| Date | November 2010 | Marks available | 2 | Reference code | 10N.2.HL.TZ0.1 |
| Level | Higher level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Deduce | Question number | 1 | Adapted from | N/A |
Question
Gibberellin promotes both seed germination and plant growth. Researchers hypothesize that the gene GID1 in rice (Oryza sativa) codes for the production of a cell receptor for gibberellin. The mutant variety gid1-1 for that gene leads to rice plants with a severe dwarf phenotype and infertile flowers when homozygous recessive. It is suspected that homozygous recessive gid1-1 plants fail to degrade the protein SLR1 which, when present, inhibits the action of gibberellin. The graphs show the action of gibberellin on the leaves and α-amylase activity of wild-type rice plants (WT) and their gid1-1 mutants.
Most rice varieties are intolerant to sustained submergence under water and will usually die within a week. Researchers have 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.
The OsGI gene causes long-day flowering and the effect of its overexpression has been observed in a transgenic variety of rice. Some wild-type rice (WT) and transgenic plants were exposed to long days (14 hours of light per day) and others to short days (9 hours of light per day).
State which variety of rice fails to respond to gibberellin treatment.
The activity of α-amylase was tested at successive concentrations of gibberellin. Determine the increment in gibberellin concentration that produces the greatest change in α-amylase activity in wild-type rice plants (WT).
Discuss the consequence of crossing gid1-1 heterozygous rice plants amongst themselves for food production.
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.
Using only this data, deduce which gene confers submersion resistance to rice plants.
State the overall effect of overexpression of the OsGI gene in plants treated with short-day light.
Compare the results between the plants treated with short-day light and the plants treated with long-day light.
State, giving one reason taken from the data opposite, if unmodified rice is a short-day plant or a long-day plant.
Evaluate, using all the data, how modified varieties of rice could be used to overcome food shortages in some countries.
Markscheme
gid1-1
between 10–8 and 10–7 mol dm–3 (units required)
a. 25% / 1 in 4 / 1:3 seeds produced would be homozygous recessive;
b. no response to/inhibits gibberellin in homozygous recessives results in less germination;
c. less growth / dwarf plants produced; (must be in context);
d. would produce plants with infertile flowers that cannot produce rice grains;
e. would lower rice production/less yield because infertile plants cannot produce seeds (that humans can eat);
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;
b. is only expressed in indica / Sub1B and SubC are expressed in both rice varieties;
c. indica is the variety showing submersion tolerance / vice versa for japonica;
it increases the length of time before flowering
a. long-day light exposure increases time before flowering only if (OsGI) gene is not overexpressed/in WT and –/–;
b. long-day light exposure decreases time before flowering for +/– and/or +/+;
c. length of day does not make much difference/makes least difference for +/+;
d. overexpression for +/– reduces time before flowering;
e. –/– acts as a control / has nearly the same length of time before flowering as WT;
Accept numerical answers if they are making a clear comparison.
is a short-day plant because WT has shortest time/shorter time before flowering in shorter days than longer days / as it takes less time to flower under short day conditions;
a. the mutant gid1-1 would not be useful because it produces sterile plants;
b. genetically modified rice/rice with Sub1A is more tolerant to submersion/can withstand seasonal flooding/torrential rain;
c. OsGI+ varieties adapted to different latitudes / day length could be produced (to overcome food shortages);
d. short flowering time possibly means more crops per year;
Examiners report
The word “increment” seemed to confuse the weaker candidates who stated a value rather than a range. In addition there were a large number who omitted or misquoted the units. In spite of being clearly stated in topic 9.3.5, very few candidates correctly gained the mark in part (iii) for saying that the amylase catalysed the breakdown of starch to maltose. Many answered glucose instead of maltose, but a surprising number did not even realise that amylase is an enzyme.
The word “increment” seemed to confuse the weaker candidates who stated a value rather than a range. In addition there were a large number who omitted or misquoted the units. In spite of being clearly stated in topic 9.3.5, very few candidates correctly gained the mark in part (iii) for saying that the amylase catalysed the breakdown of starch to maltose. Many answered glucose instead of maltose, but a surprising number did not even realise that amylase is an enzyme.
Most of the better candidates realised that it was a simple monohybrid cross (although several thought it was dihybrid) and realised that 25% would produce dwarf plants, but did not explain the consequences on potential yield in sufficient detail for the third mark.
In spite of doubts from the G2 forms, candidates had little difficulty in interpreting the photograph.
In part (i) most correctly answered Sub1C.
The answers to (ii) tended to be descriptive, not making clear differences, as asked.
Most candidates correctly identified Sub1A with a correct reason.
Most answered correctly that it increased the time before flowering.
In (ii) almost every correct answer was from the first two mark points.
In (iii) most candidates identified it as a short-day plant with reasons.
In spite of the stem saying “using all the data”, most of the answers were very vague and did not use the data. The ideas that the mutant gid1-1 should be avoided as it produces sterile plants and those modified with Sub1A would withstand seasonal flooding were missed by most candidates.
