaments are too large for motor proteins

Microtubules and actin are polarized, while intermediate filaments are not Explanation: One of the most likely explanations for why intermediate filaments have no known motor proteins is because they are nonpolar. What this means is that there are no distinct ends to the filament. Motor proteins have ways of detecting which direction they are moving along actin or microtubules because they have distinct ends (polarization). Size should not affect the ability of a motor protein to function, especially because actin is smaller than intermediate filaments and microtubules are larger (hence the name "intermediate").

Which of the following is true regarding microfilaments? Possible Answers: They are the smallest of the cytoskeletal elements They always have a 9+2 arrangement More than one of these are true They are made up of myosin and actin

They are the smallest of the cytoskeletal elements Explanation: Microfilaments are the smallest of the three cytoskeletal elements and are made up of actin filaments (myosin is not found in microfilaments). The 9+2 arrangement is found in microtubules, the biggest cytoskeletal elements. The third cytoskeletal element is called the intermediate filament and is made up of related proteins unique to each intermediate filament.