Prokaryotic cellsEdexcel International A Level Biology: Mind map

Exam questions on Prokaryotic cells

1. Penicillin-type antibiotics work by preventing the formation of cross-links in the cell wall of growing bacteria. A culture of bacteria growing in a very dilute nutrient solution is treated with such an antibiotic and the cells are seen to burst.
   Explain why an antibiotic that prevents cell wall formation does not damage human cells.2 marks
2. Two strains of a bacterium were compared. Strain S has a capsule and pili; strain R has neither. Both strains have circular DNA in the cytoplasm. When mice were injected with strain S they developed pneumonia and died. When mice were injected with strain R they survived and no bacteria were found in their lungs after a few days. Strain S bacteria were found attached to the lung epithelium of the dead mice.
   Explain why the mice injected with strain R survived.2 marks
3. A hospital laboratory isolates a bacterium that is resistant to the antibiotic ampicillin. The gene for resistance is on a small circular DNA molecule that is separate from the main bacterial chromosome. When this strain was mixed with a non-resistant strain, a thin tube formed between pairs of cells. After several hours some of the previously non-resistant cells were able to grow on agar containing ampicillin.
   Explain how the non-resistant bacteria became resistant to ampicillin.3 marks