Q24 - Nuclear-to-Cytoplasmic Ratio and the Midblastula Transition in Xenopus

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

In the African clawed frog, Xenopus laevis, the mode of cell division shifts from cleavage to somatic cell division, which has interphase, at the 12th cleavage after fertilization. This is called the mid-blastula transition (MBT). Microinjection of mRNA of genes that are required for nuclear membrane formation at one-cell stage results in the increase of the nuclear size, but cell size does not change compared with a control embryo. In this experiment, MBT occurs earlier than the 12th cleavage (Figure 1, left). Conversely, when the nuclear size is artificially reduced, the cell size does not again change but MBT occurs later than the 12th cleavage (Figure 1, right). Note: These treatments do not alter the time required for each cleavage.

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. This experiment indicates that that MBT occurs when the volume ratio of nucleus/cytoplasm is high.

B. When MBT occurs before the 12th cleavage stage, the duration from the fertilization to the 12th cleavage stage is reduced.

C. The timing of MBT depends on the number of divisions after fertilization.

D. These results indicate that MBT occurs when the amount of histone per nucleus is greater than a certain value (Note: No manipulations performed in this experiment affect amount of histone).

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Question reproduced from IBO 2020, Theoretical Exam 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)