The following is a portion of an mRNA sequence:
3'-AUCGUCAUGCAGA-5'
During transcription, was the adenine at the left-hand side of the sequence the first or the last nucleotide used to build the portion of mRNA shown? Explain how you know.

Verified step by step guidance

1. Determine the directionality of the mRNA sequence provided. mRNA sequences are conventionally written in the 5' to 3' direction. However, the sequence given is written in the 3' to 5' direction, so it must be reversed to analyze transcription properly.
2. Reverse the sequence to convert it into the 5' to 3' direction. The reversed sequence becomes 5'-AUGCAGAUCGUA-3'.
3. Recall that during transcription, RNA is synthesized in the 5' to 3' direction, meaning the RNA polymerase adds nucleotides to the 3' end of the growing mRNA strand.
4. Identify the template strand of DNA that would have been used to produce this mRNA. The template strand is complementary and antiparallel to the mRNA. For the reversed mRNA sequence (5'-AUGCAGAUCGUA-3'), the template DNA strand would be 3'-TACGTCTAGCAT-5'.
5. Analyze the position of the adenine (A) at the left-hand side of the original sequence. Since transcription proceeds from the 3' end of the template DNA strand to the 5' end, the adenine at the left-hand side of the original sequence (3'-AUCGUCAUGCAGA-5') would have been the last nucleotide added during transcription.

Key Concepts

Transcription Process

Transcription is the process by which a segment of DNA is copied into mRNA by the enzyme RNA polymerase. During this process, RNA polymerase reads the DNA template strand and synthesizes a complementary RNA strand. The direction of synthesis is 5' to 3', meaning that nucleotides are added to the 3' end of the growing mRNA strand.

mRNA Orientation

mRNA is synthesized in a specific orientation, with the 5' end being the start of the molecule and the 3' end being the end. The sequence provided in the question is written in the 3' to 5' direction, which indicates that the actual synthesis of mRNA would have occurred in the 5' to 3' direction. This orientation is crucial for understanding which nucleotide was added first.

Nucleotide Addition

During mRNA synthesis, nucleotides are added one at a time to the growing chain. The first nucleotide added corresponds to the first nucleotide of the mRNA sequence, which is determined by the template DNA strand. In the given sequence, the adenine at the left-hand side (3' end) of the mRNA is the last nucleotide added, as it is the 3' end of the synthesized strand.