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Gene to Protein
Gene to Protein

Gene to Protein Questions

Bring on the tough stuff
  1. A scientist suspects that a deadly disease is caused by the insertion of a premature stop codon in the RNA transcript. Draw the DNA, RNA, and protein sequences in someone with the disease. If the scientist is correct, which sequence(s) will be shorter?
      
  2. Pretend you are the first scientist to understand how the tRNA codes for the right amino acid. What do you think would happen if you mutated the anticodon such that the sequence no longer said GAA, but GGA?
      
  3. Explain why it is important to know exactly where translation should occur? What happens if the start site was shifted by 1 base? By 3 bases?
      
  4. Draw a eukaryotic cell and label its components. Draw the steps detailing how a protein is made from DNA. Include RNA processing and RNA transport. Make sure you label where in the cell each step is occurring.
      
  5. The genetic code is said to be degenerate. Explain why this is the case.
      
  6. Draw the lac operon, including the promoter, cAMP, the repressor protein, and RNA polymerase. Show what the operon looks like under the following conditions: low glucose, lactose available, high glucose lactose unavailable, low glucose lactose unavailable, and high glucose lactose unavailable. Make sure you indicate the expression levels of the lac genes.
      
  7. A scientist shows you a picture of a calico cat. Given that the gene for coat color is on the X chromosome, what is the sex of the calico cat? Explain why you will be unlikely to find a calico cat in the opposite gender.
      
  8. A scientist shows you the sequence of two different mRNA transcripts and insists that they are from the same gene. The sequences are mostly different. Explain how you think that the scientist would explain this phenomenon.

Answers

  1. DNA, RNA, and the protein can be drawn as lines. The students should mark that the disease DNA sequences have a change in their sequences that results in the corresponding RNA having a coding amino acid replaced with a premature stop codon (such as UAA). The protein sequence will stop at this codon in the disease individual, so the overall protein strand will be shorter than in a healthy individual.
      
  2. The GAA anticodon is found on a tRNA that codes for leucine. If the anticodon is changed to GGA, this leucine tRNA will now be capable of recognizing the codon CCU, that normally codes for proline. This mutated tRNA can therefore insert a leucine at the codon that normally codes for proline. Tricky!
      
  3. The genetic code specifies that three RNA nucleic acids (think of them as a three-letter word) code for a single amino acid. Therefore, you need to have the "correct" letters, or you will get a totally different word (the wrong amino acid). If you shift the start by 1 base, you will end up with a protein chain with completely different amino acids than you intended. However, if you shift the start base by 3, you will gain or lose 1 amino acid, but the rest of the polypeptide sequence should be preserved.
      
  4. IMAGE
      
  5. While the genetic code specifies that three RNA nucleic acids code for a single amino acid, more than one three letter code can code for the same amino acid. For example, UCC and UCG both code for the insertion of the amino acid serine.
      
  6. IMAGE
      
  7. A calico cat has a patchwork of colors on its coat because the father and mother of the animal both provided genes for different coat colors. These genes are selectively expressed throughout the cat’s coat, resulting in a patchwork pattern. Since the gene for coat color is located on the X chromosome, only females (XX animals) have a gene from each parent. The exception would be XXY males, a genetic situation that occurs very rarely. Furthermore, XXY individuals are usually sterile, which makes it difficult to breed these animals.
      
  8. The scientist would most likely explain that the two mRNA transcripts are a result of alternative splicing, meaning that the mature RNAs contained different exons. If the scientist looks at the DNA sequence of the gene, the sequences present in both mRNAs should be present. The scientist could also look at the unprocessed RNA transcript.
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