# DBT JRF 2020 — Question 60

Exam: DBT JRF 2020 · Section: B · Topic: Molecular Biology · Difficulty: Hard

## Question

Digestion of a 5 Kb linear DNA with BamHI leads to the generation of two fragments of size 2 Kb and 3 Kb, while digestion of the same DNA with HindIII generates 3 fragments of 0.7, 0.8 and 3.5 Kb. When the same DNA is cut with both BamHI and HindIII enzymes the fragments generated are of 0.7, 0.8, 1.3 and 2.2 Kb. The right order of the recognition sites for the two enzymes is

## Options

- **A.** one HindIII site between two BamHI sites
- **B.** two HindIII sites followed by one BamHI site
- **C.** one BamHI site between two HindIII sites
- **D.** two BamHI sites followed by one HindIII site

## Answer

Option C

## Explanation

In the B-form DNA double helix (the physiologically predominant form under conditions of normal hydration and ionic strength): Base pairs are PERPENDICULAR to the helical axis (they lie in planes that are perpendicular to the central axis of the helix). Adjacent base pairs STACK on top of each other parallel to each other (like stacked plates or coins), separated by a rise of 3.4 Å per base pair, with the stacking planes perpendicular to the helical axis. This is the 'stacking on top of each other, perpendicular to the helix axis' arrangement. The helix makes one complete turn every 10.5 bp (34 Å pitch / 3.4 Å per bp ≈ 10 bp per turn, commonly approximated as 10). A-form DNA has more tilted base pairs; Z-form DNA is left-handed.

Source: https://mypyqbuddy.com/questions/digestion-of-a-5-kb-linear-dna-with-6a05a8a02adafec470293254