A model that represents a process occurring in a cell of a particular organism is shown in Figure 1.

where the two strands in the middle portion of the helix have separated from each other. Twelve bases are indicated on each of the two strands. Seven complementary bases, with the sequence A U G A U C U, are shown bound to the bottom of the two DNA strands, starting from the left end of the strand. An arrow above the seven bases points from left to right.

Figure 1. Process occurring in a cell

Which of the following correctly explains the process shown in Figure 1?

Question

A model that represents a process occurring in a cell of a particular organism is shown in Figure 1.

where the two strands in the middle portion of the helix have separated from each other. Twelve bases are indicated on each of the two strands. Seven complementary bases, with the sequence A U G A U C U, are shown bound to the bottom of the two DNA strands, starting from the left end of the strand. An arrow above the seven bases points from left to right.

Figure 1. Process occurring in a cell

Which of the following correctly explains the process shown in Figure 1?

BlackTom AI · Accepted Answer

RNA nucleotides contain uracil (U) and base-pair with DNA template bases during transcription (A in DNA pairs with U in RNA; T in DNA pairs with A in RNA; G with C; C with G). The middle short strand shown has the sequence A U G A U C U (contains U), so it is RNA being synthesized complementary to a DNA template. This identifies the process as transcription initiation.

1. Relevant concepts
- Transcription: synthesis of an RNA strand from a DNA template; RNA uses uracil (U) instead of thymine (T).
- DNA replication: synthesis of DNA from DNA; new strand contains T, not U, and both strands are copied.
- Translation: synthesis of polypeptide from mRNA at ribosomes, not production of RNA from DNA.
- Alternative splicing: post‑transcriptional editing of pre‑mRNA, not the initial synthesis step.

2. Step-by-step reasoning
- The short strand shown contains U (uracil). Only RNA contains U; DNA contains T (thymine). Therefore the new strand is RNA.
- That RNA is base-paired along a single DNA strand (the two DNA strands are separated locally), consistent with RNA polymerase using one DNA strand as a template to form a complementary RNA.
- The arrow above the RNA indicates directionality (5' → 3') of RNA synthesis, showing an early/initiating segment rather than a completed, long transcript.

3. Intermediate pairing demonstration
- Example pairing at the left: DNA base T would pair with RNA A; DNA A pairs with RNA U; DNA C pairs with RNA G. The shown RNA sequence A U G A U C U therefore fits complementary pairing with the DNA template bases under it.

4. Why the correct answer is correct
- The presence of uracil and a nascent RNA strand bound to DNA is definitive for transcription initiation: RNA polymerase is synthesizing an RNA transcript from a DNA template.

5. Why the other options are incorrect
- DNA replication: would produce DNA (with T, not U) and duplicates both strands semi-conservatively; the diagram shows RNA, not DNA.
- Translation: involves ribosomes reading mRNA to make protein; it does not show RNA being synthesized from DNA.
- Alternative splicing: is modification of pre-mRNA after transcription, not the act of synthesizing RNA from DNA.

Conclusion: The process shown is initiation of transcription because a strand of RNA is being produced from a DNA template strand, which matches the provided answer.

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