All MCAT Biology Resources
Example Questions
Example Question #231 : Cell Biology, Molecular Biology, And Genetics
In which of the following structures do actin microfilaments play a crucial role?
I. Contractile ring formed during cytokinesis
II. Sarcomeres
III. Adherens junctions
IV. Eukaryotic flagella
I, II, and III
II, III, and IV
I, II, III, and IV
III and IV
I and II
I, II, and III
Eukaryotic flagella are primarily made up of microtubule doublets and singlets organized in a "9+2" manner (two singlets surrounded by nine doublets). Actin microfilaments are not present in flagella.
The contractile ring formed during cytokinesis consists of actin and myosin, and helps separate the two daughter cells to conclude mitosis. Sarcomeres consist of actin and myosin overlaps that are crucial to muscle contraction. Adherens junctions are specialized cell junctions that use the actin cytoskeleton to anchor adjacent cells.
Example Question #232 : Cell Biology, Molecular Biology, And Genetics
Desmosomes are specialized cell junctions that anchor cells to one another by connecting the __________ of the cytoskeleton to transmembrane proteins known as __________.
intermediate filaments . . . cadherins
actin . . . integrins
microtubules . . . integrins
actin . . . cadherins
intermediate filaments . . . cadherins
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 #1272 : Biology
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
I and II
I, II, III, and IV
I, II, and III
III only
III only
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 #73 : 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?
Oxygen becomes trapped in the lungs
Lung tissue becomes more elastic
Mucus accumulates in the lungs
The lungs lose their ability to expand
Lung cells can no longer divide
Mucus accumulates in the lungs
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 #233 : Cell Biology, Molecular Biology, And Genetics
The cytoskeleton acts as a scaffold for the cell and maintains cellular integrity. Which of the following is a component of the cytoskeleton?
Cilia
Spindle complexes
Flagella
Actin filaments
Myosin filaments
Actin filaments
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 #234 : Cell Biology, Molecular Biology, And Genetics
Which of the following is true about microtubules?
They are a key component of the contractile ring formed during cytokinesis
They are found in a "9+2" structure in eukaryotic axonemes
They maintain a regulated ADP/ATP gradient that assists in motility
They associate with various motor proteins, such as myosin
They are found in a "9+2" structure in eukaryotic axonemes
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 #235 : Cell Biology, Molecular Biology, And Genetics
Which of the following structures is not composed of microtubules?
Prokaryotic flagella
Eukaryotic flagella
Mitotic spindle
Cilia
Prokaryotic flagella
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 #236 : Cell Biology, Molecular Biology, And Genetics
Which of the following accurately represents the compositions of eukaryotic cilia and flagella?
Two microtubule singlets surrounded by nine microtubule doublets
Nine microtubule singlets surrounded by two microtubule doublets
Two actin singlets surrounded by nine actin doublets
Nine actin singlets surrounded by two actin doublets
Two microtubule singlets surrounded by nine microtubule doublets
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 #237 : Cell Biology, Molecular Biology, And Genetics
Which of the following choices describes a function of the eukaryotic centriole?
Condense chromosomes and repackage chromatin
Produce and organize spindle fibers used during cell division
Digest foreign pathogens in the cytosol
Catabolize very long chain fatty acids
Produce and organize spindle fibers used during cell division
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 #81 : Cellular Structures And Organelles
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?
Centriole
Centromere
Centrosome
Kinesin
Kinetochore
Kinetochore
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.
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