Which statement about high-copy versus low-copy plasmids is true?

Which statement about high-copy versus low-copy plasmids is true?

• A. High-copy plasmids have fewer copies per cell than low-copy plasmids.
• B. Copy number does not affect expression level.
• C. High-copy plasmids require smaller cells to replicate.
• D. High-copy plasmids have more copies per cell, increasing expression but increasing metabolic burden.

Answer: (D)

The main concept here is gene dosage from plasmid copy number and the trade-off with cellular burden. High-copy plasmids carry many copies of their DNA in each cell, which means there are more templates available for transcription of the gene of interest. When transcription can proceed efficiently, that extra template availability typically leads to higher mRNA levels and more protein production per cell. But pulling resources to replicate and express all those copies takes up cellular energy, nucleotides, and ribosomes, slowing growth and stressing the host. That’s why increasing copy number boosts expression but also increases metabolic burden. So the statement that high-copy plasmids have more copies per cell, increasing expression but increasing metabolic burden, captures this central idea. The other ideas don’t fit: copy number does affect expression, so claiming it doesn’t is incorrect; high-copy plasmids don’t work by forcing replication in smaller cells—their copy number is governed by the plasmid’s origin of replication, not by cell size; and high-copy plasmids do not have fewer copies, which would make them lower expression.

The main concept here is gene dosage from plasmid copy number and the trade-off with cellular burden. High-copy plasmids carry many copies of their DNA in each cell, which means there are more templates available for transcription of the gene of interest. When transcription can proceed efficiently, that extra template availability typically leads to higher mRNA levels and more protein production per cell. But pulling resources to replicate and express all those copies takes up cellular energy, nucleotides, and ribosomes, slowing growth and stressing the host. That’s why increasing copy number boosts expression but also increases metabolic burden.

So the statement that high-copy plasmids have more copies per cell, increasing expression but increasing metabolic burden, captures this central idea. The other ideas don’t fit: copy number does affect expression, so claiming it doesn’t is incorrect; high-copy plasmids don’t work by forcing replication in smaller cells—their copy number is governed by the plasmid’s origin of replication, not by cell size; and high-copy plasmids do not have fewer copies, which would make them lower expression.