In stage 4, why does the growth medium contain antibiotic?

• A. Plant tissue that has antibiotic resistance survives
• B. Antibiotic accelerates plant tissue growth
• C. Antibiotic changes color of plant tissue
• D. Antibiotic has no effect on the plant tissue

Answer: (A)

This item tests how selection works in plant tissue culture after genetic transformation. The antibiotic in the growth medium serves as a selective agent: only cells that carry and express the antibiotic resistance gene can survive in its presence. Cells that don’t have the resistance gene are inhibited or killed, so they don’t proliferate. This way, the culture becomes enriched for transformed tissue that has the desired gene.

The antibiotic isn’t used to speed up growth, to change color, or to have no effect. Its role is to discriminate between transformed and non-transformed cells, ensuring that only the former continue to grow.