MCAT Biology : Ribosomes and Cytoskeleton

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #71 : Cellular Structures And Organelles

Desmosomes are specialized cell junctions that anchor cells to one another by connecting the __________ of the cytoskeleton to transmembrane proteins known as __________.

Possible Answers:

intermediate filaments . . . cadherins

microtubules . . . integrins

actin . . . cadherins

actin . . . integrins

Correct answer:

intermediate filaments . . . cadherins

Explanation:

Desmosomes are specialized cell junctions that are important in anchoring certain cell types to one another. Intermediate filaments are used to stabilize these connections by interacting with cadherins. Cadherins are transmembrane proteins that interact with cadherins of adjacent cells on the exoplasmic face of the plasma membrane. Adherens junctions have similar function,s but make use of actin and integrins/cadherins. 

Example Question #11 : Ribosomes And Cytoskeleton

Which of the following statements is true about intermediate filaments?

I. Intermediate filaments display treadmilling

II. Intermediate filaments maintain a tightly regulated gradient of ADP/ATP bound monomers

III. Intermediate filaments play a crucial role in the function of desmosomes

IV. Intermediate filaments are a major component of the mitotic spindle

Possible Answers:

I, II, and III

I and II

III only

I, II, III, and IV

Correct answer:

III only

Explanation:

Intermediate filaments are a component of the cytoskeleton that do not display treadmilling. Both actin microfilaments and microtubules undergo treadmilling, during which the structure is built on one end and deconstructed at the other. The result is an apparently "moving" structure with a forward and reverse end. Intermediate filaments are nor polarized and have no distinct ends, making them incapable of this action.

Actin microfilaments and microtubules also maintain gradients of ADP/ATP or GDP/GTP bound monomers respectively, used to indicate their polarity, while intermediate filaments do not. The mitotic spindle is made of microtubules.

Desmosomes, however, are specialized cell junctions that use the intermediate filament cytoskeleton to anchor adjacent cells. Membrane proteins called cadherins bind to the filaments on the intracellular surface of the membrane and bind to the extracellular regions of membrane proteins on the adjacent cell. The result is two intermediate filaments, linked by the bound proteins, to form a junction.

Example Question #78 : Cellular Structures And Organelles

Primary ciliary dyskinesia (PCD) is a rare genetic lung disorder, also known as immotile cilia syndrome, and is associated with Kartegener's Syndrome. For people with PCD, the tiny hair-like structures (cilia) in the respiratory tract become non-motile.

What is the most likely clinical manifestation of this disease?

Possible Answers:

Mucus accumulates in the lungs

Oxygen becomes trapped in the lungs

Lung cells can no longer divide

The lungs lose their ability to expand

Lung tissue becomes more elastic

Correct answer:

Mucus accumulates in the lungs

Explanation:

Mucus begins to accumulate in the lungs because the cilia no longer move. Cilia function in pushing mucus up the respiratory tract so that it doesn't build up in the lungs. When they become non-motile, they lose this capability.

Example Question #71 : Cellular Structures And Organelles

The cytoskeleton acts as a scaffold for the cell and maintains cellular integrity. Which of the following is a component of the cytoskeleton?

Possible Answers:

Actin filaments

Flagella

Myosin filaments

Cilia

Spindle complexes

Correct answer:

Actin filaments

Explanation:

The cytoskeleton is comprised of actin filaments, intermediate filaments, and microtubules.

Spindle complexes are found within cells undergoing mitosis; they are made of microtubules, but are not a fundamental part of the cytoskeleton. Cilia and flagella are also largely composed of microtubules; however, these structures are also not fundamental components of the cytoskeleton. Myosin filaments work in coordination with actin filaments during muscle contraction, but are not involved in the cytoskeleton.

Example Question #80 : Cellular Structures And Organelles

Which of the following is true about microtubules?

Possible Answers:

They maintain a regulated ADP/ATP gradient that assists in motility 

They are a key component of the contractile ring formed during cytokinesis

They associate with various motor proteins, such as myosin

They are found in a "9+2" structure in eukaryotic axonemes 

Correct answer:

They are found in a "9+2" structure in eukaryotic axonemes 

Explanation:

Microtubules are found in various structures that promote cell motility. The axoneme is the name given to the cytoskeletal core that makes up whip-like appendages in eukaryotic cells. Axonemes display a "9+2" organization of microtubules.

