What would be inserted into the plasmid along with the spider gene to ensure that the spider gene was only expressed in the silk glands of the silkworms?

• A. A promoter that drives expression in all tissues
• B. An antibiotic resistance gene
• C. A silencer sequence
• D. A promoter region that drives silk-gland specific expression

Answer: (D)

The key idea is controlling where a gene is turned on by using the right regulatory DNA. To make the spider gene express only in the silk glands, you need a promoter that is active exclusively in those glands. This silk-gland–specific promoter acts as the switch that starts transcription only in silk gland cells, so the spider protein is produced there and not in other tissues.

Other options don’t provide that targeted control: a promoter that works in all tissues would cause expression everywhere, defeating specificity; an antibiotic resistance gene is for selecting transformed cells, not for directing where the gene is active; a silencer would repress expression rather than enable it. Hence, a promoter region that drives silk-gland specific expression best achieves the intended tissue-specific expression.