Science Practice Challenge Questions

11.1 The Process of Meiosis


Meiosis involves processes that are common to all eukaryotes, involving the same or similar genes. Evaluate the support for the theory of evolution provided by this evidence and, additionally, by the absence of any alternative process.


Meiotic phases of yeast cells were observed microscopically with fluorescent markers [Nachman et al, Cell, 131(3), 2007] to determine the time intervals of meiosis I and meiosis II. The data are displayed in the following figure:

A bar graph shows the number of yeast cells entering meiosis II and meiosis 1 over a 20-hour period. All cells entered meiosis II within a narrow time band, between 1 and 3 hours after the start of the measured time period, with the largest fraction of cells, 0.7, entering meiosis II at 1 hour. No meiotic activity was observed between hours 3 and 6. From hour 7 to hour 19, cells entered meiosis I. However, only a small fraction of cells, 0.1 or less, was in meiosis I at any given time.
Figure 11.11

The duration of meiosis I is measured relative to the transfer of spores to the growth medium. The duration of meiosis II is measured relative to the emergence from meiosis I. On the y-axis, the fraction of cells observed to enter each phase are shown, where the sampling has been made in increments of 0.5 hours.

  1. Qualitatively compare the mean and standard deviation for these two distributions.
  2. The gene Ime1 is transcribed at the start of meiosis I in response to nitrogen starvation. This activates Ime2 that interacts with Ime1. If, during meiosis I, the cells are supplied with nitrogen, meiosis is halted. Based on these data, justify the claim that this interaction provides a negative feedback loop.
  3. Explain the advantage provided to the population and the risk to individual cells of the timing of meiosis displayed in the graph above.

Construct an explanation as to how DNA is transmitted to the next generation via meiosis followed by fertilization.

11.2 Sexual Reproduction


In eukaryotes, sexual reproduction involves the recombination of heritable information from both parents via meiosis followed by fertilization. Meiosis reduces the number of chromosomes from diploid (2n) to haploid (1n) during the production of gametes. Meiosis begins with the duplication of DNA, producing four strands of DNA in two pairs of homologous chromosomes: 2(2n) becomes 4(n), that is, four haploid cells, where n is the number of strands of DNA in a chromosome.

A. Construct an explanation of the importance of random, independent assortment to genetic variation by creating a diagram that represents homologous chromosomes during prophase I without crossover and the possible arrangements of these chromosomes during metaphase I:

  • without recombination during prophase I
  • with recombination involving two chiasmata

B. An alternative would be to bypass the initial duplication of DNA: 2n becomes 2(n), that is, a diploid cell becomes two haploid cells. Predict the effect that this would have on genetic variation.