Haemoglobin is a protein carried in red blood cells. Fig. 10.1 shows three kinds of change that can occur in a gene. Table 10.1 shows one triplet of the hae...
Question 1 Report
Haemoglobin is a protein carried in red blood cells. Fig. 10.1 shows three kinds of change that can occur in a gene. Table 10.1 shows one triplet of the haemoglobin gene in a healthy person and in a person with a mutation, with a codon dictionary.
| Template triplet | mRNA codon | |
|---|---|---|
| Healthy | CTT | ............... |
| Mutated | CAT | ............... |
| mRNA codon | GAA | GUA |
|---|---|---|
| Amino acid | Glu | Val |
(a) Define the term gene mutation [1](b) Using Table 10.1, give the mRNA codon for the healthy triplet and for the mutated triplet [2]
(c) Name the amino acid coded for in each case, using the dictionary [2]
(d) Explain how this single base change can alter the protein that is made [3]
Answer Details
This question tests gene mutation and how the base sequence controls the amino acid sequence of a protein. A template DNA triplet is transcribed into a complementary mRNA codon (with U replacing T), and each codon is translated into one amino acid using the dictionary.
(a) [1] A gene mutation is a change in the base sequence of a gene. [1]
(b) and (c) [2 + 2] Read each template triplet off Table 10.1 and pair the bases (A-U, T-A, G-C, C-G) to get the mRNA codon, then use the dictionary. The completed table is:
| Template triplet | mRNA codon | Amino acid | |
|---|---|---|---|
| Healthy | CTT | GAA | Glu |
| Mutated | CAT | GUA | Val |
So for (b) the healthy codon is GAA [1] and the mutated codon is GUA [1]; for (c) these code for Glu (glutamic acid) [1] and Val (valine) [1].
(d) [3] Changing one base in the template changes the base sequence [1], so a different amino acid (valine instead of glutamic acid) is placed into the polypeptide chain [1]; this alters the shape and therefore the function of the haemoglobin protein [1]. (This particular Glu-to-Val change is the one that causes sickle-cell haemoglobin.)
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