Question

A DNA ladder shows bands at 10,000, 8,000, 6,000, 4,000, 2,000, and 1,000 base pairs. If an unknown DNA sample shows a single band that migrated the same distance as the 4,000 bp marker, what can you conclude?
Responses

The sample contains a DNA fragment approximately 4,000 base pairs long
The sample contains a DNA fragment approximately 4,000 base pairs long

The restriction enzyme cut the DNA at exactly one location
The restriction enzyme cut the DNA at exactly one location

The sample contains exactly 4,000 nucleotides
The sample contains exactly 4,000 nucleotides

The sample is contaminated with the DNA ladder

Answer 1

Correct choice: "The sample contains a DNA fragment approximately 4,000 base pairs long."

Explanation, briefly:
- Migration matching the 4,000 bp marker indicates the fragment's size is about 4,000 base pairs (gel estimates are approximate).
- You cannot conclude the restriction enzyme cut exactly once from a single band (other scenarios can produce a single band).
- "Exactly 4,000 nucleotides" is wrong (4,000 bp ≈ 8,000 nucleotides for double‑stranded DNA, and gel sizing is approximate).
- If the sample were contaminated with the ladder you would see multiple ladder bands, not just a single band.