SME
Grade 9 Science · Problem type
Gel electrophoresis: a piece of DNA cut once
Use this for “A 5,000 bp piece is cut once, and one fragment is 3,000 bp. How long is the other?”
Diagram
Uncut: 5,000 bp
Cut: 3,000 and 2,000 bp
Shorter pieces run farther toward +.
This is an example. Type your own values to start a new problem.
L (bp)
=
a (bp)
+
b (bp)
Formulas
L
=
a
+
b
5,000 = 3,000 + 2,000
Assumptions
•
DNA is negatively charged, so in the gel it moves toward the + end.
•
Smaller fragments slip through the gel faster and travel farther; the ladder’s known sizes give the scale.
•
A restriction enzyme cutting once splits the piece into two fragments that add back to it.
Step-by-step
WE KNOW
L = 5,000 bp, a = 3,000 bp
FIND
second fragment (b)
Step 1 · Find second fragment (b)
Use L = a + b
Take the other fragment away from the whole.
b
=
L
−
a
b
=
5,000
−
3,000
b
=
2,000
bp
Check
Put the numbers back in. Both sides should match.
5,000 = 3,000 + 2,000 ✓
Topic check
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