Ribosomes and Cytoskeleton
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MCAT Biological and Biochemical Foundations of Living Systems › Ribosomes and Cytoskeleton
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
Nine actin singlets surrounded by two actin doublets
Two actin singlets surrounded by nine actin 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.
What happens at the minus-end of actin filaments when the concentration of G-actin is above its critical concentration?
Monomers add on to it.
Monomers are lost from it.
Monomers are not lost from it or added on to it.
Monomers bind GTP.
Monomers undergo dynamic instability.
Explanation
Monomers are lost when concentration of G-actin is below its critical concentration. Monomers are gained when concentration of G-actin is above its critical concentration. If it is in between the critical concentrations, the actin filaments will undergo treadmilling, which is the addition of monomers on the (+) end and loss of monomers on the (–) end.
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
I, II, III, and IV
I and II
III and IV
II, III, and IV
Explanation
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.
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
microtubules . . . integrins
actin . . . cadherins
actin . . . integrins
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.
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.
Which statement is true of microtubules and actin?
Microtubules have a hollow center, are composed of dimers, and are thicker than actin polymers
Actin polymers have a hollow center, are composed of dimers, and are thicker than microtubules
Microtubules have a solid center, are composed of dimers, and are thicker than actin polymers
Actin polymers have a hollow center, are composed of monomers, and are thicker than microtubules
Microtubules have a hollow center, are composed of monomers, and are thicker than actin polymers
Explanation
Microtubules are composed of the protein tubulin, a GTP-binding protein, which forms a ring around a hollow center. This is in contrast to actin, a protein that forms microfilaments, which are thinner than a tubulin-based microtubule.
The cytoskeleton acts as a scaffold for the cell and maintains cellular integrity. Which of the following is a component of the cytoskeleton?
Spindle complexes
Actin filaments
Cilia
Flagella
Myosin 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.
Actin is the major protein that composes which part of the cytoskeleton?
Microfilaments
Microtubules
Cilia
Intermediate filaments
Explanation
The cytoskeleton is important for cell support and movement. It is composed of microfilaments made of actin, microtubules made of tubulin, intermediate filaments that bear tension, and cilia/flagella which are made of microtubules (tubulin).
Which of the following choices describes a function of the eukaryotic centriole?
Produce and organize spindle fibers used during cell division
Condense chromosomes and repackage chromatin
Digest foreign pathogens in the cytosol
Catabolize very long chain fatty acids
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.
Which of the following is true of microtubules?
Microtubules are larger than microfilaments
Actin forms a major component of microtubules
Microtubules are responsible for cytoplasmic streaming
Microtubules are involved in phagocytosis
Explanation
Microtubules are a component of the cell cytoskeleton formed by polymers of tubulin protein. They are larger than microfilaments and make up the internal structures of cilia and flagella. Microtubules have a positive and negative end. The negative end of a microtubule attaches to a microtubule-organizing center (MTOC) within the cell, which then allows the microtubule to grow away from the MTOC at its positive end.
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?
Anaphase
Metaphase
Telophase
Interphase
Prophase
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.