Eukaryotic organelles such as mitochondria and chloroplasts are believed to be relics of formerly free-living prokaryotes. The transition from a hypoxic (low O2) to an oxic atmosphere (21% O2) is said to have enabled primitive eukaryotic anaerobes to engulf ancient aerobic prokaryotes and consequently acquire the ability to produce energy through oxidative phosphorylation. This endosymbiotic theory of eukaryotic evolution also postulates that endosymbiosis resulted in larger eukaryotic genomes, which originated from the partial transfer of mitochondrial genes to the nuclear genome. On integration into the host genome, mitochondria-derived genes became indistinguishable from the original nuclear genes.Researchers have alternatively proposed that after a prolonged period of symbiosis, the possibility exists of gene transfer from eukaryotes to prokaryotes. This hypothesis was initially supported when copper/zinc (Cu/Zn) superoxide dismutase (SOD), a metalloprotein confined to the cytosol of eukaryotic cells, was found in Photobacterium leiognathi. The free-living bacterium P. leiognathi is also a known symbiont of ponyfish, a small fish species native to the Indian and Pacific Oceans.SODs are antioxidant enzymes that serve as the cell's first line of defense against reactive oxygen species (ROS). ROS produced by the electron transport chain damage proteins by oxidizing amino acid residues and metal ions on prosthetic groups but can accumulate during times of biochemical and environmental stress. Superoxide (O2−) radicals, a form of ROS, are sequestered by SODs and converted into less toxic hydrogen peroxide (H2O2) and O2 gas.Adapted from Bannister, J.V., & Parker, M.W. (1985). The presence of a copper/zinc superoxide dismutase in the bacterium Photobacterium leiognathi: a likely case of gene transfer from eukaryotes to prokaryotes. Proceedings of the National Academy of Sciences of the United States of America, 82(1), 149–152. Question 22Ponyfish cells containing P. leiognathi symbionts were exposed to a spindle fiber toxin that specifically inhibits microtubule polymerization. Given this information, which of the following would most likely result as a consequence of toxin exposure?A.P. leiognathi daughter cells with multiple copies of the Cu/Zn SOD geneB.Ponyfish daughter cells containing the same copy number of the Cu/Zn SOD geneC.Delayed separation of P. leiognathi cells during binary fissionD.Nondisjunction in somatic ponyfish cells undergoing nuclear division
Question
Eukaryotic organelles such as mitochondria and chloroplasts are believed to be relics of formerly free-living prokaryotes. The transition from a hypoxic (low O2) to an oxic atmosphere (21% O2) is said to have enabled primitive eukaryotic anaerobes to engulf ancient aerobic prokaryotes and consequently acquire the ability to produce energy through oxidative phosphorylation. This endosymbiotic theory of eukaryotic evolution also postulates that endosymbiosis resulted in larger eukaryotic genomes, which originated from the partial transfer of mitochondrial genes to the nuclear genome. On integration into the host genome, mitochondria-derived genes became indistinguishable from the original nuclear genes.Researchers have alternatively proposed that after a prolonged period of symbiosis, the possibility exists of gene transfer from eukaryotes to prokaryotes. This hypothesis was initially supported when copper/zinc (Cu/Zn) superoxide dismutase (SOD), a metalloprotein confined to the cytosol of eukaryotic cells, was found in Photobacterium leiognathi. The free-living bacterium P. leiognathi is also a known symbiont of ponyfish, a small fish species native to the Indian and Pacific Oceans.SODs are antioxidant enzymes that serve as the cell's first line of defense against reactive oxygen species (ROS). ROS produced by the electron transport chain damage proteins by oxidizing amino acid residues and metal ions on prosthetic groups but can accumulate during times of biochemical and environmental stress. Superoxide (O2−) radicals, a form of ROS, are sequestered by SODs and converted into less toxic hydrogen peroxide (H2O2) and O2 gas.Adapted from Bannister, J.V., & Parker, M.W. (1985). The presence of a copper/zinc superoxide dismutase in the bacterium Photobacterium leiognathi: a likely case of gene transfer from eukaryotes to prokaryotes. Proceedings of the National Academy of Sciences of the United States of America, 82(1), 149–152. Question 22Ponyfish cells containing P. leiognathi symbionts were exposed to a spindle fiber toxin that specifically inhibits microtubule polymerization. Given this information, which of the following would most likely result as a consequence of toxin exposure?A.P. leiognathi daughter cells with multiple copies of the Cu/Zn SOD geneB.Ponyfish daughter cells containing the same copy number of the Cu/Zn SOD geneC.Delayed separation of P. leiognathi cells during binary fissionD.Nondisjunction in somatic ponyfish cells undergoing nuclear division
Solution
The most likely result of toxin exposure, given that it specifically inhibits microtubule polymerization, would be C. Delayed separation of P. leiognathi cells during binary fission.
Microtubules play a crucial role in cell division, including in prokaryotes like P. leiognathi. They are involved in the separation of cells during binary fission, the process by which prokaryotes reproduce. If a toxin inhibits microtubule polymerization, it would interfere with the formation of the spindle fibers necessary for cell division, likely leading to a delay in the separation of the cells.
The toxin's effect on microtubule polymerization would not directly affect the copy number of the Cu/Zn SOD gene in either P. leiognathi or ponyfish cells (options A and B).
Option D, nondisjunction in somatic ponyfish cells undergoing nuclear division, could potentially occur if the toxin also affected the ponyfish cells. However, the question specifies that the toxin was exposed to ponyfish cells containing P. leiognathi symbionts, not the ponyfish cells themselves. Therefore, it's less likely that the toxin would directly cause nondisjunction in the ponyfish cells.
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