[Campbell Biology P.502] Referring to the electron micrograph at the top of Figure 21.10a, h... | Practice Question

Referring to the electron micrograph at the top of Figure 21.10a, how could you determine the direction of transcription if the red arrow were not present?

  • A: By observing the increasing length of the RNA transcripts, with the shorter transcripts at the beginning and longer ones at the end of the transcription unit.
  • B: By identifying the specific type of RNA polymerase involved in the transcription process.
  • C: By observing the density of dark dots (RNA polymerase molecules) along the DNA strand.
  • D: By the characteristic 'feather-like' appearance, which always points in the direction of transcription.

Explanation

In the electron micrograph, the RNA transcripts grow progressively longer as transcription proceeds. Therefore, the direction of transcription can be inferred by observing the gradient of transcript lengths: the shortest transcripts are at the initiation point, and they become progressively longer towards the termination point.