DNA, RNA & Protein Synthesis (College Board AP® Biology): Exam Questions

Researchers were investigating the process of transcription in eukaryotic cells. They isolated pre-mRNA molecules from human fibroblast cells and analyzed their lengths using gel electrophoresis. The same mRNA sequences were then isolated from the cytoplasm and analyzed using electrophoresis to separate the sequences by size. The results are shown in Figure 1.

Which of the following conclusions about eukaryotic gene expression is best supported by the data in Figure 1?

Scientists are studying the role of a newly identified enzyme in the DNA replication process of eukaryotic cells. They propose that this enzyme is crucial for initiating DNA synthesis by assisting in unwinding the double helix during replication.

Which of the following questions would best guide the researchers towards a direct test of their proposal?

In an experiment to verify the nature of DNA replication, scientists used nitrogen isotopes to label the DNA of E. coli. The bacteria were grown in a medium containing heavy nitrogen (N¹⁵) and then switched to a medium containing light nitrogen (N¹⁴). The results after one round of replication are shown in the table below.

Which of the following conclusions is best supported by this data?

Researchers were investigating the role of RNA polymerase in transcription. They set up an in vitro transcription system with purified RNA polymerase, DNA template, and nucleotides. The researchers measured the rate of RNA synthesis under various conditions. The results are shown in Figure 1.

Which of the following conclusions about transcription is best supported by the data?

The figure represents the process of expression of gene X in a eukaryotic cell.

The primary transcript in the figure is 15 kilobases (kb) long, but the mature mRNA is 7 kb in length. Describe the modification that most likely resulted in the 8 kb difference in length of the mature mRNA molecule. Identify in your response the location in the cell where the change occurs.

Predict the length of the mature gene X mRNA if the full-length gene is introduced and expressed in prokaryotic cells. Justify your prediction.

Researchers are studying the use of RNA vaccines to protect individuals against certain diseases. To develop the vaccines, particular cells are first removed from an individual. Then mRNAs coding for specific proteins from a pathogen are introduced into the cells. The altered cells are injected back into the individual, where the cells make the proteins encoded by the introduced mRNAs. The individual then produces an immune response to the proteins that will help to protect the individual from developing a disease if exposed to the pathogen in the future.

When introduced into cells, the mRNAs used for vaccines must be stable so that they are not degraded before the encoded proteins are produced. Researchers developed several modified caps that they hypothesized might make the introduced mRNAs more stable than mRNAs with the normal GTP cap. To test the effect of the modified caps, the researchers produced mRNAs that differed only in their cap structure (no cap, the normal cap, or modified caps I, II, or Ill). They introduced the same amount of each mRNA to different groups of cells and measured the amount of time required for half of the mRNAs to degrade (mRNA half-life) and the total amount of protein translated from the mRNAs (Table 1).

TABLE 1. EFFECT OF mRNA CAP STRUCTURE ON mRNA HALF-LIFE AND PROTEIN TRANSLATED FROM THE INTRODUCED mRNA

Based on the data, identify which cap structure is most likely to protect the end of the mRNAs from degradation.

Based on the data for the mRNAs with modified caps, describe the relationship between the mRNA half-life and the total amount of protein produced.

After examining the data on mRNA half-lives and the amount of protein produced, the researchers hypothesized that each mRNA molecule with modified cap I was translated more frequently than was each mRNA molecule with the normal GTP cap. Evaluate their hypothesis by comparing the data in Table 1.

The introduction of mRNAs into cells allows the cells to produce foreign proteins that they might not normally produce. Explain why the production of a foreign protein may be more likely from the introduction of mRNA than DNA into cells.