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.

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.

Nucleotide Orientation

Nucleotides in RNA are oriented in a specific direction, with a 5' phosphate group at one end and a 3' hydroxyl group at the other. This orientation is crucial for understanding how mRNA is synthesized. The sequence provided is written in the 3' to 5' direction, indicating that the actual synthesis of mRNA occurs in the opposite direction, from 5' to 3'.

mRNA Sequence and Nucleotide Position

In the context of the provided mRNA sequence, the adenine at the left-hand side (3' end) corresponds to the last nucleotide added during transcription. Since mRNA is synthesized in the 5' to 3' direction, the first nucleotide added would be at the 5' end of the mRNA strand, making the left-hand adenine the last nucleotide incorporated into the growing mRNA molecule.