The addition of promoter and terminator regions to the fragments of DNA.

The idea

A gene inserted into a new cell will not be expressed unless the cell's RNA polymerase knows where to begin and where to stop, so a promoter region is added to the start of the fragment and a terminator region to the end. The promoter is a specific base sequence upstream of the gene to which RNA polymerase, and in eukaryotes the necessary transcription factors, bind; this positions the enzyme so transcription starts at the correct base and runs in the correct direction. Without a promoter the gene is present in the host but never transcribed, so no mRNA and therefore no protein is made. The terminator is a sequence at the far end that causes RNA polymerase to detach, releasing the mRNA and preventing the enzyme running on into neighbouring DNA. Without a terminator, transcription may continue past the gene, producing run-on RNA rather than correctly ended mRNA.

What earns the marks

• Naming the region added at the start of the gene as the promoter and the region at the end as the terminator.
• The promoter being the binding site for RNA polymerase, or for transcription factors, so that transcription is initiated.
• The terminator causing RNA polymerase to stop and detach, releasing the mRNA.
• Explaining that without a promoter the gene is not transcribed, so the protein is not produced.
• Explaining that without a terminator transcription may continue past the gene, producing run-on RNA rather than correctly ended mRNA.
• A promoter determining in which cells or tissue the gene is expressed.

Common mistakes and how to put them right

• Saying the promoter is where translation begins, or confusing it with the start codon on the mRNA (correct.

  Put it right: The promoter is a DNA sequence where transcription is initiated).

• Stating that RNA polymerase binds to the terminator region (correct.

  Put it right: RNA polymerase binds the promoter and is released at the terminator).

• Believing the promoter and terminator are never transcribed (correct.

  Put it right: The terminator may be transcribed into RNA, though it is not translated into protein).

• Writing that adding a promoter 'makes more protein' with no mention of transcription (correct.

  Put it right: Link the promoter to initiation of transcription).

• Adding only a promoter and assuming transcription will stop by itself (correct.

  Put it right: A terminator is also needed to release RNA polymerase and end transcription).

Exam tips

• On a diagram of a DNA fragment, the region before the gene is usually the promoter and the region after it the terminator, but check the labels rather than assuming.
• Tie each region to RNA polymerase - starting for the promoter, detaching for the terminator.
• If asked why a transgenic gene is not expressed, a missing promoter is usually the intended answer.