Early experimentation on the single-celled organism Acetabularia led to important discoveries about the role of the nucleus in regulating cell function. Acetabularia is an enormous single cell with three distinct regions: a cap, a root-like rhizoid, and a stalk which connects the two. The following experiments were conducted to study the development of the cell:Experiment 1The stalk of an Acetabularia was cut, fragmenting the cell. The fragment which included the cap died shortly afterwards while the fragment containing the rhizoid regenerated to form a complete Acetabularia.Experiment 2The nucleus from Acetabularia mediterranea, which has a flat cap, was transplanted into Acetabularia crenulata, which has a tufted cap, following removal of the Acetabularia crenulata nucleus. The Acetabularia crenulata cap eventually assumed the flat shape.Experiment 3The nucleus of Acetabularia mediterranea was removed from the young cell before it first formed a cap. A normal cap formed several weeks later. The cell proved to be inviable and died shortly thereafter.Experiment 4A young Acetabularia was fractioned into a number of portions before it first formed a cap. Several weeks later, both the portion containing the nucleus and the portion containing the apical tip of the stalk formed caps. The other portions did not form caps.It can be inferred from the experiments in the passage that development of the cap in Acetabularia is regulated by which of the following mechanisms?
Answer(s): C
Translational regulation occurs when the translation of RNA to protein is a regulated step. In Acetabularia, the mRNA is transcribed and stored in the cap for a number of weeks before being translated. This can be concluded from Experiment 3 (in which the cap is produced a number of weeks after the nucleus is removed) and from Experiment 4 (in which the apical stalk region produces a cap a number of weeks following fragmentation of the cell).Choice A is incorrect because transcriptional regulation occurs when transcription of RNA from DNA is regulated. Choice A indicates incorrectly that transcriptional regulation occurs when mRNA lies dormant. This is translational regulation.Choice B is incorrect because translational regulation occurs when RNA translation to protein is regulated.Choice B indicates incorrectly that translational regulation occurs when mRNA transcription from DNA is regulated.Choice D is incorrect because translational regulation is occurring in Acetabularia.
Early experimentation on the single-celled organism Acetabularia led to important discoveries about the role of the nucleus in regulating cell function. Acetabularia is an enormous single cell with three distinct regions: a cap, a root-like rhizoid, and a stalk which connects the two. The following experiments were conducted to study the development of the cell:Experiment 1The stalk of an Acetabularia was cut, fragmenting the cell. The fragment which included the cap died shortly afterwards while the fragment containing the rhizoid regenerated to form a complete Acetabularia.Experiment 2The nucleus from Acetabularia mediterranea, which has a flat cap, was transplanted into Acetabularia crenulata, which has a tufted cap, following removal of the Acetabularia crenulata nucleus. The Acetabularia crenulata cap eventually assumed the flat shape.Experiment 3The nucleus of Acetabularia mediterranea was removed from the young cell before it first formed a cap. A normal cap formed several weeks later. The cell proved to be inviable and died shortly thereafter.Experiment 4A young Acetabularia was fractioned into a number of portions before it first formed a cap. Several weeks later, both the portion containing the nucleus and the portion containing the apical tip of the stalk formed caps. The other portions did not form caps.The differences in cap structure between Acetabularia mediterranea and Acetabularia crenulata are caused by differences in:
Answer(s): A
The genotype consists of the genes (DNA) which code for specific proteins. Differences in these proteins lead to a difference in the appearance of an organism. Observable differences in the structure of the organism constitute differences in the phenotype. In this case, the genes coding for different cap structures are part of the cell's genotype. The manifestation of these genes, a flat or a tufted cap, is part of the cell's phenotype.Choice B is incorrect because the difference in phenotype does not cause the difference in cap structure, it is the difference in cap structure.Choice C is incorrect because Acetabularia mediterranea and Acetabularia crenulata do not differ in genus.Their genus is AcetabulariA. Their species are mediterranea and crenulata.Choice D is incorrect because Acetabularia mediterranea and Acetabularia crenulata do not differ in their phylum, only in their species. Organisms in the same genus must be in the same phylum (and kingdom, class, etc...).
Early experimentation on the single-celled organism Acetabularia led to important discoveries about the role of the nucleus in regulating cell function. Acetabularia is an enormous single cell with three distinct regions: a cap, a root-like rhizoid, and a stalk which connects the two. The following experiments were conducted to study the development of the cell:Experiment 1The stalk of an Acetabularia was cut, fragmenting the cell. The fragment which included the cap died shortly afterwards while the fragment containing the rhizoid regenerated to form a complete Acetabularia.Experiment 2The nucleus from Acetabularia mediterranea, which has a flat cap, was transplanted into Acetabularia crenulata, which has a tufted cap, following removal of the Acetabularia crenulata nucleus. The Acetabularia crenulata cap eventually assumed the flat shape.Experiment 3The nucleus of Acetabularia mediterranea was removed from the young cell before it first formed a cap. A normal cap formed several weeks later. The cell proved to be inviable and died shortly thereafter.Experiment 4A young Acetabularia was fractioned into a number of portions before it first formed a cap. Several weeks later, both the portion containing the nucleus and the portion containing the apical tip of the stalk formed caps. The other portions did not form caps.The mRNA sequence shown below is transcribed from which of the following DNA fragments?z5-UGUAAUC-3mRNA
Answer(s): D
mRNA is transcribed through complementary base pairing from the template strand of DNA. The sequence of mRNA runs antiparallel to the strand of DNA from which it was synthesized. The base pairs in DNA bind to base pairs in RNA in the following way G-C, C-G, A-U, T-A. Note that RNA contains uracil in place of thymine.The complementary strand of DNA to the given mRNA is:5-UGUAAUC-3 mRNA3-ACATTAG-5 DNAThe DNA strand above must be rewritten in the 5 3 direction to fit the answer choices.3-ACATTAG-5 DNA = 5-GATTACA-3Choice A is incorrect because there is no uracil in DNA.Choice B is incorrect because the sequence is backwards. Note that the strands must be antiparallel.Choice C is incorrect because the base pairs are not complementary.
