# DBT JRF 2022 — Question 26

Exam: DBT JRF 2022 · Section: A · Topic: Molecular Biology › Restriction Digestion & Gel Electrophoresis · Difficulty: Medium

## Question

A plasmid has three sites for EcoRI. When digested by this enzyme, incidentally partial digestion took place, though no uncut or nicked plasmid remained in the digestion mixture. The maximum number of bands that can be visualized in an agarose gel would be:

## Options

- **A.** 1
- **B.** 3
- **C.** 7
- **D.** 10

## Answer

Option C

## Explanation

Partial restriction digestion and gel electrophoresis bands: For a circular plasmid with 3 EcoRI sites (sites 1, 2, 3): • Complete digestion gives 3 linear fragments (3 bands on gel). • In partial digestion, NOT all sites are cut; the question states no uncut/nicked plasmid remains. Possible fragments in partial digestion: • Single-cut fragments (linear forms, 1 site cut): 3 possible bands (each a linearised form of the whole plasmid is not counted since complete cuts remain — actually partial gives fragments spanning 1 or 2 adjacent junctions) • Individual complete fragments (both adjacent sites cut): 3 bands • Adjacent pairs (only middle site not cut): 3 possible larger fragments (spanning 2 original fragments) • One full-length linear (only 1 of 3 sites cut): this band is distinct Maximum distinct bands: individual fragments (3) + pairs of adjacent fragments (3) + spanning all-but-one-junction large bands = total 7 bands. Answer: 7 (Option 3).

Source: https://mypyqbuddy.com/questions/a-plasmid-has-three-sites-for-ecori-when-6a104cd5920734408bc0c6f7