Cell Structure and Function Exam #4
Which of the following statements about the regulation of Cyclin-Cdk complexes is TRUE? A) Ubiquitylation of cyclin leads to the destruction of its Cdk partner B) Ubiquitylation of cyclin is mediated by the Anaphase Promoting Complex/Cyclosome (APC/C) C) Wee1 removes inhibitory phosphates from the cyclin-Cdk complex immediately upon its formation D) Cdc25 is responsible for the addition of inhibitory phosphates to the cyclin component of the cyclin-Cdk complex
Answer = B Ubiquitylation of cyclin is mediated by the Anaphase Promoting Complex/Cyclosome (APC/C) A is INCORRECT because ubiquitylation of cyclin leads to destruction of only cyclin. B is CORRECT C is INCORRECT because Wee1 is a kinase which ADDS the inhibitory phosphates to the cyclin-Cdk complex immediately upon its formation D is INCORRECT because Cdc25 is the phosphatase which is responsible for removal of inhibitory phosphates from the Cdk member of the cyclin-Cdk complex. Make sure how you understand how I could alter these answer options to be false or true.
During fertilization in humans, A) A wave of Ca2+ ions is released in the fertilized egg's cytoplasm B) Only on sperm binds to the unfertilized egg C) A sperm moves in a random fashion until it encounters an egg D) Several sperm pronuclei compete in the cytoplasm to fuse with the egg nucleus
Answer: A A wave of Ca2+ ions is released in the fertilized egg's cytoplasm A is CORRECT B is INCORRECT because MANY sperm can bind to an egg. C is INCORRECT because cells surrounding the egg release a chemoattractant signal, attracting the sperm to the correct place; sperm are moving toward the chemoattractant signal and not moving randomly. D is INCORRECT because mechanisms exist to ensure that only one sperm fertilizes each egg, and thus only one sperm pronucleus will reach the cytoplasm of the egg.
Which of the situations below will enhance microtubule shrinkage? A) Addition of a drug that inhibits GTP exchange on free tubulin dimers B) Addition of a drug that inhibits hydrolysis of the GTP carried by tubulin dimers C) Addition of a drug that increases the affinity of tubulin molecules carrying GDP for other tubulin molecules D) Addition of a drug that blocks the ability of a tubulin dimer to bind to (gamma)-tubulin
Answer: A Addition of a drug that inhibits GTP exchange on free tubulin dimers A drug that inhibits GTP exchange on free tubulin dimers will effectively decrease the available pool of GTP-bound tubulin dimers available for addition to microtubule ends, thus tipping the balance in favor of microtubule disassembly. A drug that inhibits hydrolysis of GTP carried by tubulin dimers or that increases the affinity of GDP-bound tubulin dimers for each other will stabilize growing microtubules. Blocking the ability of a tubulin dimer to bind to γ-tubulin will decrease the rate of new microtubule formation but should not enhance microtubule shrinkage.
Which of the statements below about intermediate filaments is TRUE? A) They easily disassemble B) They can be found in only the nucleus C) They can be anchored to the plasma membrane at a cell-cell junction D) Each filament is about 10 (um) in diameter
Answer: C They can be anchored to the plasma membrane at a cell-cell junction A is INCORRECT because intermediate filaments are the most stable of the cytoskeletal filaments. B is INCORRECT because the intermediate filaments are found in both the nucleus and cytoplasm. C is CORRECT D is INCORRECT because the intermediate filaments are about 10nm in diameter.
Which of the following statements about the phases of mitosis is TRUE? A) During prophase, the duplicated chromosomes condense and the mitotic spindle begins to assemble in the cytoplasm B) During pro-metaphase, the nuclear envelope is still intact C) During metaphase, duplicated chromosomes are attached to the spindle microtubules by centrosomes D) During telophase, the nuclear envelope remains unassembled although the microtubules return to their interphase configuration
Answer: A During prophase, the duplicated chromosomes condense and the mitotic spindle begins to assemble in the cytoplasm A is CORRECT B is INCORRECT because the distinguishing mark of the start of pro-metaphase is that the nuclear envelope disassembles C is INCORRECT because chromosomes attach to the spindle microtubules via kinetochores and the connecting protein complex. D is INCORRECT because the nuclear envelope reassembles during telophase.