Microtubules are also important in vesicle trafficking and utilize motor proteins like dynein and kinesin to accomplish this. Actin filaments use the motor protein myosin. The GDP/GTP gradient helps control microtubule treadmilling, while the ADP/ATP gradient helps control actin treadmilling. The contractile ring formed during cytokinesis consists of actin and myosin rather than microtubules. 

Example Question #81 : Cellular Structures And Organelles

Which of the following structures is not composed of microtubules?

Possible Answers:

Prokaryotic flagella

Eukaryotic flagella

Cilia

Mitotic spindle

Correct answer:

Prokaryotic flagella

Explanation:

While eukaryotic flagella are composed of microtubules, prokaryotic flagella are composed of a long protein called flagellin. Microtubules are composed of the tubulin protein, and play keys roles in mitosis and cell motility. Cilia and spindle fibers are both composed of microtubules. Prokaryotes do not contain developed microtubules.

Example Question #11 : Ribosomes And Cytoskeleton

Which of the following accurately represents the compositions of eukaryotic cilia and flagella?

Possible Answers:

Two actin singlets surrounded by nine actin doublets

Two microtubule singlets surrounded by nine microtubule doublets

Nine microtubule singlets surrounded by two microtubule doublets

Nine actin singlets surrounded by two actin doublets

Correct answer:

Two microtubule singlets surrounded by nine microtubule doublets

Explanation:

Eukaryotic cilia and flagella are incredibly similar in protein composition. Their primary functions include helping cells move and maintaining fluid flow within the body. They accomplish this by maintaining a structure of 9 microtubule doublets surrounding 2 microtubule singlets (9+2). The motor protein dynein is then responsible for allowing the sliding of filaments that is necessary for movement.

Example Question #82 : Cellular Structures And Organelles

Which of the following choices describes a function of the eukaryotic centriole?

Possible Answers:

Produce and organize spindle fibers used during cell division

Catabolize very long chain fatty acids

Condense chromosomes and repackage chromatin

Digest foreign pathogens in the cytosol

Correct answer:

Produce and organize spindle fibers used during cell division

Explanation:

Centrioles have diverse functions. Particularly, they are important portions of centrosomes and help develop the mitotic spindle that aids in the separation of chromosomes during cell division.

Peroxisomes are responsible for the catabolism of very long chain fatty acids. Lysosomes are responsible for handling pathogens in the cytosol in a process called phagocytosis. The pathogens are then digested by hydrolytic enzymes in the lysosome interior. Chromosome condensation is accomplished by various proteins.

Example Question #232 : Cell Biology, Molecular Biology, And Genetics

There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.

Cisternal Maturation Hypothesis

In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.

In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.

Vesicular Transport Hypothesis

In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.

In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.

A scientist is studying microtubules in a series of mitotic cells. He notices that the microtubule structures attach to chromosomes in each mitotic cell by binding to a protein complex at the center of the chromosome. What is the name of this protein complex?

Possible Answers:

Centrosome

Centriole

Kinetochore

Centromere

Kinesin

Correct answer:

Kinetochore

Explanation:

The chromosome is formed by two sister chromatids, connected at a central point to form an "X" structure. This central location is called the centromere.

During prophase, as the chromosomes condense, a protein complex forms at the centromere and becomes bound to microtubule spindle fibers. This protein complex is known as the kinetochore.

Centrioles form the distal attachment points for the microtubules that bind to the kinetochore. During anaphase, the centrioles anchor the spindle fibers at opposite ends of the cell and allow the microtubules to pull the chromatids apart. The centrosome is composed of two centrioles.

Kinesin is a protein capable of traveling along microtubules, helping to assemble and disassemble their structures, as well as shuttling proteins along the length of the tubule.

Example Question #1305 : Mcat Biological Sciences

There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.

Cisternal Maturation Hypothesis

In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.

In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.

Vesicular Transport Hypothesis

In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.

In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.

Microtubules involved in the vesicular transport model are similar to microtubules that are involved during mitosis. Which of the following phases of mitosis is likely to involve chromosomes moving the greatest distance along microtubules?

Possible Answers:

Anaphase

Metaphase

Prophase

Interphase

Telophase

Correct answer:

Anaphase

Explanation:

Microtubules form the basis of the spindle fiber structures responsible for separating sister chromatids during mitosis. These fibers attach to chromosomes at the centromere by binding to the kinetochore during metaphase. During anaphase, the microtubules retract, pulling apart the sister chromatids and sequestering them in opposite ends of the cell. As a result, anaphase is likely to be the phase during which movement along microtubules is greatest.

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