Question 20N.3.HL.TZ0.13
| Date | November 2020 | Marks available | [Maximum mark: 7] | Reference code | 20N.3.HL.TZ0.13 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Define, Describe, Identify | Question number | 13 | Adapted from | N/A |
Plant-derived proteins are likely to be safer for human use than those derived from mammalian cell cultures, as plant pathogens are not harmful to humans. The hepatitis B vaccine has been produced in tobacco plants.
Describe how the tobacco mosaic virus is used in the production of hepatitis B vaccine.
[3]
a. fusion between genes of (viral) capsid and hepatitis B antigen/HBsAg;
b. (transformed) virus infects tobacco plant;
c. tobacco plant expresses antigen/HBsAg;
d. HBsAg producing plants fed to animals;
e. animals produce anti-HBsAg antibodies;
Most have some knowledge of the usage of the tobacco mosaic virus but had difficulty in providing clear answers.
In an experiment, soybean (Glycine max) cells were transformed using Agrobacterium tumefaciens to produce hepatitis B surface antigen (HBsAg). The amount of HBsAg made by the transformed soybean cells was measured at different times after transformation. The results are shown in the bar chart.
[Source: Reprinted by permission from Springer Nature from Plant Cell Reports.
Analysis of the limitations of hepatitis B surface antigen expression in soybean cell suspension cultures.
Ganapathi, T.R., Sunil Kumar, G.B., Srinivas, L., Revathi, C.J. and Bapat, V.A., © 2007. https://doi.org/10.1007/s00299-007-0379-7.]
Using the data, identify one limitation of using soybean cell cultures.
[1]
as time passes, the soybean produces less antigen / stabilizes to a lower level (after 6 months)
Most candidates correctly identified that the effect is reduced over time.
The open reading frame (ORF) of HBsAg used in tobacco plants was the same one used in soybean plants. Define ORF.
[1]
DNA/nucleotide sequence with a start codon and stop codon coding for a polypeptide chain;
It was difficult for many candidates to specify that a start and a stop codon were necessary in the definition of an open reading frame.
Describe one bioinformatic method that could have been used to find the gene sequence for HBsAg.
[2]
a. gene databank; (e.g. GenBank/NCBI);
b. BLASTn to search similar DNA sequences;
c. ORF finder to search for the start codon;
Alternative:
d. protein database search for antigen (sequence);
e. example of database; (e.g. NCBI/SwissPro/Uniprot/PDB/other checked database);
f. BLASTp to search similar protein sequences;
Most answers didn't contain enough elements, although many mentioned BLASTn.