Which of the following does not occur during M phase in animal cells? A) Growth of the cell B) Condensation of chromosomes C) Breakdown of the nuclear envelope D) Attachment of chromosomes to microtubules
Answer: A Growth of the cell Cell size increases throughout interphase and not during the M phase. All of the other steps are observed in the M phase.
Which of the following statements about microtubule-organizing centers is FALSE? A) Microtubules emanating from the centrosome have alternating polarity such that some have their plus end attached to the centrosome while others have their minus end attached to the centrosome B) Basal bodies serve as the microtubule-organizing centers for cilia C) Centrosomes typically contain a pair of centrioles, which is made up of a cylindrical array of short microtubules D) Centrosomes are the major microtubule-organizing center in animal cells
Answer: A Microtubules emanating from the centrosome have alternating polarity such that some have their plus end attached to the centrosome while others have their minus end attached to the centrosome Remember that microtubules emanating from the centrosome are all arranged with their minus ends at the centrosomes and the plus ends extending into the cytoplasm. Similarly, we see that with cilia, the microtubule minus ends attach to the MTOC basal bodies and plus ends extend into the cilium.
Which of the following statements about actin filaments is TRUE? A) Actin filaments, similar to microtubules, are composed of actin dimers. B) Actin filaments are found in the cell in microvilli, contractile bundles, lamellipodia, filopodia, and contractile rings C) Actin filaments, similar to intermediate filaments, do not have a polarity to their structure D) Actin filaments can only be found in the cell in bundles of filaments
Answer: B Actin filaments are found in the cell in microvilli, contractile bundles, lamellipodia, filopodia, and contractile rings A is INCORRECT because actin filaments are composed of actin monomers B is CORRECT! C is INCORRECT because actin filaments do have a polarity to their structure with a plus and minus end D is INCORRECT because actin filaments can be found in a variety of 2D and 3D structures including single filaments, bundles, and gel-like mesh structures.
During Recombination: A) Sister chromatids undergo crossing-over with each other B) Chiasmata hold chromosomes together C) One crossover event occurs for each pair of human chromosomes D) The synaptonemal complex keeps the sister chromatids together until anaphase II
Answer: B Chiasmata hold chromosomes together A is INCORRECT because non-sister chromatids undergo crossing-over; because sister chromatids are identical, there would be no exchange of genetic material if sister chromatids underwent recombination. B is CORRECT C is INCORRECT because the number of crossover events can vary for each meiosis; on average, two to three crossover events occur between each pair of human chromosomes. D is INCORRECT because although the sister chromatids do not separate until anaphase II, their arms become unglued because the cohesins holding them together are degraded during anaphase I. The synaptonemal complex is important for holding together and aligning the duplicated homologs and is not involved directly in sister chromatid cohesion.
At the end of DNA replication, the sister chromatids are held together by the: A) Kinetochores B) Securins C) Cohesins D) Histones
Answer: C
Apoptosis differs from necrosis in that necrosis: A) Requires the reception of an extracellular signal B) Causes DNA to fragment C) Causes cells to swell and burst, whereas apoptotic cells shrink and condense D) Involves a caspase cascade
Answer: C A is INCORRECT because necrosis often involves cell death due to injury B is INCORRECT as DNA fragmentation is a hallmark of apoptosis C is CORRECT D is INCORRECT as apoptosis is triggered by a caspase cascade
Which of the following statements about the cytoskeleton is FALSE? A) The cytoskeleton is made up of three types of protein filaments B) The cytoskeleton controls the location of organelles in eukaryotic cells C) Covalent bonds between protein monomers hold together cytoskeletal filaments D) The cytoskeleton of a cell can change in response to the environment
Answer: C Covalent bonds between protein monomers hold together cytoskeletal filaments C is FALSE because the protein monomers of the cytoskeleton are held together by noncovalent interactions between the protein monomers. All the other statements are true.
