Question
Topic – 5.1 Meiosis
(a) – Topic – 1.3 Introduction to Macromolecules
(b) – Topic – 5.1 Meiosis
(c) – Topic – 5.1 Meiosis
(d) – Topic – 5.1 Meiosis
During meiosis, double-strand breaks occur in chromatids. The breaks are either repaired by the exchange of genetic material between homologous nonsister chromatids, which is the process known as crossing over (Figure 1A), or they are simply repaired without any crossing over (Figure 1B). Plant breeders developing new varieties of corn are interested in determining whether, in corn, a correlation exists between the number of meiotic double-strand chromatid breaks and the number of crossovers.
Using specialized staining and microscopy techniques, scientists counted the number of double-strand chromatid breaks and the number of crossovers in the same number of meiotic gamete-forming cells of six inbred strains of corn (Table 1).
TABLE 1. NUMBER OF CHROMATID DOUBLE-STRAND BREAKS AND AVERAGE NUMBER OF
CROSSOVERS IN INBRED STRAINS OF CORN
(a) The double-strand breaks occur along the DNA backbone. Describe the process by which the breaks occur.
(b) Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand
breaks and crossovers. Based on the data, determine whether corn strains I, II, and III differ in their average number of crossovers.
(c) Based on the data, describe the relationship between the average number of double-strand breaks and the average number of crossovers in the strains of corn analyzed in the experiment.
(d) Crossing over (Figure 1A) creates physical connections that are required for proper separation of homologous chromosomes during meiosis. A diploid cell with four pairs of homologous chromosomes undergoes meiosis to produce four haploid cells. Crossing over occurs between only three of the pairs. Predict the number of chromosomes most likely present in each of the four haploid cells. Provide reasoning to justify your prediction. Explain how plant breeders can use the information in Table 1 to help develop new varieties of corn.
▶️Answer/Explanation
2(a) The double-strand breaks occur along the DNA backbone. Describe the process by which the breaks occur.
Accept one of the following:
• (Enzymatic) hydrolysis occurs between the sugars and phosphates/nucleotides.
• The covalent bonds between the sugars and phosphates/nucleotides are broken.
2(b) Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand breaks and crossovers.
• Appropriate axis scaling
Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand breaks and crossovers.
• Accurately plotted X,Y graph with separate points for the average number of crossovers for each strain Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand breaks and crossovers.
• Accurate error barz Based on the data, determine whether corn strains I , II , and III differ in their average number of crossovers.
• There is no (statistical) difference (in the average number of crossovers) between strains II and III. Strain I is higher/different (in the average number of crossovers) compared with strains II and III.
2(c) Based on the data, describe the relationship between the average number of doublestrand breaks and the average number of crossovers in the strains of corn analyzed in the experiment.
• (In general) there is a direct correlation/positive relationship (between the number of double-strand breaks and the number of chromatid crossovers).
2(d) Crossing over (Figure 1A) creates physical connections that are required for proper separation of homologous chromosomes during meiosis. A diploid cell with four pairs of homologous chromosomes undergoes meiosis to produce four haploid cells. Crossing over occurs between only three of the pairs. Predict the number of chromosomes most likely present in each of the four haploid cells.
• Two cells will have three/n−1 chromosomes; two cells will have five/n+1 chromosomes.Provide reasoning to justify your prediction.
• During meiosis I , (three homologous pairs separate normally, and) one pair does not separate/experiences nondisjunction. In meiosis II , the sister chromatids separate normally.
Explain how plant breeders can use the information in Table 1 to help develop new varieties of corn.
Accept one of the following:
• Because crossing over increases genetic diversity, the plant breeders can breed strains with high crossover numbers/double-strand breaks.
• They can increase the number of double-stranded breaks, which may lead to more crossovers that increase genetic variation.