Transcription

Objective: Identify template use, orientation, and RNA bases.

Transcription uses RNA polymerase to synthesize RNA from a DNA template, typically by adding ribonucleotides in the 5′ to 3′ direction. RNA includes uracil rather than thymine; the RNA sequence is complementary and antiparallel to the DNA template and resembles the coding strand except for U replacing T. A promoter and associated factors help position polymerase and determine the transcription start and direction. Only one of the two DNA strands serves as template for a given transcript, though different genes can use different strands. RNA polymerase reads DNA without altering its sequence as its normal product.

Objective: Describe transcription stages and compare locations across organisms.

During initiation, transcription factors and RNA polymerase engage a promoter or associated regulatory region. In elongation, polymerase moves along a template and extends the RNA at its 3′ end. Termination releases the transcript through mechanisms that differ across genes and organisms. In eukaryotes, nuclear transcription and cytoplasmic translation are usually separated by the nuclear envelope; in many prokaryotes, transcription and translation can be coupled because there is no nucleus. Gene location and cellular context influence the details, and a promoter is not ordinarily copied into the mature RNA in its entirety.

Objective: Explain how primary transcripts can yield distinct mature RNAs.

Many eukaryotic protein-coding primary transcripts are processed before export: a 5′ cap is added, introns are removed by splicing, and a poly(A) tail is added at the 3′ end. Exons retained in a mature transcript are joined together. Alternative splicing can produce different RNA isoforms from one gene, sometimes giving different protein products. Introns are not simply useless DNA; sequences can participate in regulation or other functions. Processing patterns vary across RNA classes and organisms; not every transcript follows the same scheme. A mature mRNA sequence should be distinguished from the original genomic DNA and from the unprocessed RNA.