Q.44 A linear DNA contains five restriction sites for EcoRI and three restriction sites for BamHI. The number of fragments that will be generated after digestion with EcoRI is ______.
A linear DNA with five EcoRI restriction sites produces six fragments upon complete digestion.
Core Concept
Restriction enzymes like EcoRI cut DNA at specific recognition sequences (GAATTC for EcoRI). For linear DNA, n cut sites create n+1 fragments because each cut divides one piece into two, starting from one intact molecule.
Detailed Calculation
Visualize a linear DNA as a single line with five EcoRI sites marked:
5’—[site1]—[site2]—[site3]—[site4]—[site5]—3′
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EcoRI makes cuts at all five sites simultaneously (complete digestion).
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This yields: end fragment 1, internal fragment 2, internal 3, internal 4, internal 5, and end fragment 6.
Thus, 5 sites = 6 fragments.
BamHI Information
The query mentions three BamHI sites, but asks only for EcoRI digestion results. BamHI (GGATCC recognition) would separately produce 3+1=4 fragments from the same linear DNA, but this is irrelevant here.
Common Errors
Trainees often confuse linear vs. circular DNA:
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Linear: fragments = sites + 1 (e.g., 3 sites → 4 fragments).
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Circular: fragments = sites (e.g., 3 sites → 3 fragments).
Partial digestion yields fewer fragments than complete.
Introduction: Solving Linear DNA EcoRI Digestion for CSIR NET
In molecular biology and CSIR NET Life Sciences, understanding linear DNA EcoRI restriction sites fragments is essential for recombinant DNA technology questions. A linear DNA with five EcoRI restriction sites generates exactly six fragments upon complete digestion. This follows the universal rule: fragments = restriction sites + 1.
Step-by-Step Fragment Calculation
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Identify the DNA type: Linear DNA has two free ends.
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Count recognition sites: Five EcoRI sites mean five precise cuts.
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Apply formula: Fragments=n+1=5+1=6.
Example Visualization:
Intact linear DNA: ————————————
After 5 EcoRI cuts: — | — | — | — | — | —
Fragments: 1 2 3 4 5 6
Why Not Other Numbers?
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4 fragments? Wrong—applies to 3 sites (3+1).
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5 fragments? Matches 4 sites.
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BamHI irrelevant: Query specifies EcoRI digestion only.
CSIR NET Exam Tips
Practice with variations: partial digestion (fewer fragments), circular DNA (n fragments), or double digests. Always confirm “linear” for +1 rule. This concept appears in Unit 13 (Biotechnology).
