Passage
Trypanosoma brucei is a eukaryotic parasite that causes sleeping sickness in Sub-Saharan Africa. The cell cycle of T. brucei differs from that of higher eukaryotes. For example, the chromatin does not condense during mitosis, and the nuclear envelope remains intact. In addition, the nucleus divides before cell division, and the cell divides along the longitudinal axis (Figure 1) .
Figure 1 Mitosis in T. bruceiA culture of T. brucei was arrested in S phase of the cell cycle by overnight incubation with the reversible inhibitor hydroxyurea. The cells were then washed, and their progression through the cell cycle was monitored by flow cytometry and microscopy. A subset of the culture was first exposed to the drug VX-680, which arrests cells upon completion of metaphase. The results are shown in Figure 2.
Figure 2 Percentages of a population of T. brucei cells in each phase of the cell cycle, measured at different time points in the (A) absence and (B) presence of VX-680
Adapted from Li Z, Umeyama T, Wang CC. The Aurora Kinase in Trypanosoma brucei plays distinctive roles in metaphase-anaphase transition and cytokinetic initiation. PLoS Pathog. 2009;5(9) :e1000575.
-Based on the passage, VX-680 most likely inhibits:

Question

Passage
Trypanosoma brucei is a eukaryotic parasite that causes sleeping sickness in Sub-Saharan Africa. The cell cycle of T. brucei differs from that of higher eukaryotes. For example, the chromatin does not condense during mitosis, and the nuclear envelope remains intact. In addition, the nucleus divides before cell division, and the cell divides along the longitudinal axis (Figure 1) .

Figure 1 Mitosis in T. bruceiA culture of T. brucei was arrested in S phase of the cell cycle by overnight incubation with the reversible inhibitor hydroxyurea. The cells were then washed, and their progression through the cell cycle was monitored by flow cytometry and microscopy. A subset of the culture was first exposed to the drug VX-680, which arrests cells upon completion of metaphase. The results are shown in Figure 2.

Figure 2 Percentages of a population of T. brucei cells in each phase of the cell cycle, measured at different time points in the (A) absence and (B) presence of VX-680
Adapted from Li Z, Umeyama T, Wang CC. The Aurora Kinase in Trypanosoma brucei plays distinctive roles in metaphase-anaphase transition and cytokinetic initiation. PLoS Pathog. 2009;5(9) :e1000575.
-Based on the passage, VX-680 most likely inhibits:

Quizplus · Accepted Answer

11ecba2d_101b_ba86_a3bf_cda5ff3e8a30_MD0008_00 Mitosis consists of four phases in most eukaryotes: Prophase, in which the nuclear envelope disintegrates and chromatin condenses. (Note: This phase does not occur in T. brucei.) Metaphase, in which chromosomes align on the metaphase plate. In this phase, the chromosomes become attached to spindle fibers. Anaphase, in which the spindle fibers shorten, causing sister chromatids to separate and migrate toward opposite poles of the cell. Telophase, in which a nuclear envelope reforms around each set of sister chromatids and the chromatin returns to its uncondensed form. (This phase does not occur in T. brucei because the nuclear envelope never disintegrates and chromatin does not condense, so the reverse processes cannot happen either.) The passage states that the drug VX-680 arrests cells upon completion of metaphase. Figure 2B confirms this by showing that in the presence of VX-680, T. brucei are able to begin the mitotic (M) phase but cannot complete it to progress back to G₁. Therefore, with VX-680 treatment, T. brucei cells complete metaphase but fail to enter anaphase. Because sister chromatids migrate toward opposite poles of the cell in anaphase, VX-680 most likely prevents anaphase by inhibiting this migration. (Choice A) Transcription factors regulate gene expression by promoting or repressing RNA synthesis. These factors are synthesized primarily during interphase and are unlikely to be inhibited by VX-680, which interferes with M phase. (Choice B) Genome replication occurs during S phase of the cell cycle. Figure 2B shows that in the presence of VX-680, cells are able to complete S phase but become stuck in M phase after many hours. Therefore, VX-680 does not inhibit chromosome replication. (Choice C) Chromosomes align in the center of the cell during metaphase. The passage states that VX-680 arrests cells after metaphase is complete, so alignment of chromosomes in the center of the cell is not inhibited. Educational objective: Mitosis typically consists of four phases: Prophase, in which the nuclear envelope disintegrates and chromatin condenses; metaphase, in which chromosomes align on the metaphase plate; anaphase, in which sister chromatids migrate toward opposite poles of the cell; and telophase, in which nuclear envelopes reform and chromatin reverts to its uncondensed form.

Passage Trypanosoma Brucei Is a Eukaryotic Parasite That Causes Sleeping Sickness