This is a black-and-white photograph of the gel, used to check the DNA extractions from gel slices. The gel was run by groups B from the wet lab.
 Lanes 2 - 6 (
A) and lanes 8 - 10 (B) show 10 ml samples from DNA extraction preps, after 2 hours electrophoresis. DNA size markers to compare with the extracted DNA fragments were run in lanes 1 and 7; they show EcoRI and BamHI digests, 0.5 μg Lambda DNA each.

It is clear that in lane 2, fragment EcoA  is present (C).
In lane 3 there is EcoB (
D), in lane 4 a mixture of EcoC and EcoD (E), in lane 5 EcoE was expected, but apparently this was lost during the isolation procedure.. (F). In lane 6, EcoF was easily recognised (G

In lane 8 BamA is visible (H), a mixture of BamB, BamC and BamD in lane 9 (I), and a mixture of BamE and BamF in lane 10 (J).

Some contaminations are detected: (K):
there is some EcoCD in the EcoB prep; some EcoE in EcoCD; and some BamEF in the BamBCD prep...

Marker digests for lanes 1 and 7 were not heated prior to loading in the gel slots, so there must have been some EcoAF cos-site containing DNA in lane 1 (L); and for the same reason the BamCF cos-site containing band is visible in lane 7 (M).

To estimate (as an example) the DNA concentration in the EcoB prep, of which 10μl was run in lane 3, you could do the following:
Estimate by eye that the 10
μl EcoB DNA in in lane 3 gave a band (
N) of about the same intensity as the EcoE band in the size markerlane (O). This means there is the same amount of DNA in those bands.
In the EcoE band in lane 1, there is (4878/48,502) x  0.5
μg of DNA (the size of EcoE is 4878 basepairs, total Lambda DNA is 48,502 bp, and in total there was 0.5μg DNA in lane 1). This gives about 50 ng of DNA in lane 3. That was in 10μl EcoB prep sample, so the concentration of EcoB was about 5 ng/μl.