[Campbell Biology P.404] Refer to Figure 17.24, which illustrates coupled transcription and ... | Practice Question

Refer to Figure 17.24, which illustrates coupled transcription and translation in bacteria. Based on the diagram and the direction of transcription, which mRNA molecule started being transcribed first, and on that specific mRNA, which ribosome started translating first?

  • A: The mRNA molecule on the far left; the ribosome closest to the 3' end of that mRNA.
  • B: The mRNA molecule on the far right; the ribosome closest to the 5' end of that mRNA.
  • C: The mRNA molecule on the far right; the ribosome closest to the 3' end of that mRNA.
  • D: All mRNA molecules started transcribing simultaneously; all ribosomes started translating simultaneously on each mRNA.

Explanation

In Figure 17.24, the direction of transcription is shown from left to right. This means that the RNA polymerase molecules move from left to right along the DNA, producing progressively longer mRNA molecules. Therefore, the mRNA molecule on the far right, being the longest, must have started transcribing first. On any given mRNA molecule, translation begins from the 5' end. As the mRNA is being transcribed from left to right, its 5' end would be on the left. Thus, the ribosome closest to the 5' end the leftmost ribosome on that particular mRNA string would have started translating first.