mRNA transcript

Meaning: RNA polymerase reads the template (antisense) strand 3′→5′ and builds the transcript 5′→3′. The mRNA is therefore complementary and antiparallel to the template and has the same sequence as the coding (sense) strand, with uracil in place of thymine. Ribosomes read its codons 5′→3′. In eukaryotes the first product, the primary transcript (pre-mRNA), receives a 5′ cap and a poly-A tail and has introns removed by the spliceosome before the mature mRNA leaves the nucleus. Prokaryotes generally do not splice, and translation can begin while transcription is still under way.

On the MCAT: Expect to write the mRNA from a template strand, then read codons to find amino acids, or to predict how a point mutation, deletion or splice-site change alters the transcript and the protein. Experiments often measure transcript level (northern blot, RT-PCR) to infer gene expression, or compare transcripts made by alternative splicing of one gene.

Common confusion: Students copy the template strand or keep T in the mRNA. The transcript matches the coding strand (U for T) and pairs with the template. Also, more transcript does not always mean more protein, because translation and protein degradation are regulated separately.