Interpret Gel Bands
Read a gel: smaller DNA fragments travel farther (lower). Compare a band to the labeled ladder to estimate its size.
- Control logic: Molecular results need positive and negative controls.
- Signal interpretation: Bands, colors, curves, and E-values must be compared to a rule.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Compare an unknown band to the labeled ladder to estimate its fragment size.
In the gel, Lane A's band lines up with the ladder mark labeled 500 bp. What is the best size estimate for the DNA in Lane A?
Reviewed- A.About 500 bp
- B.About 100 bp
- C.About 1000 bp
- D.It cannot be estimated from a gel
Show the worked solution ▾
Answer: A. About 500 bp
- Step 1: Match the height: Read straight across from Lane A's band to the ladder.
- Step 2: Read the label: That height is the 500 bp mark, so Lane A is about 500 bp.
Why it's right: A band that lines up with the 500 bp ladder mark is about 500 bp.
- B: 100 bp is the lowest ladder mark; Lane A sits higher than that.
- C: 1000 bp is the highest mark, near the wells; Lane A is lower than that.
- D: A gel does estimate size by comparing to the ladder.
Aligned to Biotechnology Research and Experiments · reading level ~grade 9
Using the same gel, which lane contains the SMALLEST DNA fragment?
Reviewed- A.Lane B (band lines up with 100 bp, the lowest mark)
- B.Lane C (band near the wells)
- C.Lane A (band at the middle)
- D.All three lanes are the same size
Show the worked solution ▾
Answer: A. Lane B (band lines up with 100 bp, the lowest mark)
- Step 1: Smaller = lower: The smallest fragment travels farthest, ending lowest on the gel.
- Step 2: Find the lowest band: Lane B's band is at the 100 bp mark, the lowest of the three, so it is the smallest.
Why it's right: The lowest band traveled farthest, which means it is the smallest fragment: Lane B at 100 bp.
- B: Lane C is near the wells (1000 bp), so it is the largest, not smallest.
- C: Lane A is at 500 bp, between B and C.
- D: The bands sit at different heights, so the sizes differ.
Aligned to Biotechnology Research and Experiments · reading level ~grade 9
- In Unit 2.1 Testing Methods (PCR/Gel/Microarray), this skill turns class evidence into a result another person can check.
Fill these in as you work through the lesson.
- Well (the slot at the top where DNA is loaded):
- Ladder (lane of known fragment sizes used as a ruler):
- Migration distance (how far a band traveled down the gel):
- Base pairs (bp) (the size unit for a DNA fragment):
Smaller fragments travel (lower); to estimate a band's size, line it up with the and read the matching .
- Which way do small fragments move compared with large ones?
- How do you use the ladder to estimate an unknown band's size?
- What can a gel NOT tell you about a fragment?
Lane A lines up with the 500 bp ladder mark, Lane B with 100 bp, Lane C with 1000 bp. Rank the lanes from smallest to largest and give each size.