Biology 9700/43 — October/November 2014
Cambridge A-Level · A Level Structured Questions · worked solutions for every part, with the mark scheme
Topics Inheritance · Genetic Technology · Selection and Evolution · Classification, Biodiversity and Conservation · Energy and Respiration · (outdated) Biotechnology · +4 more
Phenylketonuria (PKU) is a genetic disease which results in a raised concentration of the amino acid phenylalanine in the blood. If left untreated in a newborn baby, it can lead to brain damage. For this reason, babies may be tested for PKU soon after birth.
In people without PKU, excess phenylalanine is converted to the amino acid tyrosine by the enzyme phenylalanine hydroxylase (PAH). One of the mutations that can lead to PKU is caused by a base substitution in the gene coding for PAH. This results in the replacement of the amino acid arginine by the amino acid glutamine.
Table 1.1 shows the triplet base codes for arginine and glutamine.
Table 1.1
| amino acid | DNA triplet |
|---|---|
| arginine | GCA |
| GCG | |
| GCT | |
| GCC | |
| TCT | |
| TCC | |
| glutamine | GTT |
| GTC |
Using the information in Table 1.1, explain how a single base substitution can replace arginine with glutamine in PAH.
Suggest the consequence to the structure of PAH as a result of a single base substitution in its gene.
For some genetic diseases, the proportion of individuals that have the disease can vary in different parts of the world. Sometimes this is due to different environmental conditions. The number of cases of PKU is approximately ten times higher in Europe than in Sub-Saharan Africa.
- Individuals who are carriers of the recessive allele for PKU are more resistant to ochratoxin A, produced by a fungus found on contaminated grain products.
- Ochratoxin A has been linked to a form of renal cancer.
- The fungi that produce ochratoxin A are less common in dry regions such as Sub-Saharan Africa.
Suggest and explain the mechanism which has resulted in the different number of cases of PKU in European and Sub-Saharan populations.
The rest of this paper
9 more questions- Q2Classification, Biodiversity and Conservation9M
- Q3Genetic Technology9M
- Q4Energy and Respiration14M
- Q5(outdated) Biotechnology14M
- Q6Homeostasis · Control and Coordination8M
- Q7Inheritance8M
- Q8Photosynthesis15M
- Q9Genetic Technology15M
- Q10(outdated) Aspects of Human Reproduction15M