Kinesins and Dyneins... A) Have tails that bind to the filaments B) Move along both microtubules and actin filaments C) Move in opposite directions to each other D) Derive their energy from GTP hydrolysis
Answer: C Move in opposite directions to each other A is INCORRECT because the globular heads of motor proteins bind to the filaments. B is INCORRECT because both kinesin and dynein move along microtubules only. C is CORRECT D is INCORRECT because the motors use ATP hydrolysis for energy.
Which of the following structures shortens during muscle contraction? A) Myosin filaments B) Flagella C) Sarcomeres D) Actin filaments
Answer: C Sarcomeres Sarcomeres contain actin filaments and myosin filaments that slide past each other during muscle contraction, leading to shortening of the sarcomere; the actin filaments and myosin filaments do not change in length. Flagella are microtubule-based structures that are not present on muscle cells.
Which of the following descriptions is consistent with the behavior of a cell that lacks a protein required for a checkpoint mechanism that operates in G2? A) The cell would be unable to enter M phase B) The cell would be unable to enter G2 C) The cell would enter M phase under conditions when normal cells would not D) The cell would pass through M phase more slowly than normal cells
Answer: C The cell would enter M phase under conditions when normal cells would not Normal cells arrest at the G2 checkpoint if DNA replication is incomplete or DNA is damaged. Cells without this mechanism may enter M phase with unreplicated or damaged DNA, whereas normal cells would not.
Which of the following are true statements about skeletal muscle fibers? A) Skeletal muscle fibers are large multi-nucleated cells B) Skeletal muscle fibers include a specialized portion of the endoplasmic reticulum called the sarcoplasmic reticulum C) Each skeletal muscle fiber will include many myofibrils within its cytoplasm D) All of the above
Answer: D All of the above All of these statements are true
Which of the following statements about the structure of microtubules is FALSE? A) Microtubules are built from protofilaments that come together to make a hollow structure B) The two ends of a protofilament are chemically distinct, with alpha-tubulin exposed at one end and beta-tubulin exposed at the other end. C) Within a microtubule, all protofilaments are arranged in the same orientation, giving the microtubule structural polarity. D) Alpha-Tubulin and Beta-Tubulin are covalently bound to make the tubulin dimer that then assembles into protofilaments
Answer: D Alpha-Tubulin and Beta-Tubulin are covalently bound to make the tubulin dimer that then assembles into protofilaments Remember that α-Tubulin and β-tubulin bind with each other through non-covalent interactions. This is a recurring feature of cytoskeletal bonding that makes the cytoskeleton easily able to assemble and reassemble when needed.
Which of the following statements about apoptosis is TRUE? A) Cells that constitutively express Bcl2 will be more prone to undergo apoptosis B) The procaspases are catalytically active upon initial expression in the cell C) Bax and Bak promote apoptosis by binding to procaspases in the apoptosome D) Apoptosis can be promoted by the release of cytochrome c into the cytosol from mitochondria
Answer: D Apoptosis can be promoted by the release of cytochrome c into the cytosol from mitochondria A is INCORRECT because Bcl2 tends to inhibit rather than promote apoptosis. B is INCORRECT because procaspases are initially inactive and require activation. C is INCORRECT because, when activated, Bax and Bak promote apoptosis by stimulating the release of cytochrome c from mitochondria into the cytosol, not by binding to procaspases in the apoptosome. D is CORRECT as once cytochrome c is released into the cytosol, cytochrome c then binds to an adaptor protein, causing it to assemble into a seven-armed complex called the apoptosome. Apoptosome activity enables the activation of caspase cascades leading to apoptosis.
