Give a detailed account of a transcription unit.

Explanation

A transcriptional unit in DNA is defined by three regions, a promoter, the structural gene and a teminator. The promoter is located towards the 5. end. It is a DNA sequence that provides binding site for RNA polymerase. The presence of promoter in a transcription unit, defines the template and coding strands. The terminator region located towards the 3 end of the coding strand contains a DNA sequence that causes the RNA polymerase to stop transcribing In eukaryotes the promoter has AT rich regions called TATA box (Goldberg- Hogness box) and in prokaryotes this region is called Pribnow box. Besides promoter, eukaryotes also require an enhancer. The two strands of the DNA in the structural gene of a transcription unit have opposite polarity. DNA dependent RNA polymerase catalyses the polymerization in only one direction, the strand that has the polarity `3. rarr 5.` acts as a template, and is called the template strand. The other strand which has the polarity `5. rarr 3.` has a sequence same as RNA (except thymine instead of uracil) and is displaced during transcription. This strand is called coding strand (Fig. 5.7).
The structural gene may be monocistronic (eukaryotes) or polycistronic (prokaryotes). In eukaryotes, each mRNA carries only a single gene and encodes information for only a single protein and is called monocistronic mRNA. In prokaryotes, clusters of related genes, known as operon, often found next to each other on the chromosome are transcribed together to give a single mRNA and hence are polycistronic.