Transcription Unit and the Gene

A gene is the functional unit of inheritance. While we know that genes are physically located on DNA, arriving at a precise definition of a gene in terms of a DNA sequence is surprisingly difficult. The textbook makes this point clearly: a DNA sequence that codes for a tRNA or an rRNA molecule also qualifies as a gene, even though it does not code for a protein. So the definition must be broader than just "a segment of DNA that codes for a polypeptide."

To handle this complexity, the book introduces the term cistron — a segment of DNA that codes for a polypeptide. Using this idea, structural genes within a transcription unit can be classified as either monocistronic or polycistronic.

• Monocistronic genes are typical of eukaryotes. Each structural gene codes for a single polypeptide.
• Polycistronic genes are typical of bacteria (prokaryotes). A single structural gene contains the coding sequences for multiple polypeptides.

In eukaryotes, monocistronic structural genes have an additional complication: they are split genes. The coding sequences are not continuous. The parts that actually appear in the mature, processed RNA are called exons (expressed sequences). These exons are interrupted by non-coding stretches called introns (intervening sequences), which are removed during RNA processing and do not appear in the final RNA. …