Levels of Cdk activity change during the cell cycle, in part because: A) The Cdks phosphorylate each other B) The Cdks activate the cyclins C) Cdk degradation precedes entry into the next phase of the cell cycle D) Cyclin activity and concentrations change during the cycle
Answer: D Cyclin activity and concentrations change during the cycle Cyclin concentrations and activity fluctuate throughout the cell cycle and are required for Cdk activity. M-cyclins are degraded in a cell-cycle-dependent fashion (not Cdks), and they are required for Cdk activity. Cdks do not phosphorylate each other. The Cdks do not activate the cyclins.
During sexual reproduction, novel mixtures of alleles are generated. This is because: A) In all diploid species, two alleles exist for every gene B) A diploid individual has two different alleles for every gene C) Every gamete produced by a diploid individual has several different alleles of a single gene D) During meiosis, the segregation of homologs is random such that different gametes end up with different alleles of each gene.
Answer: D During meiosis, the segregation of homologs is random such that different gametes end up with different alleles of each gene. A is INCORRECT because MANY alleles (not just 2) alleles can exist for any gene B is INCORRECT because a diploid individual can have either two different or two identical alleles of any given gene. They can be either heterozygous or homozygous for a particular allele. C is INCORRECT because each gamete has only a single allele of any gene. D is CORRECT
The G1 DNA damage checkpoint: A) Causes cells to proceed through S phase more quickly B) Involves the degradation of p53 C) Is activated by errors caused during DNA replication D) Involves the inhibition of cyclin-Cdk complexes by p21
Answer: D Involves the inhibition of cyclin-Cdk complexes by p21
Which of the following statements is FALSE? A) Cytokinesis in plant cells must include the formation of new cell way B) Small membrane vesicles derived from the Golgi apparatus deliver new cell-wall material for the new wall of the dividing cell C) The phragmoplast forms from the remains of interpolar microtubules of the mitotic spindle D) Motor proteins walking along the cytoskeleton are important for the contractile ring that guides the formation of the new cell wall.
Answer: D Motor proteins walking along the cytoskeleton are important for the contractile ring that guides the formation of the new cell wall. D is false because no contractile ring is formed during plant cytokinesis.
Keratins, neurofilaments, and vimentins are 3 of the 4 categories of intermediate filaments. Which of the following properties is NOT true of these types of intermediate filaments? A) They strengthen cells against mechanical stress B) Dimers associate by non-covalent bonding to form a tetramer C) They are found in the cytoplasm D) Phosphorylation causes disassembly during every mitotic cycle
Answer: D Phosphorylation causes disassembly during every mitotic cycle D is NOT TRUE because keratins, neurofilaments, and vimentins are cytoplasmic intermediate filaments, which tend to be very stable once formed. The nuclear intermediate filaments are disassembled and reformed during mitosis; this process is regulated by phosphorylation.
Which of the following proteins is an accessory protein that assists intermediate filaments in binding to other cytoskeletal components and links nuclear and cytoskeletal proteins? A) Calmodulin B) Lamin A C) Keratin D) Plectin
Answer: D Plectin Remember that we saw the electron micrograph showing plectin linking an intermediate filament to microtubules. And Figure 17-10 showed plectin as part of protein complexes linking the nucleus and cytoplasm.
What would be the most obvious outcome of repeated cell cycles consisting of S phase and M phase only? A) The cells would not be able to replicate their DNA B) The mitotic spindle could not assemble C) The cells would get larger and larger D) The cells produced would get smaller and smaller
Answer: D The cells produced would get smaller and smaller If a cell is only going through the S phase where it replicates DNA and M phase where it divides, the cells produced would get smaller and smaller, as they would not have sufficient time to double their mass before dividing. This is often the case with cell divisions taking place during early embryonic development. The organism starts as a relatively large fertilized egg and through many rapid divisions, we see a rapid increase in the number of cells, but a decrease in their average cell size.