Nitric oxide, NO, has recently been found to have widespread physiological effects, acting as a major regulator in the nervous, cardiovascular, and immune systems. The production of NO in the body is regulated by specific NOS enzymes which exist in at least three different isoforms bNOS, eNOS, and macNOS. Each of these isoforms differ in location and function and serve to mediate different physiological responses to NO. Some physiological roles of NO have been demonstrated as follows:I). In the central nervous system, NO production is regulated by bNOS. Calcium ion concentrations of 200-400 nM in the central nervous system activate bNOS to catalyze the formation of NO. NO exerts definite effects on brain function although its specific roles are not well established. bNOS inhibitors have been found to block the release of neurotransmitter from presynaptic neurons. Excess levels of NO are also thought to contribute to neurodegenerative disorders such as Alzheimer's disease.II). In the blood vessels, NO is produced by eNOS which is activated by Ca2+ concentrations of 200-400 nM. NO acts as the major endogenous vasodilator in blood vessels. It diffuses into smooth muscle cells and leads to muscle relaxation by stimulating cGMP formation through activation of guanylyl cyclase. In addition, NO regulates the vascular system by inhibiting platelet aggregation and adhesion.III). The role of NO in the immune system is regulated by macNOS through a pathway that is not Ca2+ dependent. Rather, exposure to cytokines, including interleukin-1 and interferon- , leads to synthesis of large amounts of NO by activation of macNOS in response to inflammatory stimulI). The NO produced plays a definitive role in the mediation of the activities of macrophages and neutrophils. NO also acts to inhibit the mechanism of viral replication.Based on information in the passage, which of the following is a possible effect of NO in the brain?
Answer(s): B
The passage states that bNOS inhibitors have been found to block the release of neurotransmitter. Since bNOS inhibitors would decrease the production of NO, this implies that NO functions to promote the release of neurotransmitter. The passage does not describe the mechanism by which NO promotes neurotransmitter release, so any mechanism that would increase such release is a possibility.Choice A is incorrect because NO promotes the release of neurotransmitter. Inhibiting the fusing of vesicles at the presynaptic membrane would inhibit the release of neurotransmitter.Choice C is incorrect because the passage states that NO acts as a vasodilator, not as a vasoconstrictor.Besides, vasoconstriction to maintain blood pressure is not necessarily related to brain function.Choice D is incorrect because NO promotes the release of neurotransmitter. While the passage does not imply a role for NO in neurotransmitter synthesis, decreasing neurotransmitter synthesis would most likely tend to decrease the amount of neurotransmitter released.
Nitric oxide, NO, has recently been found to have widespread physiological effects, acting as a major regulator in the nervous, cardiovascular, and immune systems. The production of NO in the body is regulated by specific NOS enzymes which exist in at least three different isoforms bNOS, eNOS, and macNOS. Each of these isoforms differ in location and function and serve to mediate different physiological responses to NO. Some physiological roles of NO have been demonstrated as follows:I). In the central nervous system, NO production is regulated by bNOS. Calcium ion concentrations of 200-400 nM in the central nervous system activate bNOS to catalyze the formation of NO. NO exerts definite effects on brain function although its specific roles are not well established. bNOS inhibitors have been found to block the release of neurotransmitter from presynaptic neurons. Excess levels of NO are also thought to contribute to neurodegenerative disorders such as Alzheimer's disease.II). In the blood vessels, NO is produced by eNOS which is activated by Ca2+ concentrations of 200-400 nM. NO acts as the major endogenous vasodilator in blood vessels. It diffuses into smooth muscle cells and leads to muscle relaxation by stimulating cGMP formation through activation of guanylyl cyclase. In addition, NO regulates the vascular system by inhibiting platelet aggregation and adhesion.III). The role of NO in the immune system is regulated by macNOS through a pathway that is not Ca2+ dependent. Rather, exposure to cytokines, including interleukin-1 and interferon- , leads to synthesis of large amounts of NO by activation of macNOS in response to inflammatory stimulI). The NO produced plays a definitive role in the mediation of the activities of macrophages and neutrophils. NO also acts to inhibit the mechanism of viral replication.A "knock out" mouse with a mutant bNOS protein was generated by recombination techniques. The mutant protein was identical to the wild-type protein except for the identity of amino acid 675; the mutated bNOS has Tryptophan instead of Cysteine at position 675. Which of the following is responsible for the mutant protein?
A point mutation is the substitution of a single nucleotide. Point mutations can result in missense mutations in which a single amino acid is substituted for another in the translated protein, or in nonsense mutations. The mutation described in the question is a missense mutation.Choice A is incorrect because a frame shift mutation changes the reading frame of the mRNA and would change the amino acid sequence of the entire protein from that point on.Choice B is incorrect because deletion of a single base pair would be a frame shift mutation leading to changes in the amino acid sequence from that point on.Choice D is incorrect because a nonsense mutation is a point mutation that introduces a stop codon into the mRNA. This leads to translation of a shorter protein.
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