| Date | November 2010 | Marks available | 2 | Reference code | 10N.2.SL.TZ0.1 |
| Level | Standard level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Calculate | Question number | 1 | Adapted from | N/A |
Question
Rice (Oryza sativa) is usually intolerant to sustained submergence under water, although it grows rapidly in height for a few days before dying. This is true for one variety, Oryza sativa japonica. The variety Oryza sativa indica is much more tolerant to submergence.
Three genetically modified forms of O. sativa japonica, GMFA, GMFB and GMFC, were made using different fragments of DNA taken from O. sativa indica.
The plants were then submerged for a period of 11 days. The heights of all the plants were measured at the beginning and at the end of the submergence period.
In the same experiment, the researchers hypothesized that the capacity to survive when submerged is related to the presence of three genes very close to each other on rice chromosome number 9; these genes were named Sub1A, Sub1B and Sub1C. The photograph below of part of a gel shows relative amounts of messenger RNA produced from these three genes by the submergence-intolerant variety, O. sativa japonica, and by the submergence-tolerant variety, O. sativa indica, at different times of a submergence period, followed by a recovery period out of water.
State which group of rice plants were the shortest at the beginning of the experiment.
Calculate the percentage change in height for the O. sativa japonica unmodified variety during the submergence period. Show your working.
Deduce the general relationship between the growth of all the japonica varieties and their stated tolerance level.
Outline the use of the binomial system of nomenclature in Oryza sativa.
Determine which gene produced the most mRNA on the first day of the submergence period for variety O. sativa japonica.
Outline the difference in mRNA production for the three genes during the submergence period for variety O. sativa indica.
Compare the mRNA production for the three genes during the submergence period between the two varieties.
Deduce, using all the data, which gene was used to modify GMFC.
Evaluate, using all the data, how modified varieties of rice could be used to overcome food shortages in some countries.
Markscheme
(GMF) C
inversely proportional / the higher the tolerance, the less the growth / vice-versa
a. first name/Oryza for genus / second name/sativa for species;
b. (all) members of Oryza satica share special/unique features;
c. two names make a unique combination to designate species / worldwide recognizable nomenclature;
d. varieties (japonica and indica) have some (consistent) differences (in tolerance);
Sub1C
a. Sub1A is expressed strongly/the most / Sub1A produces the most RNA;
b. Sub1B (always) has the lowest expression/produces least mRNA;
c. Sub1A expressed/produces mRNA for the longest time/days 1 to 10;
d. Sub1C expressed/produces mRNA for the shortest time/days 3 to 7;
a. Sub1A only expressed/produces mRNA in indica / not/never expressed/ never produces mRNA in japonica;
b. Sub1C expressed/produces mRNA from day 1 in japonica, but not indica;
c. Sub1B has lower expression/production of mRNA than Sub1C in both varieties;
Award [1 max] for other accurate comparisons between japonica and indica.
a. Sub1A;
b. is only expressed in indica;
c. indica is the variety showing submersion tolerance;
a. genetically modified rice/rice with Sub1A is more tolerant to submersion;
b. can withstand seasonal flooding/torrential rain;
c. GMF/tolerant rice ensures greater harvest/provides more food during flooding;
Examiners report
Generally well done. A few wrote only GMF.
Little understanding shown. Many divided the difference in height by 50 instead of 22.
Many candidates worded generalized relationships such as the higher the tolerance, the less the growth or growth and tolerance were inversely proportional. Sometimes “height” was given rather than “growth”.
The designation of Oryza as genus and sativa as species was the only marking point that many candidate got correct in this question, although some candidates mixed up the terms calling Oryza the species and sativa the genus. Very few candidates went beyond to mention that O. sativa shared special features. Even fewer candidates mentioned that the varieties japonica and indica had differences in tolerance. Occasionally, a candidate mentioned that binomial nomenclature helps scientists communicate about the same plant or the worldwide acceptance for the terminology.
Some candidates did not appreciate that the actual production of each gene was indicated by the intensity of the bands shown on the photograph of electrophoresis.
Since the question asked for differences in mRNA production for the three genes, it was important that candidates used quantitative wording such as Sub1A produces the “most” mRNA or that Sub1B produces the “lowest” or “least” mRNA to convey a sense of comparison. A few candidates noted that Sub1A produced mRNA for the “longest” time/days 1 to 0 and/or that Sub1C produced mRNA for the “shortest” time/days 3 to 7.
Many valid comparisons could be made comparing the mRNA production for the three genes. Most often given was that Sub1A only produced mRNA in japonica and/or never in indica. The two mark maximum was achieved frequently.
The question was poorly answered. Though Sub1A was sometimes correctly identified as the gene to modify GMFC, reasoning to support that answer was usually incorrect or missing.
Many candidates missed the question by trying to relate GMFs to drought conditions rather than flooding. GMFs offered tolerance to submersion enabling them to withstand flooding so that greater harvests/food production were ensured during flooding.
