Transcription and Translation
A newly discovered genetic disease has been observed in several large families. Characteristics of the disease include mental retardation and coronary defects. Researchers studying this disease have isolated the responsible gene. The functional protein encoded by this gene consists of a single polypeptide chain. Experiments were done to determine the exact sequence of the gene with the following results:
Sequencing result 1: In a family with no history of the disease, the gene was found to be 849 nucleotides long, encoding a protein containing 283 amino acids. The mRNA sequence coding for the 22-26th amino acids is shown below:
5’ AUUCCUAGUUGCACG 3’
Sequencing result 2: The genetic disease runs in family A. However, in this family, the disease is not severe and most affected individuals lead a fairly normal life. The mRNA transcribed form this gene is shown below (corresponding to the same region of the normal gene in result 1):
5’ AUUCCUAGAUGCACG 3’
Sequencing result 3: The disease is also present in family B, but affected individuals are much worse off than those in family A. The gene was sequenced, and the corresponding mRNA is shown below (same region as sequence results 1 and 2):
5’ AUUCCUAGUUGAACG 3’
An in vitro transcription/translation system was set up to express the isolated genes. Which alteration in the system would allow the gene from family A to be translated into a fully functional protein?
|A||Include a ribosome that would not recognize the codon AGA|
|B||Use a tRNA with the anticodon UCU that carries the amino acid serine|
|C||Replace the normal splicing machinery with one that will remove the incorrect codon|
|D||Substitute the normal RNA polymerase with one that randomly makes mistakes to correct the mutation|
If the corresponding tRNA was mutated to now allow incorporation of the normal amino acid into the sequence, the protein would function correctly. Responses A and C would not allow for insertion of the correct amino acid at the critical position in the protein, and response D would not guarantee correction of the mutation. As a matter of fact, it would cause more mutations.