During transcription, which RNA base pairs with adenine on the DNA template strand?

Introduction / Context:
Base pairing rules are fundamental in genetics and molecular biology. This question tests your understanding of how RNA bases pair with DNA bases during transcription, the process by which mRNA is synthesized from a DNA template. Recognizing the correct pairing helps explain how genetic information is accurately copied from DNA to RNA.

Given Data / Assumptions:
• DNA uses the bases adenine (A), thymine (T), cytosine (C), and guanine (G). • RNA uses the bases adenine (A), uracil (U), cytosine (C), and guanine (G). • Transcription forms an RNA strand complementary to a DNA template strand.

Concept / Approach:
In DNA, adenine pairs with thymine and cytosine pairs with guanine. However, when RNA is synthesized, thymine is replaced by uracil. During transcription, whenever the DNA template has adenine, the RNA polymerase adds uracil in the growing RNA chain. Therefore, the correct RNA base that pairs with adenine in DNA is uracil, not thymine. Remember that thymine is specific to DNA, while uracil is specific to RNA.

Step-by-Step Solution:
Step 1: Recall the base pairs in double stranded DNA: A pairs with T, and C pairs with G. Step 2: Recall that in RNA, the base thymine is absent and replaced by uracil. Step 3: During transcription, RNA polymerase reads the DNA template strand and inserts complementary RNA bases. Step 4: When the DNA template shows adenine, the complementary RNA base is uracil, forming an A U pair. Step 5: Therefore, identify uracil as the RNA base that pairs with adenine in DNA during transcription.

Verification / Alternative check:
You can check this by writing a short DNA sequence and then writing the complementary mRNA sequence. For example, if the DNA template segment reads T A C, the mRNA will read A U G. Notice that where DNA has A, RNA has U. This simple exercise confirms that uracil pairs with adenine when RNA is formed from DNA.

Why Other Options Are Wrong:
Option A (Thymine): Thymine pairs with adenine within DNA, but thymine is not present in RNA. Option B (Cytosine): Cytosine pairs with guanine, not with adenine. Option C (Guanine): Guanine pairs with cytosine, so it is not correct for pairing with adenine in this context. Option E (Adenine): Adenine does not pair with itself during transcription; it pairs with uracil in RNA.

Common Pitfalls:
A common error is to memorize only the DNA DNA base pairs and to forget that RNA uses uracil instead of thymine. Students may automatically think of adenine thymine pairing even when the question involves RNA. To avoid this, always check whether the context is DNA DNA pairing or DNA RNA pairing. Another pitfall is to confuse which base is absent from which nucleic acid; remember that thymine is absent in RNA and uracil is absent in DNA.

Final Answer:
During transcription, the RNA base that pairs with adenine in DNA is uracil.