Which of the following statements is FALSE? A) The cleavage furrow is a puckering of the plasma membrane caused by the constriction of a ring of filaments attached to the plasma membrane. B) The cleavage furrow will not begin to form in the absence of a mitotic spindle C) The cleavage furrow always forms perpendicular to the interpolar microtubules D) The cleavage furrow always forms in the middle of the cell
Answer: D The cleavage furrow always forms in the middle of the cell Although the furrow always forms perpendicular to the interpolar microtubules about midway between the spindle poles, if the spindle were in an asymmetrical position (which we discussed can occur normally during development), cell division would not always occur in the middle of the cell.
Which of the following statements about actin is FALSE? A) ATP hydrolysis decreases actin filament stability B) Actin at the cell cortex helps govern the shape of the plasma membrane C) Actin filaments are nucleated at the side of existing actin filaments in lamellipodia D) The dynamic instability of actin filaments is important for cell movement
Answer: D The dynamic instability of actin filaments is important for cell movement D is FALSE because dynamic instability is a phenomenon associated with microtubules and not actin. Actin disassembly and assembly are both important for cell movement. However, the process of disassembly and reassembly differs from dynamic instability in that the growth of actin filaments occurs at the leading edge; this growth occurs in a directed fashion because of actin-binding proteins that promote the formation of new filaments at the leading edge.
Organisms that reproduce sexually: A) Must be haploid, unlike organisms that reproduce asexually B) Can reproduce only with a partner that carriers the same alleles C) Create zygotes that are genetically identical to each other D) Undergo a sexual reproductive cycle that involves an alternation of haploid cells with the generation of diploid cells
Answer: D Undergo a sexual reproductive cycle that involves an alternation of haploid cells with the generation of diploid cells A is INCORRECT because organisms that reproduce sexually are generally diploid B is INCORRECT because sexually reproducing organisms can reproduce with partners carrying different alleles - this is one advantage of sexual reproduction C is INCORRECT because sexual reproduction produces zygotes that are genetically unique - again this is an advantage of sexual reproduction D is CORRECT
Short Answer: Mitotic Spindle Formation In the lab, you are able to manipulate an animal cell experimentally to create the following conditions. In each condition, would the typical mitotic spindle form? If yes, how? If no, why not? Condition 1: The cell has no centrosome Condition 2: The cell has no centrosome and no kinetochores
For Condition 1, Yes, the cell could still produce a mitotic spindle. We discussed earlier that cells that normally don't have centrosomes and cells that have been experimentally manipulated to not have centrosomes form their mitotic spindles by nucleating microtubules from the chromosomes and with assistance from motor proteins. For Condition 2, No, the cell would not be likely to produce a mitotic spindle. Without a centrosome, the microtubules can't use their usual method of nucleation at the centrosome to form, so would rely on chromosomes as their nucleation point. But we learned that kinetochores are the attachment point for microtubules to chromosomes, so this alternate mechanism for spindle formation is also not available.
Short Answer: Ciliopathies Ciliopathies are diseases caused by the dysfunction of cilia. Based on what we learned about the normal function of cells in the human respiratory tract, what respiratory-related outcomes would you expect in patients with ciliopathies? The best answers will start by explaining what we know about healthy cells in the respiratory tract - their features and functions - and then will explain what could occur in the disease state.
We learned that epithelial cells in the human respiratory tract show thick tufts of cilia extending from these ciliated cells. Healthy ciliated cells in the respiratory tract will use those cilia to move along layers of mucus that have trapped dust particles, dead cells, debris, and pathogens (like bacteria and viruses) up toward the throat where we will swallow or cough out the mucus for elimination from the body. However, in the case of a person with ciliopathy, their cilia are not able to move the mucus along and are therefore not able to clear out debris or pathogens. This situation leads to a higher susceptibility to bronchial infections.